Growth and Yielding of Ten Cultivars of Plums (Prunus domestica L.) Grown in Organic System

Abstract

In recent years, Polish producers have been increasingly interested in organic fruit production. In this growing system, it is very important to choose cultivars that are less susceptible to diseases and pests. In a study conducted between 2016 and 2024 in central Poland, the suitability of ten plum cultivars (‘Cacanska Lepotica’, ‘Cacanska Najbolja’, ‘Kalipso’, ‘Katinka’, ‘Jubileum’, ‘Presenta’, ‘Silvia’, ‘Tophit’, ‘Tophit Plus’, and ‘Vision’) for organic cultivation was assessed. The results demonstrated that organic plum cultivation is feasible; however, it remains challenging. Based on long-term observations and experimental data, it was found that early-ripening plum cultivars such as ‘Katinka’, ‘Kalipso’ and ‘Cacanska Lepotica’ are more suitable for organic farming than others. Fruits of these cultivars were either not infested or only occasionally affected by the plum fruit moth and exhibited only sporadic symptoms of brown rot. The medium-early-ripening ‘Cacanska Najbolja’ cultivar is also worth noting, as the trees yield well in organic orchard conditions; however, the fruit is sometimes affected by rot-causing diseases and by the plum fruit moth. Fruits of late-ripening cultivars (‘Presenta’, ‘Tophit’, ‘Tophit Plus’, and ‘Vision’) were significantly more frequently infested by plum fruit moth caterpillars and exhibited a higher incidence of rot. The highest level of plum fruit moth infestation was observed in the fruit of ‘Tophit Plus’, whereas ‘Jubileum’ was the most susceptible to brown rot. These findings provide long-term evidence supporting cultivar selection as a key non-chemical strategy for improving the reliability of organic plum production under temperate climate conditions. The results may support cultivar selection strategies aimed at improving the sustainability and reliability of organic plum production in temperate climates.

1. Introduction

Plums are among the major fruit crops in Poland, where they occupy over 9000 ha, with an annual production of approximately 100,000 tonnes [1]. Orchards are dominated by cultivars of European plum (Prunus domestica L.), one of the principal fruit species grown in Central Europe. Plum fruits are used both for fresh consumption and processing. Under Central European climatic conditions, plum production is strongly influenced by weather conditions during flowering, fruit set and fruit ripening, and fluctuations in temperature and precipitation may substantially affect yield level and yield stability [2,3].

In Poland, many Prunus domestica cultivars perform well under conventional and integrated production systems [4,5]. However, organic plum production remains relatively limited in scale. In 2024, the area of organic plum orchards in Poland amounted to slightly less than 500 ha, with an estimated fruit production of 1033 tonnes [6]. This indicates that organic production accounts for only a small fraction of the total plum-growing area and total plum fruit production in the country. Although the area and production of organic plums increased steadily until 2022, a slowdown and even a decline have been observed in recent years. These changes may be attributed to the numerous biological, agronomic and economic challenges associated with fruit production under organic management systems [7,8,9,10]. Weed management may also represent an additional challenge in young organic plum orchards; however, previous studies indicate that different in-row orchard floor management systems may have a limited effect on plum tree growth, yield and fruit quality [11].

Organic plum production is also limited in other European countries. Available data indicate that Romania, Bulgaria and Spain are among the largest producers of organic plums in the European Union, however, organic production still represents less than 5% of total plum production in these countries. Previous studies on organic plum production conducted in Romania [12,13], Bulgaria [14], Norway [15,16] and Italy [17,18,19] focused mainly on pest and disease management, fruit quality and orchard management practices. Only a limited number of studies have evaluated the suitability of European plum cultivars for organic orchard production systems [20].

A major limitation of organic plum production is the limited availability of effective plant protection methods [14]. Consequently, diseases and pests may significantly reduce tree productivity and fruit quality in organic orchards [15]. Plum cultivation is further constrained by the widespread occurrence of plum pox virus (PPV) [21], and the brown rot of stone fruit trees (Monilinia spp.) [2,8], as well as by pests damaging fruitlets and fruits, such as plum sawflies (Hoplocampa minuta, Hoplocampa flava), the plum fruit moth (Grapholita funebrana), aphids and mites [7,16].

Sharka disease, caused by plum pox virus (PPV), has posed a major challenge to plum cultivation in Poland since the second half of the twentieth century. The spread of PPV has strongly affected plum production and has made cultivar selection one of the most important preventive strategies. In organic orchards, where the control of aphid vectors is particularly difficult, the use of resistant or tolerant cultivars is especially important. Several European plum cultivars characterised by tolerance to PPV, good fruit quality and diverse ripening periods have performed well in conventional and integrated production systems [4,5]. However, their performance under organic orchard conditions, where pest and disease pressure may be higher and plant protection options are limited, requires separate evaluation.

Winter hardiness is another important trait determining the suitability of plum cultivars for organic orchards in Central Europe. European plum cultivars are generally well adapted to Polish climatic conditions and, during deep dormancy, may tolerate temperatures as low as −30 °C [4]. However, winter hardiness alone does not determine cultivar suitability, because tree condition, disease pressure, flowering time and fruit susceptibility to pests and pathogens may also strongly influence the reliability of production under organic management.

Consumer preferences in the European Union have increasingly shifted towards healthier and more sustainable food products. Consequently, organically produced food has gained growing importance, and plum cultivation under organic management has attracted increasing attention [22,23]. Although fruits produced under organic systems may be more infested with pests and diseases than those from conventional systems [24,25,26,27], several studies indicate that organic cultivation may improve the health-promoting value of fruit, particularly with regard to polyphenol and vitamin C content [28]. Nevertheless, the quantity and quality of bioactive compounds in plum fruits strongly depend on genotype [29,30].

Under the limited plant protection possibilities in organic orchards, cultivar selection becomes particularly important. Cultivars intended for organic production should, in addition to early fruiting and high yield, also be resistant or tolerant to plum pox virus (PPV), and have low susceptibility to fungal diseases and pest infestation. For many years, the cultivar ‘Węgierka Zwykła’ (‘Common Prune’ type) was widely grown in conventional plum orchards in Poland due to its high productivity, relatively regular fruiting and very good fruit flavour [31]. However, its high susceptibility to PPV makes it unsuitable for organic orchards, where the control of aphids, which are vectors of this virus, is challenging. Currently, several cultivars can be recommended for organically managed orchards, including ‘Jojo’, which is resistant to PPV [10]. The cultivar ‘Herman’, which shows tolerance to this virus, can also be successfully cultivated under organic conditions [8].

Available studies concerning organic plum production are still limited and usually involve short-term observations, individual cultivars or selected aspects of fruit quality and productivity. Long-term comparative evaluations of multiple European plum cultivars under organic orchard conditions in Central Europe remain scarce.

Therefore, a long-term experiment was conducted in an organically managed orchard. The aim of this study was to compare the tree growth, yield, health status and fruit quality of ten Prunus domestica cultivars grown under organic production conditions. In addition, the influence of weather conditions on tree health, flowering, yield, ripening time and fruit quality was evaluated.

2. Materials and Methods

The study was conducted between 2016 and 2024 in the Experimental Ecological Orchard (EEO) in Nowy Dwór-Parcela (central Poland; altitude 159 m above sea level; latitude 51°52′13″ N; longitude 20°14′49″ E), which belongs to the National Institute of Horticultural Research in Skierniewice.

The trees were planted on sandy loam podzolic soil with a loamy subsoil, classified as class IVb of the rye–potato agricultural complex soil category. Before planting, soil samples were collected from multiple points in a randomised manner in the 0–20 cm soil layer and pooled into composite samples. Chemical soil analyses were performed at the Chemical Analysis Laboratory of the National Institute of Horticultural Research in Skierniewice.

The following parameters were analysed: soil pH, macroelement content (P, K and Mg), and organic matter content. The soil pH was 6.7, which is considered suitable for plum cultivation. The macroelement content (P—12.5; K—29.9; and Mg—10.3 mg·100 g⁻¹ DW soil) corresponded to a high soil fertility class, while the organic matter content was 2.2%.

Ten cultivars of Prunus domestica characterised by tolerance to plum pox virus (PPV) were selected for the field experiment. The cultivars ‘Kalipso’, ‘Katinka’, ‘Silvia’, ‘Cacanska Lepotica’, ‘Cacanska Najbolja’, ‘Jubileum’, ‘Vision’, ‘Tophit’, ‘Tophit Plus’, and ‘Presenta’, grafted on ‘Wangenheim Prune’ seedlings, were planted at a spacing of 4.5 × 2.5 m in four replications in a randomised block design with three trees per plot. The evaluated cultivars were grown in the presence of other flowering plum cultivars located within the Experimental Ecological Orchard, which ensured appropriate pollination conditions. In addition, mason bees (Osmia rufa) were introduced into the orchard annually to support pollination during the flowering period.

The trees were irrigated using a drip irrigation system. Irrigation lines were suspended under the tree crowns, which were trained in a spindle form. During the first two years after planting, the soil in the orchard was maintained as mechanical fallow. In the third year, naturally occurring grass was introduced in the inter-rows, while mechanical fallow was maintained in the tree rows. Depending on weather conditions, the grass was mown 6–7 times during each growing season.

The trees were fertilised in spring with cow manure at a rate of 30 t·ha⁻¹.

Preparations permitted for use in organic plum production were applied to protect trees and fruit against diseases and pests. Disease control included 2–3 treatments with plant protection products based on copper oxychloride (WG or SC formulation) applied annually at a rate of 1.5–3 kg or L·ha⁻¹.

Pest control included a single treatment with Treol 770 EC (paraffin oil, 770 g L⁻¹), applied at a concentration of 1.5% against the red spider mite (Panonychus ulmi) and the brown scale (Parthenolecanium corni). Aphids were controlled using one or several treatments, depending on infestation intensity, using a mixture of potassium horticultural soap with extracts of tansy, horsetail or garlic (concentration 2%) and ethanol (concentration 1%). In years with high pest pressure, preparations containing azadirachtin were additionally applied.

2.1. Measurements, Observations and Analyses

2.1.1. Health Condition of the Trees

Winter hardiness was assessed annually in spring according to a 9-point scale developed by COBORU in Słupia Wielka [32], where 1 indicated severely damaged or dead trees and 9 indicated the absence of visible frost injury symptoms.

Tree susceptibility to plum pox virus (PPV) was assessed twice per year, in late June and mid-September. The condition of infected trees was evaluated visually using a 0–3 scale describing symptoms on leaves: 0—trees without symptoms, 1—symptoms present in one part of the crown, 2—symptoms present on several scaffold branches, and 3—symptoms present throughout the tree [33].

Tree susceptibility to colonisation by aphids was assessed annually in June. All cultivars were subjected to the same aphid control measures throughout the study period. Twelve trees per cultivar were evaluated. The occurrence of aphid colonies on shoot apices was assessed using a five-point scale adapted for the purposes of this study, based on ordinal scales commonly used in studies of aphid infestation in fruit trees. Aphids such as Brachycaudus helichrysi and Hyalopterus pruni are among the most common species colonising plum trees [34]. The scale described the percentage of colonised shoot apices as follows: 0—no occurrence; 1—1–20% of shoot apices colonised; 2—21–40%; 3—41–60%; 4—61–80%; and 5—81–100%.

2.1.2. Tree Growth, Flowering, Yielding, and Cumulative Yield Efficiency

Tree growth vigour was expressed as trunk diameter, measured annually in autumn after the completion of vegetative growth. Measurements were taken at a permanently marked point 10 cm above the graft union.

Based on these measurements, trunk cross-sectional area (TCSA) was calculated, which is widely regarded as a reliable indicator of tree vigour [35].

Phenological observations were carried out annually by recording the beginning, full bloom and end of flowering according to the methodology described by Tzonev and Yamaguchi [36].

Damage to flower buds and flowers caused by spring frosts was assessed after frost events by determining the percentage of damaged buds or flowers on a selected branch of one tree in each replication, according to the methodology developed for plum by COBORU in Słupia Wielka [32].

The occurrence of plum sawflies (Hoplocampa flava and Hoplocampa minuta) was monitored using white sticky traps installed prior to flowering. The number of captured adults was monitored every 2–3 days. The percentage of damaged fruitlets for each cultivar was assessed after flowering (April–May) on a selected branch of one tree in each replication [7].

To determine yield (kg), fruits were harvested annually from each tree separately after the trees had entered the bearing period and were then weighed.

Cumulative yield efficiency (kg·cm⁻²) was calculated at the end of the experiment by dividing the cumulative yield obtained in 2018–2024 by the trunk cross-sectional area measured in the final year of the experiment [37].

2.1.3. Fruit Quality

Fruits for laboratory analyses were harvested at commercial maturity. The harvest date was determined based on the skin and flesh colour typical for each cultivar. Fruits were collected from different parts of the canopy.

Fruit quality was evaluated based on four parameters.

Fruit weight (g) was determined from a 2 kg fruit sample collected randomly from three trees in each replication. In years with low yield, the entire harvest was used as the sample.

Total soluble solids content (°Brix) was measured using an ATAGO PR-101 refractometer (ATAGO Co., Ltd., Tokyo, Japan). Twenty-five fruits were randomly selected from the previously weighed sample for each cultivar.

Damage caused by plum fruit moth (Grapholita funebrana) was assessed using 200 fruits per cultivar (4 replications × 50 fruits). Results are presented as the percentage of damaged fruits.

Infection by brown rot was assessed on 200 fruits per cultivar (4 replications × 50 fruits). Results are presented as the percentage of infected fruits.

2.2. Weather Conditions

Weather data for 2016–2024 were collected using an iMetos 1 automatic meteorological station (Pessl Instruments, Weiz, Austria) located in the Experimental Ecological Orchard in Nowy Dwór-Parcela at a height of 2 m above ground level.

Table 1. Air temperatures on the experimental plot in April–October, 2016–2024 [°C].

	2016	2017	2018	2019	2020	2021	2022	2023	2024
Air temperatures in April
Mean	9.2	7.2	13.3	12.6	9.1	6.5	7.4	8.4	10.8
Min	−2.2	−4.5	−2.7	−5.6	−4.3	−4.8	−3.2	−3.4	−1.4
Max	25.1	23.1	29.7	28.6	25.0	21.7	21.4	23.4	28.0
Air temperatures in May
Mean	15.0	14.0	16.5	12.9	11.4	12.5	13.6	13.0	16.6
Min	1.7	−3.0	1.6	−2.8	−1.6	0.8	1.3	−0.7	0.0
Max	30.3	29.7	33.6	28.3	26.4	28.3	27.6	24.8	28.9
Air temperatures in June
Mean	18.6	17.8	18.5	22.1	18.3	19.5	18.8	18.1	19.3
Min	2.8	6.7	1.2	7.0	3.5	3.5	5.2	2.8	5.9
Max	34.2	31.4	35.5	39.0	32.3	35.8	33.7	31.9	34.5
Air temperatures in July
Mean	19.1	18.6	20.6	18.7	18.6	21.2	19.0	20.2	21.6
Min	6.7	5.6	9.4	5.7	7.3	10.3	7.1	8.7	9.9
Max	34.0	33.4	35.2	36.8	33.3	38.5	35.6	34.2	36.5
Air temperatures in August
Mean	18.3	19.3	20.2	20.1	19.5	16.7	20.6	20.5	20.7
Min	5.0	5.8	5.9	5.6	7.0	5.1	8.5	8.1	8.9
Max	34.2	37.4	35.4	36.8	35.9	32.7	32.7	35.6	36.1
Air temperatures in September
Mean	15.3	13.5	15.2	14.0	14.6	13.5	11.9	18.0	17.3
Min	−2.6	1.6	1.3	2.0	1.9	1.5	1.6	6.3	0.6
Max	34.2	25.0	30.6	32.5	29.5	28.8	23.1	31.9	33.1
Air temperatures in October
Mean	7.3	9.8	9.6	10.3	10.0	8.8	11.1	10.8	9.7
Min	−2.4	0.7	−2.1	−4.1	0.4	−4.1	−0.6	−2.5	−1.7
Max	22.9	21.7	22.8	24.4	22.2	22.7	21.9	26.9	23.3

presents the mean, minimum and maximum temperatures for the period April–October, as weather conditions during these months significantly influenced flowering, fruit set and fruit quality.

Table 2. Average, minimum, and maximum annual temperatures in 2016–2024 [°C].

	2016	2017	2018	2019	2020	2021	2022	2023	2024
Mean	10.1	9.8	9.4	11.1	10.7	9.5	10.0	11.0	11.7
Min	−17.9	−20.9	−20.6	−11.5	−10.1	−22.4	−22.3	−12.9	−12.8
Max	34.2	37.4	35.5	39.0	35.9	38.5	38.4	35.6	35.5

presents mean, minimum and maximum annual temperatures, while

Table 3. Total precipitation recorded in 2016–2024 [mm].

Month	Total Precipitation Recorded in 2016–2024 [mm]									Mean
	2016	2017	2018	2019	2020	2021	2022	2023	2024
January	14.0	12.0	16.8	24.8	24.2	25.2	35.0	49.6	59.2	29.0
February	61.4	22.8	6.8	23.2	44.6	23.0	28.0	47.6	47.2	33.8
March	21.0	34.2	10.6	23.2	9.2	9.0	1.4	47.6	40.4	21.8
April	19.6	48.8	22.6	9.7	7.8	40.2	40.0	57.0	29.4	30.6
May	37.4	37.0	39.0	44.2	59.0	47.2	47.4	43.6	27.2	42.4
June	98.6	102.4	21.2	31.8	117.4	56.0	54.6	30.6	43.8	61.8
July	81.6	36.0	58.4	43.4	53.8	108.0	101.0	38.8	25.2	60.7
August	34.2	45.4	48.0	40.4	82.8	121.8	87.6	54.6	53.0	63.1
September	7.4	123.8	55.0	57.4	28.4	25.0	41.6	10.8	41.8	43.5
October	60.4	53.4	41.6	16.8	72.2	10.6	26.6	75.6	31.0	43.1
November	28.8	28.4	9.0	12.8	10.4	38.0	24.8	68.4	27.0	27.5
December	39.4	48.2	35.2	32.0	22.6	22.8	63.6	60.6	18.2	38.1
Total	503.8	592.4	364.2	359.7	532.4	526.8	551.6	584.6	443.4	495.4

presents monthly and annual precipitation totals.

Late spring frosts occurred in April and, in some years, also in May, shortly before or during the plum flowering period. They damaged some of the flower buds and flowers in 2019–2022.

Temperatures during fruit development and ripening (June–October) varied among the study years and influenced fruit ripening times, fruit quality, and the occurrence of pests and diseases.

Average annual air temperatures showed high variability between years. Most winters were relatively mild. In 2021 and 2022, temperatures dropped below −22 °C, but these frost events were short-term and did not cause frost damage to trees, shoots or buds.

Annual precipitation varied considerably between years and months. The highest annual precipitation was recorded in 2023, whereas 2018 and 2019 were relatively dry years. The highest rainfall occurred during the summer months (June–August).

2.3. Statistical Analysis

Data were subjected to statistical analysis using analysis of variance (ANOVA). Percentage data were transformed using the Bliss transformation (arcsine square root transformation) prior to statistical analysis in order to stabilise variance and improve normality. The experimental factor was cultivar, while observations were repeated across the study years. Cultivar was treated as a fixed factor, whereas years were treated as repeated observations in the statistical model. The experimental design consisted of four field replications, with three trees per plot.

Mean values were calculated based on four field replications. Mean values presented in the tables represent averages calculated across all study years. Percentage data were checked for normality and homogeneity of variance before analysis.

When significant differences were detected, mean values were separated using Tukey’s honestly significant difference (HSD) test.

Differences between means were considered significant at p ≤ 0.05. The results are presented as mean values ± standard error (SE).

All statistical analyses were performed using Statistica 13.3 software (TIBCO Software Inc., Palo Alto, CA, USA).

3. Results and Discussion

3.1. The Health Status of Evaluated Trees of the European Plum Cultivars

3.1.1. Winter Hardiness

During the study period (2016–2024), winters were relatively mild. Temperatures occasionally fell lower than minus 20 °C (Table 2). Trees of all cultivars survived these conditions well. In the spring, there was no evidence of damage to their trunks or shoots or discoloration of the bark characteristic of frost damage. No symptoms of silver leaf (Chondrostereum purpureum) were observed on the trees either. In contrast to observations reported for some other fruit species grown organically, no serious problems associated with winter injury or secondary disease development were observed in the evaluated plum cultivars.

3.1.2. Susceptibility to Plum Pox Virus

During the 9-year study, the health status of most evaluated European plum cultivars remained good. Typical symptoms of plum pox virus (PPV) infection, manifested as leaf discoloration, were observed only on two out of 12 trees of the ‘Vision’ cultivar and on one tree of ‘Cacanska Lepotica’. The infected trees were removed from the orchard in order to limit further spread of the virus. Earlier reports classified ‘Vision’ and ‘Cacanska Lepotica’ as slightly susceptible to PPV infection, whereas ‘Katinka’ and ‘Presenta’ were considered more sensitive [3,38]. However, under the organic orchard conditions of the present experiment, PPV symptoms occurred only sporadically. These differences indicate that the expression of PPV susceptibility may depend not only on genotype, but also on environmental conditions and virus strain variability.

3.1.3. Susceptibility to Colonisation by Aphids

Aphids are among the most important pests limiting tree growth and productivity in organically managed plum orchards [7]. Their harmfulness depends on the species and the density of the population. Aphids such as leaf-curling plum aphid (Brachycaudus helichrysi) and mealy plum aphid (Hyalopterus pruni) are among the most common species occurring on plum trees [34].

In the present study, aphids were regularly observed on trees of the evaluated plum cultivars grown under organic orchard conditions. The intensity of infestation varied considerably among years and cultivars, with the highest aphid pressure recorded in 2022, when weather conditions favoured aphid development. The most heavily colonised cultivars included ‘Cacanska Najbolja’ and ‘Jubileum’, whereas ‘Silvia’, ‘Tophit’ and ‘Presenta’ showed the lowest susceptibility to aphid colonisation during the study period (

Table 4. Degree of aphid infestation of shoot apexes in the studied plum cultivars (on a five-point scale *) in 2017–2024.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2017	0	1	1	1	2	0	0	0	0	0
2018	1	2	1	1	1	1	0	1	0	1
2019	0	2	1	1	1	0	0	2	0	2
2020	1	3	2	2	2	0	0	3	0	3
2021	1	2	1	1	1	1	1	1	1	1
2022	2	4	2	3	4	1	1	2	1	3
2023	1	2	3	1	2	0	0	1	0	1
2024	1	1	2	2	1	0	0	1	0	1

*—0: no occurrence of aphids, 1: 1–20%; 2: 21–40%; 3: 41–60%; 4: 61–80%; and 5: 81–100% of shoot apexes colonised.

). A similar diversity in susceptibility to aphid colonisation was observed among Japanese plum cultivars evaluated under organic cultivation conditions in Spain [39]. Lower susceptibility to aphid infestation may be particularly valuable in organic orchards, where direct control options are limited and aphids may contribute to the spread of PPV. However, all evaluated cultivars were colonised to some extent, which confirms earlier reports indicating the lack of fully aphid-resistant plum cultivars [40].

3.2. Growth and Yield of Plum Trees

3.2.1. Flowering Period

Flowering dates of the evaluated plum cultivars varied considerably among years and cultivars. In 2018 and 2020, flowering occurred at the typical time for Prunus domestica, namely in the second decade of April. In contrast, in 2019, 2022 and 2023, flowering began several to more than ten days later, whereas in 2021 flowering was delayed until the first half of May due to prolonged low spring temperatures. The earliest flowering was recorded in 2024. Early- and medium-early-flowering cultivars (‘Cacanska Lepotica’, ‘Cacanska Najbolja’, ‘Katinka’, ‘Kalipso’, ‘Silvia’ and ‘Presenta’) generally began flowering 1–5 days earlier than late-flowering cultivars (‘Jubileum’, ‘Tophit’, ‘Tophit Plus’ and ‘Vision’).

The flowering period of the evaluated plum cultivars usually lasted from 6 to 10 days, which is consistent with observations reported by Szabo and Nyeki [41]. However, flowering duration differed among cultivars and years. In 2020, the flowering period of some cultivars (‘Kalipso’, ‘Katinka’ and ‘Jubileum’) reached 13 days, whereas the longest flowering period (16 days) was recorded for ‘Tophit Plus’. In contrast, the shortest flowering period (4–6 days) was observed in 2024, when flowering started particularly early (

Table 5. Beginning and ending of flowering of 10 plum cultivars in the years 2018–2024.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2018	16–22.04	16–23.04	15–21.04	15–22.04	17–24.04	16–22.04	18–27.04	19–28.04	16–22.04	19–24.04
2019	20–27.04	20–26.04	19–26.04	19–25.04	21–28.04	19–25.04	21–28.04	22–29.04	18–25.04	21–27.04
2020	17–24.04	16–27.04	13–25.04	13–25.04	15–27.04	13–24.04	17–27.04	18–28.04	13–24.04	17–25.04
2021	07–12.05	06–12.05	07–13.05	07–12.05	05–14.05	09–13.05	08–15.05	08–15.05	08–15.05	07–13.05
2022	28.04–4.05	25.04–3.05	28.04–2.05	26.04–2.05	28.04–9.05	25.04–1.05	29.04–10.05	27.04–12.05	26.04–05.05	27.04–6.05
2023	21.04–1.05	22.04–1.05	23.04–1.05	22–30.04	22.04–3.05	21–29.04	23.04–4.05	24.04–6.05	22.04–1.05	24.04–1.05
2024	06–10.04	05–09.04	05–09.04	05–09.04	06–12.04	05–09.04	07–12.04	08–13.04	06–11.04	07–11.04

). Under organic orchard conditions, ‘Vision’ had the shortest flowering period, whereas ‘Tophit Plus’ and ‘Jubileum’ flowered for the longest time. The prolonged flowering period observed in ‘Jubileum’ was previously described as a cultivar-specific trait [42]. A longer effective pollination period (EPP) may be particularly advantageous under the unstable spring weather conditions typical of Central Europe, because it may increase the probability of successful pollination and adequate fruit set despite temporary unfavourable conditions.

3.2.2. Damage to Flower Buds and Flowers Caused by Late Spring Frosts

During the study period, late spring frost damage to flower buds and flowers was recorded mainly in 2019–2022. The most severe frost injury occurred in years characterised by frost events shortly before or during flowering.

Considerable differences in frost susceptibility were observed among cultivars and years (

Table 6. Damage [%] to plum flower buds and flowers due to spring frosts.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2019	9.8 ± 1.11 abc	2.3 ± 0.48 d	11.3 ± 1.15 ab	14.5 ± 0.65 a	4.3 ± 0.85 cd	15.5 ± 1.04 a	6.3 ± 0.75 bcd	10.0 ± 1.29 abc	12.3 ± 1.49 ab	12.3 ± 1.71 ab
2020	9.3 ± 0.75 de	3.0 ± 0.41 f	9.3 ± 0.85 ab	16.8 ± 0.85 bc	6.5 ± 0.65 ef	28.3 ± 1.49 a	13.8 ± 1.11 cd	15.8 ± 1.38 c	21.0 ± 1.47 b	13.3 ± 0.75 cd
2021	4.0 ± 0.57 ef	3.8 ± 0.48 ef	5.3 ± 0.63 de	5.8 ± 0.48 de	2.5 ± 0.29 f	13.8 ± 0.75 a	7.8 ± 0.48 cd	10.8 ± 0.85 b	13.5 ± 0.65 a	8.5 ± 0.65 bc
2022	6.8 ± 0.63 ef	4.3 ± 0.48 f	12.0 ± 0.71 cd	16.8 ± 0.85 b	8.8 ± 0.85 de	13.5 ± 0.55 bc	10.0 ± 0.91 cde	1.3 ± 0.85 cd	24.5 ± 1.89 a	8.3 ± 0.75 def
Average	7.4 ± 0.24 ef	3.5 ± 0.32 g	9.4 ± 0.40 cde	13.4 ± 0.85 b	5.5 ± 0.34 fg	17.8 ± 0.53 a	8.3 ± 1.15 de	11.9 ± 0.43 bc	17.8 ± 0.64 a	10.7 ± 0.51 cd

Data are presented as the mean ± SE with ANOVA p-value; means in rows followed by the different letters are significantly different at the 5% level of probability (p < 0.05).

). The lowest level of damage was usually recorded in ‘Cacanska Najbolja’ and ‘Jubileum’, whereas ‘Silvia’, ‘Presenta’ and ‘Katinka’ were generally more susceptible to spring frost injury. In 2022, flower bud damage ranged from 1.3% for the late-flowering cultivar ‘Tophit Plus’ to 24.5% for the early-flowering cultivar ‘Presenta’.

There is limited information available on the susceptibility of flower buds and flowers of different cultivars of European plum to damage caused by spring frosts, because it is highly variable and depends on many factors. To a certain degree, the susceptibility to spring frost depends on the genotype. In addition, phenology and physiological aspects, the topography of the orchard, the rate of temperature drop, and the duration of low temperatures have a significant influence on the extent of frost damage [43]. The most important factor is the developmental stage of flower buds at the time of the frost event as well as the quality of the flowers, which in organic orchards tends to be poorer than in other cultivation systems. This may be explained by the poorer condition of the flower buds on trees weakened by diseases and pests, which is a significant problem in organic farming [27].

Earlier reports described ‘Cacanska Najbolja’ as sensitive to unfavourable weather conditions during flowering, whereas ‘Cacanska Lepotica’ and ‘Katinka’ were considered relatively hardy [3]. However, under the organic orchard conditions of the present study, ‘Cacanska Najbolja’ showed relatively low susceptibility to frost damage, whereas the greatest flower injury was observed in the early-flowering cultivars ‘Silvia’ and ‘Presenta’. This confirms that flowering time strongly influences the risk of spring frost injury in plum orchards. In the present study, frost damage did not exceed 30% in any cultivar or year. Despite the occurrence of spring frosts, satisfactory fruit set was still possible, which confirms that moderate levels of flower injury do not necessarily result in severe yield reduction in plum orchards [41]. However, fruit set may additionally be influenced by other environmental factors, including rainfall, wind conditions and pollinator activity during flowering [44].

3.2.3. Damage to Fruitlets by Hoplocampa flava and Hoplocampa minuta

Plum sawflies (Hoplocampa flava and Hoplocampa minuta) are among the most damaging pests of European plum [45]. In organic plum orchards, plum sawflies may substantially reduce fruit set and yield under favourable conditions for pest development [46,47]. In the present study, the level of fruitlet damage varied considerably among cultivars and years. The lowest percentage of damaged fruitlets was usually recorded in the early-flowering cultivars ‘Cacanska Lepotica’, ‘Katinka’ and ‘Kalipso’, whereas the highest damage was observed in ‘Tophit Plus’. Intermediate levels of fruitlet injury were recorded for ‘Cacanska Najbolja’, ‘Jubileum’, ‘Presenta’, ‘Silvia’, ‘Tophit’ and ‘Vision’ (

Table 7. Damage [%] to plum fruitlets due to plum sawflies (Hoplocampa minuta, Hoplocampa flava).

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2019	5.0 ± 1.08 c	2.3 ± 0.25 c	16.3 ± 1.11 b	28.8 ± 1.75 e	2.0 ± 0.58 c	31.3 ± 1.49 a	6.0 ± 0.91 c	4.5 ± 0.65 c	5.3 ± 0.85 c	3.5 ± 0.65 c
2020	12.5 ± 1.04 de	28.5 ± 2.53 b	6.0 ± 0.91 e	18.3 ± 1.38 d	30.0 ± 1.47 b	19.8 ± 2.06 cd	40.3 ± 2.14 a	39.0 ± 2.27 a	27.8 ± 2.06 bc	40.3 ± 2.14 a
2021	9.5 ± 1.32 e	8.0 ± 0.91 e	11.5 ± 1.32 cd	14.0 ± 0.91 cd	16.8 ± 1.65 c	14.0 ± 1.68 cd	25.3 ± 2.29 b	38.8 ± 1.31 a	9.3 ± 1.11 e	26.3 ± 1.11 b
2022	4.5 ± 0.65 b	6.0 ± 0.71 ab	8.8 ± 0.63 a	3.5 ± 0.65 b	5.8 ± 0.85 ab	4.5 ± 0.65 b	6.0 ± 1.08 ab	3.8 ± 0.48 b	4.0 ± 0.91 b	6.3 ± 0.85 ab
2023	2.8 ± 0.48 d	3.8 ± 0.48 d	2.8 ± 0.48 d	3.5 ± 0.65 d	18.0 ± 1.29 b	14.0 ± 1.68 bc	16.8 ± 1.55 b	24.0 ± 1.96 a	16.5 ± 0.96 b	10.5 ± 1.32 c
2024	35.8 ± 3.25 b	44.0 ± 4.38 ab	21.0 ± 1.41 c	21.5 ± 1.85 c	36.8 ± 2.32 b	21.8 ± 4.02 bc	36.3 ± 2.10 b	50.5 ± 0.65 a	21.3 ± 2.93 c	33.5 ± 2.90 bc
Average	11.7 ± 0.57 fg	15.4 ± 0.98 efg	11.0 ± 0.81 g	14.9 ± 0.53 efg	18.2 ± 0.34 bcd	17.5 ± 1.05 cde	21.8 ± 1.11 b	26.8 ± 0.79 a	14.0 ± 0.58 fgh	20.0 ± 0.87 bc

Data are presented as the mean ± SE with ANOVA p-value; means in rows followed by the different letters are significantly different at the 5% level of probability (p < 0.05).

).

Similar cultivar-related differences in plum sawfly damage have also been observed previously in European plum orchards under different production conditions [3,46,47].

The lower susceptibility of early-ripening cultivars to plum sawfly damage may be associated with partial asynchrony between flowering time and peak pest activity. In contrast, late-flowering cultivars remained exposed to oviposition for a longer period, which probably contributed to the higher level of fruitlet injury observed in ‘Tophit Plus’. Under organic orchard conditions, where direct control options are limited, cultivar phenology may therefore play an important role in reducing pest pressure. Throughout the experimental period, plum sawfly abundance consistently exceeded the economic threshold, with total trap catches ranging from over 40 to more than 100 individuals per trap during flowering. H. minuta was the dominant species at the experimental site (Supplementary Materials).

Despite the occurrence of plum sawfly injury during the study period, the observed damage levels generally remained lower than those reported from heavily infested unmanaged orchards [46,47]. This indicates that appropriate cultivar selection may partially reduce the negative impact of plum sawflies in organic production systems.

3.2.4. Growth and Yield

Tree growth was determined based on the calculation of the trunk cross-sectional area (TCSA). After nine years under organic orchard conditions, the highest TCSA values were achieved by trees of the ‘Cacanska Najbolja’ and ‘Vision’ cultivars. The cultivars ‘Katinka’ and ‘Kalipso’ showed the weakest growth (

Table 8. Yield [kg∙tree⁻¹] in the years 2018–2024, trunk cross-sectional area [cm²] and cumulative yield efficiency [kg∙cm⁻²] of examined plum cultivars.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2018	2.0 ± 0.90 ab	0.8 ± 0.11 a-d	0.2 ± 0.06 d	0.2 ± 0.03 d	1.8 ± 0.10 abc	0.6 ± 0.06 cd	2.1 ± 0.20 a	0.6 ± 0.05 bcd	0.5 ± 0.04 cd	0.2 ± 0.06 d
2019	1.6 ± 0.24 bc	3.8 ± 0.32 a	1.5 ± 0.13 c	0.5 ± 0.10 c	4.9 ± 0.24 a	0.4 ± 0.08 c	5.0 ± 0.14 a	4.4 ± 0.24 a	3.6 ± 0.38 ab	0.6 ± 0.05 c
2020	3.8 ± 0.33 abc	4.5 ± 1.00 ab	2.9 ± 0.32 bcd	1.5 ± 0.21 def	5.5 ± 0.52 a	0.3 ± 0.06 f	0.6 ± 0.19 ef	2.5 ± 0.24 cde	0.2 ± 0.02 f	0.2 ± 0.05 f
2021	7.5 ± 0.21 d	15.3 ± 0.64 c	10.7 ± 0.85 d	6.6 ± 0.84 d	20.3 ± 0.49 b	1.3 ± 0.10 e	26.0 ± 1.25 a	18.8 ± 1.61 bc	7.6 ± 0.69 d	7.6 ± 0.45 d
2022	3.2 ± 0.39 bc	3.7 ± 0.19 ab	3.9 ± 0.53 ab	6.2 ± 1.44 a	3.6 ± 0.23 abc	3.4 ± 0.28 bc	3.3 ± 0.37 bc	1.0 ± 0.25 c	0.8 ± 0.21 c	2.1 ± 0.20 bc
2023	28.9 ± 1.29 a	25.4 ± 1.88 ab	25.1 ± 1.09 ab	26.9 ± 1.04 a	15.8 ± 0.85 c	1.9 ± 0.39 e	22.2 ± 0.65 b	9.0 ± 1.02 d	9.2 ± 0.49 d	7.3 ± 0.41 d
2024	3.6 ± 0.18 e	30.5 ± 1.30 a	11.8 ± 0.78 c	20.0 ± 0.23 b	18.2 ± 1.04 b	11.3 ± 0.60 cd	7.9 ± 0.91 d	11.8 ± 0.61 c	9.0 ± 1.14 cd	3.4 ± 0.43 e
Total yield	50.7 ± 1.39 ef	83.9 ± 2.25 a	56.1 ± 1.58 de	61.9± 1.06 cd	69.9 ± 2.16 b	19.1 ± 0.65 h	67.0 ± 2.05 bc	47.9 ± 1.23 f	30.9 ± 1.72 g	21.3 ± 1.54 h
TCSA	54.5 ± 3.15 de	97.2 ± 1.81 a	46.7 ± 1.58 e	48.1 ± 0.58 e	64.1± 3.47 cd	65.3 ± 1.73 c	63.6 ± 1.94 cd	72.3 ± 1.15 bc	80.0 ± 2.13 b	94.3 ± 1.23 a
Yield efficiency	0.94 ± 0.03 cd	0.86 ± 0.02 d	1.20 ± 0.02 a	1.29 ± 0.04 a	1.10 ± 0.07 b	0.290 ± 0.00 f	1.06 ± 0.02 bc	0.663 ± 0.02 e	0.386 ± 0.02 f	0.225 ± 0.02 f

Data are presented as the mean ± SE with ANOVA p-value; means in rows followed by the different letters are significantly different at the 5% level of probability (p < 0.05); TCSA—trunk cross-sectional area.

).

Information regarding the growth vigour of European plum trees under organic orchard conditions remains limited. Previous studies conducted on other fruit species demonstrated that tree growth in organic orchards may be reduced compared with conventional systems, mainly because of lower nutrient availability and differences in orchard management [27,48,49].

In plum orchards, tree growth and productivity depend strongly on cultivar characteristics, environmental conditions and orchard management practices [20]. Similar relationships have also been reported for other fruit species cultivated under organic and conventional systems [50,51].

In the present study, trees grown in the organic orchard did not exhibit visible symptoms of nutrient deficiency. Annual fertilisation with fermented cattle manure probably ensured an adequate nitrogen supply. Tree growth of the evaluated cultivars was influenced mainly by aphid infestation, which damaged young shoot tips in some years and proved difficult to control under organic orchard conditions.

These results indicate that pest pressure may substantially influence vegetative growth and long-term orchard performance in organically managed plum production.

Early fruiting and regular yield are among the most important traits of modern plum cultivars. In the present study, the first fruits were harvested in the third year after planting. Yield varied significantly among cultivars and years. The highest yields were generally recorded in early- and medium-early-ripening cultivars, whereas late-ripening cultivars produced lower yields (Table 8). Particularly high yields were obtained in 2023 for ‘Cacanska Lepotica’, ‘Katinka’, ‘Cacanska Najbolja’, and ‘Kalipso’. Among late-ripening cultivars, ‘Tophit’ showed the highest productivity.

Previous studies showed that European plum cultivars may achieve high productivity under conventional orchard conditions already in the early years after planting [3]. However, yields obtained in organic orchards are usually lower because of higher pest and disease pressure and the limited effectiveness of available protection methods [8]. Similar observations under organic orchard conditions were also reported in Romania by Butac et al. [20].

In the present study, total yield and cumulative yield efficiency differed significantly among cultivars. The highest cumulative yields were obtained from ‘Cacanska Najbolja’. This cultivar, together with ‘Cacanska Lepotica’, was also identified as highly productive under organic orchard conditions in Romania [20]. However, under Polish climatic conditions, ‘Cacanska Lepotica’ produced lower yields, which suggests a strong influence of local environmental conditions on cultivar performance.

The cultivars ‘Kalipso’ and ‘Katinka’ showed the highest cumulative yield efficiency because of their weaker vegetative growth. In contrast, ‘Silvia’ and ‘Vision’ produced the lowest yields and showed the lowest yield efficiency values (Table 8). Yield variability among years was associated mainly with weather conditions during flowering and fruit set, pest pressure and the limited effectiveness of available organic protection methods. These results indicate that cultivar suitability for organic plum production depends not only on productivity, but also on resistance to environmental stress and biotic factors.

3.3. Fruit Quality

3.3.1. Fruit Ripening Time

In the present study, the evaluated plum cultivars represented a wide range of fruit ripening periods, from early (‘Katinka’, ‘Kalipso’, ‘Cacanska Lepotica’, and ‘Silvia’), through medium-early (‘Cacanska Najbolja’ and ‘Jubileum’) to late (‘Vision’ and ‘Tophit’) and very late (‘Tophit Plus’ and ‘Presenta’), which ensured a supply of fresh plums for over three months. The fruits ripened from the third week of July until the beginning of October. The exception was 2024, when fruit ripening occurred 2–4 weeks earlier than in the previous years. The fruits of the earliest-ripening cultivar, ‘Katinka’, were harvested in the first half of July, whereas the latest-ripening cultivar, ‘Presenta’, was harvested at the beginning of October (

Table 9. The ripening date of 10 plum cultivars in the years 2018–2024.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2018	24.07	07.08	23.07	23.07	31.07	23.07	05.09	20.09	24.09	30.08
2019	06.08	20.08	29.07	22.07	20.08	01.08	12.09	16.09	21.09	10.09
2020	10.08	26.08	07.08	29.07	18.08	10.08	18.09	27.09	01.10	10.09
2021	19.08	03.09	10.08	03.08	30.08	20.08	15.09	25.09	03.10	10.09
2022	12.08	30.08	06.08	28.07	01.09	15.08	25.09	27.09	06.10	27.09
2023	09.08	26.08	01.08	26.07	20.08	10.08	14.09	25.09	26.09	07.09
2024	24.07	11.08	12.07	08.07	04.08	24.07	13.08	30.08	06.09	15.08

).

Fruits of the evaluated plum cultivars grown in organic orchard conditions (central Poland) ripened within a period similar to that in other growing systems [4,5].

Fruit ripening time may strongly influence cultivar suitability for organic production systems. Early-ripening cultivars are usually less exposed to late-season fungal infections and plum fruit moth infestation, whereas late-ripening cultivars remain exposed to pest and pathogen pressure for a longer period. Therefore, fruit ripening phenology should be considered one of the important traits in the selection of plum cultivars for organic orchards.

3.3.2. Fruit Weight

Fruit size is one of the most important quality traits determining the market value of plums. Fruit weight is influenced by genotype, environmental conditions and cultivation system [13,27].

Considerable differences in fruit weight were observed among cultivars and years (

Table 10. The weight [g] of examined plum cultivars in the years 2018–2024.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2018	44.7 ± 0.98 c	52.7 ± 0.19 b	23.4 ± 1.03 e	18.5 ± 0.25 e	66.5 ± 2.59 a	42.8 ± 1.17 c	70.0 ± 1.17 a	52.4 ± 1.07 b	30.7 ± 0.43 d	52.2 ± 1.50 b
2019	37.3 ± 0.45 c	56.6 ± 0.86 b	29.5 ± 0.88 d	18.6 ± 0.81 e	66.6 ± 0.46 a	41.6 ± 0.74 c	66.8 ± 0.55 a	40.7 ± 1.47 c	27.3 ± 0.75 d	54.7 ± 0.09 b
2020	49.9 ± 0.31 d	61.8 ± 3.61 bc	32.2 ± 0.68 e	23.3 ± 0.93 e	73.5 ± 3.50 a	50.5 ± 0.65 d	65.5 ± 0.63 abc	67.5 ± 0.63 ab	30.6 ± 0.60 e	56.8 ± 0.74 cd
2021	38.7 ± 0.35 f	58.5 ± 0.50 de	34.2 ± 2.19 g	23.8 ± 1.11 h	80.0 ± 1.69 a	55.4 ± 0.45 e	60.5 ± 0.33 cd	63.4 ± 1.34 bc	23.8 ± 0.20 h	67.2 ± 0.76 b
2022	55.1 ± 0.77 e	74.1 ± 0.47 b	41.7 ± 0.69 f	25.8 ± 0.18 h	83.2 ± 0.07 a	63.7 ± 1.48 d	63.2 ± 1.02 d	66.0 ± 0.41 cd	32.1 ± 0.15 g	68.8 ± 1.52 c
2023	36.4 ± 0.38 e	59.2 ± 0.27 c	35.6 ± 0.59 e	19.3 ± 0.29 g	72.6 ± 0.47 ab	47.3 ± 1.03 d	60.1 ± 0.23 c	74.4 ± 0.42 a	30.5 ± 0.28 f	70.7 ± 0.90 b
2024	55.9 ± 0.77 bc	55.3 ± 1.00 bc	30.5 ± 0.53 e	18.3 ± 0.29 g	52.5 ± 1.11 c	46.3 ± 0.54 d	60.4 ± 1.00 a	57.7 ± 0.20 ab	23.8 ± 0.40 f	58.1 ± 0.38 ab
Average	45.4 ± 0.29 e	59.7 ± 0.66 c	32.4 ± 0.18 f	21.1 ± 0.16 h	70.7 ± 1.06 a	49.7 ± 0.19 d	63.8 ± 0.29 b	60.3 ± 0.56 c	28.4 ± 0.10 g	61.2 ± 0.56 c

Data are presented as the mean ± SE with ANOVA p-value; means in rows followed by the different letters are significantly different at the 5% level of probability (p < 0.05).

). Fruit weight was generally highest in 2022, when crop load was relatively low. Significant differences between cultivars occurred in every year of the study. In 2018–2023, the largest fruits were produced by ‘Jubileum’, whereas in 2024 the largest fruits were harvested from ‘Tophit’. In contrast, ‘Katinka’ consistently produced the smallest fruits throughout the study period.

Fruit size may be strongly influenced by crop load and environmental conditions during fruit development. Cultivars characterised by lower yield intensity often produced larger fruits, whereas high crop load was usually associated with reduced fruit size. Under organic orchard conditions, where possibilities for thinning and intensive fertilisation are limited, cultivar-dependent differences in fruit size may therefore be particularly important for marketability.

Previous studies have indicated that plums grown under organic conditions often produce smaller fruits than those cultivated in integrated or conventional orchards [8,13]. This is usually associated with lower nutrient availability and the absence of intensive fertilisation. However, excessive crop load in favourable years may also negatively affect fruit size and quality [3]. In the present study, overcropping was not a major problem because spring frost injury and plum sawfly damage reduced fruit set in some seasons. As a result, fruit weight depended mainly on genotype and weather conditions during fruit development. The obtained fruit weights were generally similar to, and in some cases even higher than, those reported for the same cultivars grown under conventional and integrated management systems [4,52].

The cultivars ‘Jubileum’ and ‘Tophit’ consistently produced fruits weighing more than 50 g, whereas ‘Tophit Plus’ and ‘Vision’ were also characterised by relatively large fruits. However, the fruits of these late-ripening cultivars were more susceptible to plum fruit moth infestation and fungal diseases under organic orchard conditions.

3.3.3. Total Soluble Solids Content

Soluble solids are an important index of plum quality. The content of these substances is highly correlated with sugar content, which largely determines fruit taste.

In the discussed experiment, soluble solids content varied significantly among cultivars and years. This characteristic depended largely on weather conditions during fruit ripening (July–October The lowest soluble solids content (12.3 °Brix) was recorded in fruits of the ‘Katinka’ cultivar in 2021, which was probably associated with prolonged rainy weather during fruit ripening in July. The highest soluble solids content (26.2 °Brix) was found in fruits of the ‘Presenta’ cultivar harvested in the same year, when September and October were warm and sunny. Earlier fruit ripening in 2024 was generally less favourable for sugar accumulation in the evaluated plum cultivars (

Table 11. The content of total soluble solids [°Brix] * in examined plum cultivars in the years 2021–2024.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2021	12.7 ± 0.51 f	12.9 ± 0.18 f	13.9 ± 0.20 ef	12.3 ± 0.24 f	20.4 ± 0.09 bc	18.0 ± 0.39 d	15.1 ± 0.27 b	20.7 ± 0.31 b	26.2 ± 0.51 a	18.8 ± 0.56 cd
2022	18.1 ± 0.47 c	16.7 ± 0.20 cd	16.6 ± 0.16 cd	15.2 ± 0.41 d	20.8 ± 0.26 b	15.8 ± 0.32 d	21.4 ± 0.61 b	21.0 ± 0.47 b	23.7 ± 0.55 a	21.0 ± 0.47 b
2023	16.1 ± 0.25 d	18.5 ± 0.26 c	16.9 ± 0.22 cd	16.8 ± 0.33 cd	21.6 ± 0.67 ab	18.0 ± 0.68 cd	17.5 ± 0.61 cd	21.0 ± 0.30 b	23.3 ± 0.57 a	18.2 ± 0.44 c
2024	15.1 ± 0.33 de	14.3 ± 0.29 ef	15.3 ± 0.32 cde	13.2 ± 0.38 fg	16.2 ± 0.55 cd	12.7 ± 0.19 g	18.3 ± 0.24 ab	16.8 ± 0.45 bc	19.5 ± 0.46 a	16.4 ± 0.25 cd
Average	15.5 ± 0.20 d	15.6 ± 0.10 d	15.7 ± 0.06 d	14.4 ± 0.14 e	19.7 ± 0.21 b	16.1 ± 0.21 d	18.1 ± 0.24 c	19.9 ± 0.17 b	23.2 ± 0.33 a	18.6 ± 0.20 c

* the value expressed in degrees, indicating how much sugar the liquid contains. Data are presented as the mean ± SE with ANOVA p-value; means in rows followed by the different letters are significantly different at the 5% level of probability (p < 0.05).

).

Kadar [53] reported that fruits of early-ripening plum cultivars should contain at least 12 °Brix soluble solids, whereas late-ripening cultivars should exceed 17 °Brix. The values for the cultivars evaluated in our study were within this range. Despite unfavourable conditions for sugar accumulation in 2021, soluble solids content in early-ripening cultivars did not fall below 12 °Brix, whereas average values for 2021–2024 ranged from 14.4 to 16.1 °Brix. In late-ripening cultivars, the average soluble solids content ranged from 18.1 to 23.2 °Brix. These results were consistent with observations reported by Kovács et al. [54], who found that fruits of early-ripening cultivars such as ‘Katinka’ and ‘Silvia’ contained approximately 12 °Brix, whereas late-ripening cultivars such as ‘Tophit’ and ‘Presenta’ exceeded 18 °Brix. In the present study, the highest average soluble solids content (23.2 °Brix) was recorded in fruits of the latest-ripening cultivar ‘Presenta’.

Previous studies have reported inconsistent effects of the cultivation system on soluble solids content in fruits. Cara et al. [13] observed higher sugar content in organically grown plums than in fruits produced under integrated and conventional systems. According to Milošević et al. [55], this may be associated with a higher humus content and more balanced nutrient availability in organically managed soils. In contrast, Peck et al. [50] did not confirm a significant influence of the cultivation system on soluble solids content, while Głowacka et al. [27] reported lower sugar content in organically grown sour cherries than in fruits from conventional orchards. These inconsistent findings suggest that soluble solids accumulation depends mainly on genotype and weather conditions during fruit ripening rather than on the cultivation system alone. Under organic orchard conditions, high soluble solids content may partially compensate for a smaller fruit size and improve fruit sensory quality.

3.3.4. Fruit Damage Caused by Grapholita funebrana

In addition to aphids and plum sawflies, the plum fruit moth (Grapholita funebrana) is one of the most important pests of plum orchards in Europe [56]. Pest control in organically managed orchards is considerably more difficult because the number of effective control measures is limited [57].

Fruit infestation by plum fruit moth caterpillars varied considerably among cultivars and years. The lowest pest pressure was observed in 2021, whereas the highest level of fruit damage occurred in 2024. Significant cultivar-dependent differences in susceptibility to Grapholita funebrana infestation were observed throughout the study period (

Table 12. Damage [%] of examined plum cultivars by plum fruit moth (Grapholita funebrana) in the years 2018–2024.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2018	0.0 ± 0.00 e	17.5 ± 1.04 bc	0.0 ± 0.00 e	0.0 ± 0.00 e	29.0 ± 2.48 a	0.0 ± 0.00 e	21.8 ± 3.04 ab	27.3 ± 2.01 a	13.3 ± 1.75 cd	8.3 ± 1.49 d
2019	0.0 ± 0.00 f	7.3 ± 1.25 e	0.0 ± 0.00 f	0.0 ± 0.00 f	23.8 ± 2.39 c	0.0 ± 0.00 f	15.0 ± 0.81 d	36.3 ± 1.75 a	31.3 ± 1.65 ab	26.0 ± 1.68 bc
2020	0.0 ± 0.00 d	29.8 ± 2.29 c	0.0 ± 0.00 d	0.0 ± 0.00 d	41.5 ± 1.71 b	0.0 ± 0.00 d	57.3 ± 1.11 ac	51.0 ± 1.68 a	42.8 ± 2.39 b	30.8 ± 1.80 c
2021	1.8 ± 0.48 d	8.5 ± 0.65 bc	0.0 ± 0.00 d	0.0 ± 0.00 d	15.0 ± 1.78 a	0.0 ± 0.00 d	16.3 ± 1.65 a	4.3 ± 1.10 cd	2.0 ± 0.40 d	11.5 ± 1.32 ab
2022	0.0 ± 0.00 d	7.5 ± 0.65 bc	0.0 ± 0.00 d	0.0 ± 0.00 d	33.3 ± 1.31 a	0.0 ± 0.00 d	11.0 ± 1.78 ab	3.5 ± 0.65 cd	1.0 ± 0.41 d	7.3 ± 1.03 bc
2023	0.0 ± 0.00 c	3.3 ± 0.48 c	0.0 ± 0.00 c	0.0 ± 0.00 c	20.0 ± 2.29 b	0.0 ± 0.00 c	27.5 ± 2.50 a	29.5 ± 0.96 a	16.5 ± 2.50 b	15.0 ± 2.04 b
2024	17.0 ± 1.08 cd	10.3 ± 0.85 de	0.0 ± 0.00 e	0.0 ± 0.00 e	16.0 ± 1.63 d	11.3 ± 1.49 d	33.3 ± 5.68 b	58.3 ± 0.63 a	28.3 ± 2.69 b	27.3 ± 2.29 bc
Average	2.7 ± 0.15 e	11.4 ± 0.20 d	0.0 ± 0.00 f	0.0 ± 0.00 f	22.6 ± 0.73 b	1.6 ± 0.21 ef	24.8 ± 1.17 b	30.0 ± 0.30 a	19.3 ± 0.67 c	18.0 ± 0.49 c

Data are presented as the mean ± SE with ANOVA p-value; means in rows followed by the different letters are significantly different at the 5% level of probability (p < 0.05).

).

Fruits of the earliest-ripening cultivars, ‘Katinka’ and ‘Kalipso’, remained free from infestation during all study years. Very low infestation levels were also recorded for ‘Silvia’ and ‘Cacanska Lepotica’. In contrast, medium-early cultivars (‘Cacanska Najbolja’ and ‘Jubileum’) and all late-ripening cultivars (‘Vision’, ‘Tophit’, ‘Tophit Plus’, and ‘Presenta’) were infested every year. The highest infestation level (58.3%) was recorded in fruits of the ‘Tophit Plus’ cultivar in 2024. Average infestations during 2018–2024 ranged from 0% in ‘Katinka’ and ‘Kalipso’ to approximately 30% in ‘Tophit Plus’.

The observed differences in susceptibility to plum fruit moth infestation were strongly associated with fruit ripening time. Early-ripening cultivars probably escaped the period of highest pest activity, whereas late-ripening cultivars remained exposed to oviposition and larval feeding for a longer period. This indicates that fruit phenology may play an important role in reducing pest pressure in organically managed plum orchards.

Under organic orchard conditions, cultivar selection may therefore represent an important component of integrated pest management strategies aimed at reducing fruit damage caused by Grapholita funebrana.

Detailed data concerning plum fruit moth trap catches recorded during the experimental period are presented in the Supplementary Materials.

Effective control of the plum fruit moth remains a major challenge in organically managed plum orchards. Limited information is available regarding cultivar susceptibility to Grapholita funebrana infestation. Rizzo et al. [19] demonstrated that susceptibility of plum fruits to plum fruit moth damage may depend on fruit characteristics such as shape, skin colour, and sugar and acid content. However, the results of the present study indicate that fruit ripening time was probably the most important factor influencing susceptibility to infestation. Fruits of the earliest-ripening cultivars, ‘Katinka’ and ‘Kalipso’, remained free from infestation, whereas the highest infestation level was recorded in the late-ripening cultivar ‘Tophit Plus’.

Various biological and behavioural methods have been tested to reduce plum fruit moth damage in organic orchards, including mating disruption, microbial products, parasitoids, entomopathogenic fungi and nematodes [16,21]. However, the effectiveness of these methods is often limited and strongly depends on treatment timing and pest population dynamics. Studies conducted with Cydia pomonella granulovirus (CpGV) also showed lower effectiveness against Grapholita funebrana than against codling moth in apple orchards [56,58]. Therefore, under organic orchard conditions, the cultivation of early-ripening plum cultivars may currently represent one of the most effective approaches for reducing fruit losses caused by plum fruit moth infestation.

3.3.5. Susceptibility to Monilinia spp.

The brown rot, caused by the fungi Monilinia spp., is one of the most important diseases of plums. Disease severity depends strongly on environmental conditions, particularly humidity and temperature during fruit ripening [2].

Fruit infection by Monilinia spp. was observed in all study years, although disease severity differed considerably among years and cultivars (

Table 13. Degree of infection [%] of examined plum cultivars by Monilinia spp. in the years 2018–2024.

Year	Cultivar
	Cacanska Lepotica	Cacanska Najbolja	Kalipso	Katinka	Jubileum	Silvia	Tophit	Tophit Plus	Presenta	Vision
2018	0.0 ± 0.00 e	2.0 ± 0.41 c	0.0 ± 0.00 c	0.0 ± 0.00 c	47.0 ± 2.20 a	0.0 ± 0.00 c	45.3 ± 2.17 a	35.5 ± 1.94 b	46.0 ± 2.04 a	36.0 ± 2.68 b
2019	0.0 ± 0.00 f	7.3 ± 1.30 d	0.0 ± 0.00 e	0.0 ± 0.00 e	74.3 ± 2.48 a	0.0 ± 0.00 e	43.8 ± 1.65 b	16.5 ± 1.71 c	15.8 ± 1.25 c	78.8 ± 1.31 a
2020	4.8 ± 0.85 de	11.0 ± 0.91 d	0.0 ± 0.00 e	0.0 ± 0.00 e	70.3 ± 1.70 a	2.3 ± 0.48 de	52.8 ± 4.23 d	57.8 ± 2.29 b	41.3 ± 2.39 c	76.8 ± 1.65 a
2021	0.5 ± 0.29 e	4.5 ± 1.19 e	0.0 ± 0.00 e	0.0 ± 0.00 e	62.3 ± 0.63 a	5.3 ± 0.75 e	13.3 ± 1.18 d	28.0 ± 3.11 c	52.5 ± 2.72 b	16.8 ± 0.85 d
2022	1.8 ± 0.48 e	12.8 ± 2.17 d	0.0 ± 0.00 e	0.0 ± 0.00 e	40.8 ± 2.66 a	2.8 ± 0.48 e	18.5 ± 1.85 cd	24.0 ± 2.55 bc	27.0 ± 1.08 b	41.0 ± 2.86 a
2023	8.0 ± 0.41 ef	15.5 ± 1.32 de	0.0 ± 0.00 f	0.0 ± 0.00 e	59.0 ± 4.60 a	3.0 ± 0.71 f	46.8 ± 0.63 b	51.3 ± 1.44 ab	32.8 ± 2.87 c	23.0 ± 1.78 d
2024	32.8 ± 3.27 cd	24.3 ± 1.89 de	11.3 ± 1.18 f	15.0 ± 0.91 f	38.0 ± 1.78 c	29.3 ± 3.71 cd	68.0 ± 3.72 a	50.8 ± 2.72 b	28.0 ± 2.12 cd	58.0 ± 1.87 ab
Average	6.8 ± 0.41 f	11.0 ± 0.14 e	1.6 ± 0.17 g	2.1 ± 0.13 g	55.9 ± 0.55 a	6.1 ± 0.63 f	41.2 ± 1.16 c	37.7 ± 1.16 c	34.8 ± 0.73 d	47.2 ± 0.38 b

Data are presented as the mean ± SE with ANOVA p-value; means in rows followed by the different letters are significantly different at the 5% level of probability (p < 0.05).

). The highest percentage of rotten fruits was recorded in 2024, whereas the lowest level of infection occurred in 2022. Significant cultivar-dependent differences in susceptibility to brown rot were observed throughout the study period.

Fruits of the cultivars ‘Katinka’ and ‘Kalipso’ showed the lowest susceptibility to infection and developed symptoms only in 2024. Relatively low infection levels were also recorded for ‘Cacanska Lepotica’ and ‘Silvia’. In contrast, the highest percentage of rotten fruits (more than 70%) was observed in ‘Vision’ and ‘Jubileum’ harvested in 2019 and 2020.

The observed differences in susceptibility to brown rot were probably associated with fruit ripening time, fruit skin characteristics and weather conditions during the ripening period. High rainfall and elevated humidity during fruit maturation favoured disease development, especially in late-ripening cultivars that remained exposed to infection for a longer period.

Under organic orchard conditions, cultivar susceptibility to Monilinia spp. may strongly influence fruit marketability and storage potential. Therefore, the selection of cultivars with a lower susceptibility to brown rot appears particularly important for organic plum production.

The available literature contains relatively limited comparative data regarding the susceptibility of European plum cultivars to infection by Monilinia spp. Hartmann and Neumüller [3] reported that differences between years were often greater than differences between cultivars, which was also confirmed in the present study. Disease severity depended strongly on weather conditions during fruit ripening, and the highest percentage of rotten fruits was observed in years characterised by lower temperatures and high rainfall, which is consistent with the observations of Grabowski [59].

Despite the strong influence of weather conditions, significant cultivar-dependent differences in susceptibility to brown rot were observed throughout the study period. Late-ripening cultivars were generally more severely affected than early-ripening cultivars. According to Hartmann and Neumüller [3], cultivars with a high sugar content may be more susceptible to Monilinia spp. infection because of their greater tendency to fruit cracking and the creation of more favourable conditions for fungal development. In the present study, high soluble solids content was recorded in ‘Presenta’, ‘Jubileum’, ‘Tophit’, ‘Tophit Plus’, and ‘Vision’, and fruits of these cultivars were generally more susceptible to rot.

Particularly high levels of fruit rot were observed in ‘Jubileum’, whose fruits were also frequently damaged by plum fruit moth caterpillars. Skin damage caused by the pest probably facilitated fungal infection and disease development, which is consistent with previous observations [8,10].

Copper-based products approved for organic production were used to reduce brown rot incidence; however, their effectiveness was insufficient during rainy seasons with high disease pressure.

4. Summary and Conclusions

The results of the present study demonstrated that organic production of European plum is possible under Central European conditions; however, effective pest and disease management remains a major challenge. Aphids, plum sawflies, plum fruit moth, and fungal diseases caused significant limitations to fruit production and quality in organically managed orchards.

Considerable cultivar-dependent differences were observed in susceptibility to pests and diseases. Early-ripening cultivars such as ‘Katinka’, ‘Kalipso’, and ‘Cacanska Lepotica’ were generally less susceptible to infestation by the plum fruit moth and infection by Monilinia spp., which resulted in higher fruit quality and lower production losses. In contrast, medium- and late-ripening cultivars remained exposed to pest and pathogen pressure for a longer period and were therefore more severely affected.

The obtained results indicate that fruit ripening time is one of the key factors influencing cultivar suitability for organic plum production. Early-ripening cultivars probably escape the periods with the highest pest activity and favourable conditions for fungal disease development. Among the evaluated cultivars, ‘Katinka’ and ‘Kalipso’ showed the highest suitability for organic cultivation systems.

Biological and preventive methods currently available for organic orchards were not sufficiently effective during years characterised by high pest pressure and favourable conditions for disease development. Therefore, regular orchard inspections, sanitary and agrotechnical measures, and careful cultivar selection remain essential components of organic plum production.

Further research should focus on the evaluation of additional plum cultivars, particularly cultivars tolerant or resistant to plum pox virus (PPV) and less susceptible to fungal diseases, especially Monilinia spp. It is also necessary to develop and improve biological and environmentally safe methods for pest and disease control in organic orchards.