Effective Yield Protection in Organic Potato Cultivation Through the Application of Diverse Strategies Utilizing Basic Substances

Abstract

Basic substances of plant or animal origin are permitted for use in the protection of organic crops. Experiments were performed under laboratory, greenhouse, and field conditions using emulsified sunflower oil solution at 10%, water onion extract (Allium cepa L.) at 25%, chitosan at 2%, two commercial strains of Saccharomyces cerevisiae (US 05 and Coobra), and a low dose of copper (2 kg/ha) to inhibit the growth of Phytophthora infestans, to contribute to the extension of the vegetation period, and to maintain the photosynthetic capacity responsible for the quantity of yield. Potato varieties with different levels of resistance to the pathogen were planted, and preventive treatments were performed. In the greenhouse experiment, inoculation of potato plants with the pathogen was carried out. The aim of the study was to develop strategies for the combined or alternating use of basic substances and copper in order to delay the appearance of potato late blight symptoms and keep them below the level of economic damage. The main factor determining the effectiveness of the tested strategies was the yield. Protective treatments contributed to an increase in yield compared with untreated plants. Strategies combining copper with sunflower oil, onion extract, or chitosan reduced late blight symptoms; however, the final effect on plant health and yields depended on the susceptibility of the variety to the pathogen. Strategies based solely on basic substances were effective in protecting potato varieties that were less susceptible to P. infestans (e.g., Red Sonya, Lilly, Tajfun). For more-susceptible varieties (e.g., Vineta, Satina, Lord) copper pesticide must be included in the treatment strategy; however, copper can be applied either as the first four foliar sprays followed by two treatments with basic substances or, alternately, with them.

1. Introduction

Organic farming (OF) requires specific methods and protection measures to achieve satisfactory yields. The entire farm management process is based on agrotechnical, preventive, and non-chemical methods, which should be implemented in an integrated manner. The basis of plant protection in OF is crop rotation and the selection of suitable potato varieties. However, this is not always possible or sufficient. Therefore, studies are still being conducted to assess the efficacy of plant protection products (PPPs) approved for use in OF, as well as other protective treatments, basic substances (BSs), biological methods, and any approach that improves plant nutrition and strengthens or stimulates the plant’s defense system. All agricultural treatments in OF should be applied both directly and indirectly, with a comprehensive approach that ensures good plant health and high yields.

The potato (Solanum tuberosum L.) is an important food crop and ranks third in global importance after rice and wheat. Organic potatoes are often purchased by consumers, but protecting organic potatoes is challenging due to the limited range of pesticides available for controlling late blight and early blight. Late blight is the most devastating disease of potato worldwide, causing annual production losses estimated at USD 6.7 to 15 billion [1]. Late blight causes severe yield losses, especially under weather conditions favorable for epidemics. Among the pesticides approved for use in OF, currently only copper-based products are permitted to control P. infestans (Mont.) de Bary. This pathogen belongs to the following taxonomic classification: Kingdom Chromista, Phylum Oomycota, Class Oomycetes, Order Peronosporales, Family Peronosporaceae, Genus Phytophthora. Despite its fungal-like appearance and behavior, P. infestans is phylogenetically closer to brown algae and diatoms, placing it in the Stramenopila group and the Kingdom Chromista. At present, the primary method for controlling late blight in organic farming is the application of copper products; however, their efficacy is limited and depends on the timing of the first treatment, the frequency of spraying, and the prevailing weather conditions. However, discussions have been ongoing for several years in the EU regarding the removal of copper from organic production. Therefore, the development of effective substances for controlling potato late blight remains an urgent need [2].

1.1. Basic Substances

Basic substances in plant protection are non-toxic products that can be used to help protect plants from pests and diseases [2,3,4,5,6]. These are not considered pesticides in the traditional chemical sense but can still play a protective role (Table 1) [7]. They are approved by regulatory authorities, such as the EU, for use in agriculture including organic farming (EU Regulation 889/202) and are listed under EU Regulation (EC) No. 1107/2009 as “basic substances” [8]. These are substances commonly used in other industries (e.g., food or cosmetics) but have potential for plant protection without leaving harmful residues and without the need for a maximum residue limit (MRL) [9,10]. Approved basic substances are listed in the European pesticide database [11]. Their use can be complementary to biological control, cultural practices, and the limited application of natural pesticides approved for use.

However further studies are needed, because, while promising, basic substances have several challenges such as limited commercial incentives due to low profit margins, lack of awareness among farmers and advisors, and variable efficacy compared with synthetic products. Some more strategies developed towards future directions include expanding the list of approved substances, educating stakeholders, and supporting research on novel applications and combinations [12,13,14]. Recent reviews (2025) highlight the role of plant-based extracts in sustainable pest management for potatoes, with particular attention to onion extract as a useful complementary measure. Essential oils (EOs) and extracts from medicinal plants have demonstrated strong activity against both the mycelium and spores of P. infestans, which is attributed to their high content of bioactive compounds, including polyphenols, flavonoids, sterols, terpenoids, and alkaloids [15].

Table 1. Examples of representative/selected basic substances used in plant protection under EU regulations.

Substance	Source/Description	Primary Use
Baking soda (Sodium bicarbonate)	A common household compound	Fungicide, especially against powdery mildew
Vinegar (Acetic acid)	Natural product from fermentation	Bactericide and herbicide
Chitosan hydrochloride	Derived from chitin (shells of crustaceans)	Elicitor of plant defense mechanisms
Fructose	Natural sugar	Improves microbial activity in compost teas
Lecithins	Derived from soybeans or sunflower	Fungicide, improves plant resistance
Nettle extract (Urtica dioica)	Plant extract	General plant tonic and insect repellent
Whey	By-product of cheese production	Fungicide against downy mildew
Horsetail (Equisetum arvense)	Traditional plant extract	Fungicide, strengthens plant cell walls
Sucrose	Table sugar	Attracts beneficial insects, feeds microbes

Own elaboration [16].

Table 1. Examples of representative/selected basic substances used in plant protection under EU regulations.

Substance	Source/Description	Primary Use
Baking soda (Sodium bicarbonate)	A common household compound	Fungicide, especially against powdery mildew
Vinegar (Acetic acid)	Natural product from fermentation	Bactericide and herbicide
Chitosan hydrochloride	Derived from chitin (shells of crustaceans)	Elicitor of plant defense mechanisms
Fructose	Natural sugar	Improves microbial activity in compost teas
Lecithins	Derived from soybeans or sunflower	Fungicide, improves plant resistance
Nettle extract (Urtica dioica)	Plant extract	General plant tonic and insect repellent
Whey	By-product of cheese production	Fungicide against downy mildew
Horsetail (Equisetum arvense)	Traditional plant extract	Fungicide, strengthens plant cell walls
Sucrose	Table sugar	Attracts beneficial insects, feeds microbes

Own elaboration [16].

Basic substances can be applied mostly preventively, directly on plants, to the soil, or/and used in compost.

1.2. Key Basic Substances for Potato Protection

Detailed information on the usefulness of basic substances in protection of potato and tomato against late and early blight is shown in

Table 2. Basic substances listed as promising to limit the main pathogens of potato and tomato crops, according to the European Commission’s active substances, safeners, and synergists list (ec.europa.eu/food/plant/pesticides/eu-pesticides-database/start/screen/active-substances).

Basic Substance	Crop	Pathogen	Preparation/Concentration of the Spray Liquid, Decoction	Application Method	No. of Treatments
Allium cepa extract	Tomato, potato	Alternaria solani	100% decoction of 50 g onion/L water	Leaf spraying BBCH21 to BBCH85	3–5
	Tomato	P. infestans	100% decoction of 50 g onion/L water	Leaf spraying BBCH21 to BBCH75	3–5
Equisetum arvense L.	Tomato	Alternaria solani	2 g/L plant homogenate extracted with hot water and filtered to be used	Leaf spraying BBCH51-BBCH59	2
	Potato	P. infestans	2.25 g/L plant homogenate extracted with hot water and filtered to be used	Leaf spraying BBCH51-BBCH59	4–8
	Potato	Alternaria solani	2.25 g/L plant homogenate extracted with hot water and filtered to be used	Leaf spraying BBCH1-BBCH9	4–8
Lecithin	Tomato	P. infestans	990–1030 g/L	Leaf spraying BBCH10-BBCH90	3–12
Sunflower oil	Potato	P. infestans	0.1–0.5/100 L water	Leaf spraying BBCH19–BBCH60 and BBCH69–BBCH70	1–7
Urtica spp.	Potato	P. infestans	75 g fresh plant or 15 g dried/L of boiling water	Leaf spraying until BBCH49	1–6

, based on data from the EU Pesticides Database—European Commission.

1.3. Own Elaboration

In the literature, studies have been conducted with chitosan (including chitosan hydrochloride), a biopolymer derived from shellfish, which, as an elicitor, can boost plant defenses, inhibit spore germination, and form a protective film on leaves. Depending on concentration and timing, it was reported to reduce late blight by up to 60–99% in field trials [4]. Chitosan significantly inhibits the mycelial growth and in vitro spore germination of P. infestans, induces resistance to the pathogen in potato pieces and leaves [17], and forms a mechanical barrier to pathogen penetration [18]. It also has a synergistic effect with plant protection products, allowing reduced used of chemical plant protection products. An example of chitosan hydrochloride use in potatoes and tomatoes to reduce late blight and grey mold (Botrytis cinerea) by eliciting systemic acquired resistance was presented by the European Food Safety Authority (EFSA) [19].

The combination of basic substances with different PPPs or plant extracts can be an effective alternative to chemical plant protection methods and can be applied in OF. However, such combinations are not often used in practice, as their field efficacy depends on various factors, including the crop, the pathogen or pest, the origin of the basic substance, and the timing and method of application. The composition of the basic substance is also highly relevant.

The aim of the study was to develop strategies for the combined or alternating use of basic substances and copper in order to delay the appearance of potato late blight symptoms and keep them below the level of economic damage in order to obtain a higher yield. The substances were applied alone or in combination with other agents with different varieties in different experiment conditions.

2. Materials and Methods

Experiments were conducted under laboratory, greenhouse, and field conditions in two regions of Poland. Various concentrations of BSs, applied either alone, in combination with other BSs, or together with a low dose of copper, were evaluated. Additionally two commercial yeast strains of Saccharomyces cerevisiae (US 05 and Coobra), which effectively minimized early blight symptoms on potato plants in previously tests [20], were included in the treatments.

In field trials, the efficacy of protective treatments was assessed primarily based on the yield obtained. Potato varieties with differing sensitivity to late blight were used, and the disease severity on leaves was evaluated using a 9-point scale, in which 9 represented the highest resistance. The varieties used were as follows: Red Sonya (6/9); Tajfun, and Jelly (both moderately sensitive, 5/9); Lilly (low sensitive, 6/9); Vineta (very high sensitive 2/9); Satina (sensitive 3/9), Lord (sensitive 3/9), and Jurek (more sensitive 4/9). Foliar sprays were applied 4–6 times at 7–10-day intervals.

2.1. Laboratory Experiments

In the in vitro experiment, onion bulb extract and sunflower oil (commercial product for use in food: Carrefour Classic, EAN: 5905784350325, manufactured by EOL Polska sp. z o.o., Szamotuły, Poland) at concentrations of 5% and 1% (

Table 3. In vitro effect of selected basic substances added to Rye-B Agar (RBA) on Petri dish surface colonization by P. infestans (%), over 8 days.

Days	Unmodified RBA (Control)	RBA Modified with 1% Sunflower Oil	RBA Modified with 5% Onion Bulb Extract
5	52.96 ± 4.98 a *	32.60 ± 4.04 b	29.62 ± 3.28 b
6	63.71 ± 2.52 a	44.07 ± 4.22 b	44.82 ± 3.30 b
7	67.04 ± 2.89 a	48.89 ± 3.86 b	50.73 ± 4.17 b
8	75.56 ± 2.52 a	50.38 ± 4.01 b	53.71 ± 4.39 b

* Values in lines followed by the same letter are not statistically different at p < 0.05.

), respectively, were added into RBA (Rye B Agar) medium. P. infestans strain 12/124 was obtained from the Pathogen Bank of the Institute of Plant Protection—National Research Institute, Poznań, Poland.

Discs (5 mm in diameter) of P. infestans mycelium, taken from the actively growing margin of fresh colonies, were placed in the center of each plate. The experiment was conducted with five replications, each consisting of five plates. In each replicate, a new culture of P. infestans was used because the susceptibility to the pathogen can vary. No plant protection product was used because the aim of the laboratory experiment was to select the growth-inhibition potential of the pathogen depending on the presence of the basic substance; a medium not modified by the addition of basic substances was used as a control. All plates were incubated at 23 °C. The percentage of Petri dish colonization by P. infestans was recorded daily from day 5 to day 8 of the experiment.

2.2. Greenhouse Experiments

In the in planta greenhouse experiment, twenty unfertilized potato plants (var. Lord) at the BBCH 19 growth stage were used in each replicate; four replicates were used. Plants were inoculated with a conidial suspension (1 × 10⁵ conidia/mL) of P. infestans isolate (strain 12/124) grown on RBA. Foliar spray inoculation of the leaves (with occasional coverage of stems) was performed with a hand sprayer, with approximately 5 mL per plant being applied. Following inoculation, to ensure high humidity, plants were covered with plastic bags and maintained at 25 °C under a 12/12 h light/dark photoperiod for 24 h. Plants were then transferred to the greenhouse, where each was sprayed with aqueous onion extract at 5% or 25% dilution and sunflower oil at 1% and 10% dilutions. Inoculated plants sprayed with distilled water served as controls. After one week, when chlorotic and necrotic symptoms became visible, disease severity was assessed on five randomly selected leaves per plant from the middle part of the plants and confirmed by microscopic observation. The following scale was used: grade 0—disease free; grade 1—1–10% of leaf area infected; grade 2—11–25% infected; grade 3—26–50% infected; grade 4—51–75% infected; grade 5—>76% infected [21].

2.3. Field Experiment Design in 2022

In general, the field experiments in 2022 and 2023 were two one-year trials with different potato varieties, in different regions, and with different treatments, but all the trials were conducted in an organic system in order to obtain as many results as possible as a basis for developing harmonized recommendations. Each field experiment was performed according to the same details; each combination consisted of 4 plots, and each plot was 22 m² with 4 ridges. The yield was harvested from two central ridges in each plot, due to the necessity of maintenance buffer zones and prevention of drift. In 2022, field trials were conducted in western Poland at the experimental agricultural station of the Institute of Plant Protection—National Research Institute (IPP–NRI), Poznań (52.2° N; 17.4° E). Three potato varieties were used: Tajfun, Lilly, and Vineta (all susceptible to P. infestans to varying degrees). The effectiveness of protective treatments, applied 4–6 times during the growing season at 7–10-day intervals, was evaluated primarily on the basis of the harvested yield. Different basic substances were tested: sunflower oil at 10% (20 L oil per 200 L water), onion bulb extract at 25% (1 kg of onion bulb per 4 L water, boiling 10 min, put away, filter); chitosan was also used at 2% (2 L chitosan per 100 L water), and Cu was applied at a total dose of 2 kg/ha. Protective treatments (Ts) were applied in the following combinations: (a) copper fungicide (Cu) at a total pure copper dose of 2 kg (T1–T4); (b) copper fungicide (T1–T4) followed by sunflower oil treatments (T5–T6; first with 5–10% oil emulsion); (c) copper fungicide (T1–T4) followed by onion extract treatments (T5–T6); (d) onion extract 25% (T1–T4); (e) sunflower oil emulsion (T1–T4); (f) copper fungicide (T1–T4) followed by chitosan treatments (T5–T6); (g) chitosan 2% (T1–T4); (h) untreated control.

A parallel experiment was carried out in eastern Poland on a private organic farm, using the potato cultivar Red Sonya, which is more resistant to P. infestans. Four treatments (T1–T4) were performed. The strategies tested included sunflower oil at 10%, onion extract at 25%, chitosan at 2%, copper at a total dose of 2 kg/ha, and combinations of different substances (details of strategies of treatments: sunflower oil emulsified solution (T1–T4), onion water extract (T1–T4), chitosan (T1–T4), Cu (2 kg) (T1–T4), Cu (T1, T3)/sunflower oil (T2, T4), Cu (T1, T3)/onion (T2, T4), Cu (T1, T3)/chitosan (T2, T4), untreated control).

2.4. Field Experiment Design in 2023

Field trials were conducted in western Poland in 2023 under organic farming conditions at the experimental agricultural station of the Institute of Plant Protection—National Research Institute (IPP–NRI), Poznań (52.2° N; 17.4° E). Three potato varieties with differing susceptibility to P. infestans were planted: Satina, Jelly, and Jurek. Protective treatments consisted of four applications (T1–T4) and included the following strategies: (a) copper fungicide (Cuprozin Progress) applied at a total seasonal dose of 2 kg/ha pure copper (T1–T4); (b) yeast treatments: two commercial strains of Saccharomyces cerevisiae (US 05 and Coobra), each suspended in distilled water at a concentration of 2 × 10⁷ CFU/mL (T1–T4/200 L water/ha); (c) copper + sunflower oil—copper fungicide (T1–T2) followed by two treatments (T3–T4) with emulsified sunflower oil at 10%; (d) untreated control. Treatments were carried out at 7–10-day intervals. The effectiveness of the strategies was evaluated primarily on the basis of the harvested yield.

2.5. Statistical Analysis

Mean values were calculated. Final experimental data were represented as the mean. The results were analyzed statistically by two-way (cultivar and treatment) ANOVA. The significance of differences between the mean values was verified by Tukey’s test at a level of p < 0.05 using Statistica 12.

3. Results

3.1. Laboratory Experiments

Both basic substances were effective in limiting the growth of the pathogen on Petri plates. Statistically significant differences were observed between the unmodified control medium and the media supplemented with the basic substances. The inhibitory efficacy of sunflower oil (1%) and onion bulb extract (5%) was approximately 50% (Table 3).

3.2. Greenhouse Experiment

Greenhouse experiments showed that neither a 5% nor a 25% aqueous extract of white onion bulb could suppress the development of disease symptoms in potato leaves. A tendency to inhibit the rate of disease development on potato leaves was observed only when a 25% solution was applied compared with a 5% solution and the control (Figure 1). Thus, the self-application of onion extract under field conditions can be ineffective.

In the greenhouse, results indicated potential for the growth suppression of the pathogen when 25% onion extract was applied; these results were not statistically significant but indicated a trend. Therefore, under field conditions, this extract was also applied to varieties with different susceptibilities and in combined treatments.

On the other hand, plants treated with 10% sunflower oil exhibited a significant reduction in disease symptoms compared with the untreated control, and the application of 1% sunflower oil did not result in a statistically significant decrease in symptoms (Figure 2).

3.3. Field Experiments in 2022

The ability to reduce late blight symptoms on potato plants was evaluated based on the harvested yield, which reflected the effectiveness of the protection strategies used. Yields varied significantly among cultivars (16.6–22.6 t/ha), confirming that variety choice is a critical factor for successful potato production, particularly under organic farming and the used protection methods. The ability to quickly set tubers and protect the yield is crucial.

Vineta, among the used varieties, produced the highest yield (22.6 t/ha) regardless of the protection strategy applied, whereas the yield of Tajfun was the lowest (17 t/ha). All protective treatments reduced disease symptoms and prolonged plant vitality compared with the untreated control, which is reflected in the yield. The yield harvested in combination with chitosan application (21.9 t/ha) was significantly better than that obtained for copper treatment (18.7 t/ha); however, it was not significantly higher than the yield from other treatments. Among them the most promising combination was copper followed by sunflower oil spraying (21.4 t/ha) (

Table 4. Effect of basic substances and copper application on mean yield ± SD (t/ha) of three potato varieties in organic field trials in 2022 in western Poland.

Combination	Treatment	Potato Variety—Yield (t/ha)
		Lilly	Vineta	Tajfun	Mean
a	Cu	18.33 ± 1.29 ab *	21.46 ± 2.64 ab	16.29 ± 1.21 ab	18.69 b
b	Cu/sunflower oil	22.60 ± 2.13 a	23.02 ± 3.50 ab	18.64 ± 2.08 a	21.42 a
c	Cu/onion	18.42 ± 3.00 ab	24.37 ± 1.64 a	17.83 ± 0.97 a	20.20 ab
d	Onion	19.86 ± 1.97 ab	24.11 ± 2.75 a	15.89 ± 1.54 ab	19.95 ab
e	Sunflower oil	18.79 ± 3.56 ab	23.86 ± 2.33 a	18.91 ± 1.20 a	20.52 ab
f	Cu/chitosan	22.09 ± 2.81 a	22.50 ± 2.84 ab	16.98 ± 0.76 ab	20.52 ab
g	Chitosan	22.36 ± 3.14 a	25.46 ± 2.52 a	17.86 ± 1.55 a	21.89 a
h	Untreated control	14.34 ± 0.56 b	16.54 ± 0.64 b	13.35 ± 1.79 b	14.74 c
	Mean	19.60 B	22.66 A	16.97 C

* Values in lines and columns followed by the same letter are not statistically different at p < 0.05.

).

At the second location, where the cultivar Red Sonya (less susceptible to P. infestans than Tajfun, Lilly, and Vineta) was grown, the effects of the tested protective treatments were less visible. Final effectiveness was evaluated based on the harvested yield. Untreated plants had a lower yield (23.2 t/ha) than treated plants; however, the differences were not statistically significant. Copper applied at a total seasonal dose of 2 kg/ha proved sufficiently effective (28.8 t/ha), clearly indicating that the combination of a more resilient potato variety and a low-dose copper treatment, applied preventively, can be an effective protection strategy in organic farming (

Table 5. Effect of basic substances and copper application on mean yield ± SD (t/ha) of Red Sonya variety in organic field trials in 2022 in eastern Poland.

Combination	Treatment	Yield (t/ha)
a	Sunflower oil emulsified solution	24.1 ± 1.52 b *
b	Onion water extract	24.8 ± 1.40 ab
c	Chitosan	24.7 ± 1.24 ab
d	Cu (2 kg)	28.8 ± 2.04 a
e	Cu (T1, T3)/sunflower oil (T2, T4)	26.2 ± 1.25 ab
f	Cu (T1, T3)/onion (T2, T4)	26.9 ± 1.61 ab
g	Cu (T1, T3)/chitosan (T2, T4)	26.2 ± 1.50 ab
h	Untreated control	23.2 ± 1.67 b

* Values followed by the same letter are not statistically different at p < 0.05.

).

3.4. Field Experiments in 2023

In the previous year, protective combinations of sunflower oil with a low dose of copper proved to be effective and easier to implement than combinations with onion extract. Therefore, the 2023 study focused on sunflower oil alone and on the introduction of additional microorganisms—yeast—whose efficacy had been confirmed in earlier studies [20]. Three potato cultivars (with different levels of resistance to P. infestans) were used: Satina (3/9), Jelly (5/9), and Jurek (4/9). Statistically significant yield differences were noted between cultivars (9.9–20.5 t/ha), with Jelly producing the lowest yield (

Table 6. Effect of basic substances, copper application, and yeast on mean yield ± SD (t/ha) of different varieties in organic field trials in 2023 in western Poland.

Treatment	Potato Cultivar—Yield (t/ha)
	Satina	Jelly	Jurek	Mean
Cu	19.75 ± 3.28 ab *	13.57 ± 1.41 a	23.53 ± 1.97 a	18.95 a
Yeast	20.21 ± 0.68 a	9.75 ± 2.58 ab	22.41 ± 1.97 a	17.46 ab
Cu/sunflower oil	21.46 ± 1.01 a	8.20 ± 2.51 b	22.80 ± 2.17 a	17.48 ab
Untreated control	15.38 ± 1.72 b	7.99 ± 1.64 b	13.56 ± 1.44 b	12.31 b
Mean	19.2 ab	9.88 b	20.57 a

* Values followed by the same letter are not statistically different at p < 0.05.

). Similar to results obtained in the previous year for Red Sonya, a low copper dose (2 kg/ha) was effective in some less-susceptible cultivars, i.e., Jelly and Jurek (with the mean for all cultivars being 18.9 t/ha). The lowest yields were obtained from untreated plants, regardless of variety (mean 12.3 t/ha); however, statistically, it was significantly only compared with copper treatment. In treatments with yeast and copper combined with sunflower oil, the mean yield was 17.5 t/ha. Statistical analysis confirmed that copper at a dose of 2 kg/ha, applied alone or in combination with sunflower oil or yeast, can be an effective, practical, inexpensive, and environmentally friendly protection strategy for some potato cultivars.

4. Discussion

P. infestans is the most devastating pathogen of potato. Considering the increasing demand for reducing inputs of synthetic and copper-based pesticides and regarding the emergence of novel pesticide-resistant strains, alternative control methods are a necessity [22]. Effective management of P. infestans includes sanitation, host resistance, and efficient and effective use of agrochemicals. By limiting the pathogen’s survival, reproduction, dispersion, and penetration, agricultural practices can help decrease the pathogen’s population. Agricultural practices include the removal of plant debris that can act as a potential inoculum and appropriate ridge management practices, harvesting, and post-harvest soil tillage. It is important to remove infected leaves and stems and destroy them or to use mechanical means to prevent the tubers from becoming infected [23].

Several alternative approaches to combat P. infestans have been reported. These include the use of beneficial microbes as biological control agents (BCAs) for controlling phytopathogens including P. infestans [15]. Another successful approach includes intercropping potatoes with garlic, which has been found to efficiently manage late blight of potato crops due to chemical secretions from garlic roots that negatively impact late blight disease [24]. Also many yeasts have been evaluated for biocontrol applications because of their antagonistic ability, low cultivation requirements, and limited biosafety concerns [15,20,23,24,25]. Ten wild yeast strains including Metschnikowia pulcherrima, Curvibasidium pallidicorallinum, Candida saitoana applied to potato leaves before inoculation significantly reduced late blight symptoms—suggestive of induced plant immunity. M. pulcherrima was particularly effective under narrow concentration ranges, inducing phytoalexin production [26]. A comprehensive screening was carried out of 149 fungal candidates (including yeasts) sourced from culture collections in Belgium and the Netherlands. Candidates were also evaluated in in vitro and in planta assays against P. infestans, highlighting the importance of large-scale screening and in vivo verification for potential biocontrol strains [1]. In other reviews, a range of biocontrol agents—bacteria, filamentous fungi, and yeasts (e.g., Aureobasidium, Curvibasidium, Metschnikowia)—used as promising agents against P. infestans in solanaceous crops like potato and pepper were noted [27]. For chosen potato cultivars, the high potential of yeasts in protection of potato was also confirmed in our trials; the application of two commercial strains of S. cerevisae was effective, and the yield was comparable to that of the combination, in which the plants were protected by copper (Table 6).

Biocontrol products based on plant extracts also appear to be a promising solution. To evaluate the in vitro inhibitory potential of a plant extract-based biocontrol product on the different stages of the P. infestans lifecycle, including mycelial development, the formation and germination of infection structures (sporangia and zoospores) has been described. Interestingly, at non-inhibitory doses, zoospore germination exhibited disturbances, such as an increase in abnormal germination phenotypes. Overall, the plant extract showed significant inhibitory potential against the oomycete [28]. Our in vitro test confirmed that both onion extract and sunflower oil present inhibitory effects on pathogen growth (Table 3).

Most studies on the management of late blight of potato caused by P. infestans through botanical aqueous extracts focus on garlic (Allium sativum), which is closely related to onion and shows strong antifungal activity. Garlic extract at 15% concentration inhibited mycelial growth by 58% in vitro and reduced disease incidence to 5.8% in greenhouse trials—versus 61% in controls [29]. These results strongly suggest that allium extracts, which share similar sulfur compounds, can suppress P. infestans. Onion (A. cepa) also contains many of the same bioactive compounds as garlic (e.g., alk(en)yl sulfides, thiosulfinates), so it is plausible that onion extract could work similarly. However, direct studies on onion extract vs. P. infestans are scarce. Application is recommended preventively; it should start before the first signs of blight and be repeated fortnightly during humid weather. Another study was carried out to evaluate the efficacy of garlic (A. sativum) versus neem (Azadirachta indica), turmeric (Curcuma longa), and mint (Mentha) at 5, 10, and 15% concentrations as bio-fungicides against late blight of potato. The in vitro effect of aqueous plant extracts was evaluated based on the percent inhibition and radial growth of the pathogen. In comparison to the control, A. sativum and A. indica at 15% concentrations were found to be more effective in inhibiting P. infestans mycelial growth by 58.4% and 43.9%, respectively. The use of A. sativum and A. indica aqueous plant extracts at a concentration of 30% was found to be the most promising and effective measure against the late blight pathogen. In our study onion water extract applied at a 25% concentration and sunflower oil at a 10% concentration were also promising towards the control/limitation of symptoms of potato late blight (Figure 1 and Figure 2). In another paper, it was indicated that it was useful to soak sweet potato in onion extract (50 g/L) for an hour before planting followed by spraying once a week 4–6 week after planting, which could inhibit sweet potato scab disease intensity 70 to 80%, increase the weight and size of tubers by 46%, and prevent yield loss by up to 33% [30]. Our strategy involved copper applied with onion extract; it also confirmed that using onion extract combined with copper (usually copper sulfate or copper hydroxide) is a promising integrated, low-risk strategy for protecting potatoes from fungal and bacterial diseases, especially late blight (Phytophthora infestans) and bacterial soft rot (Pectobacterium spp., Dickeya spp.). This is in line with our statement (Table 4 and Table 5).

In Romania, 2.2% and 3.3% water solutions of the onion crop showed significant protection against A. solani in potato fields [31]. Dry extracts of onion (concentration 20.0 mg/mL) showed antifungal activity against A. alternata and P. infestans. This is in line with our results. Another substance, chitosan (CS) is a natural cationic biopolymer, and previous studies have explored the antibacterial activity of CS, and more recently different types of CS derivates have been synthesized to enhance its natural antibacterial activity [32]. Additionally, chitosan treatment regulates several genes in plants, especially the activation of plant defense signaling pathways, including the acquisition of plant antitoxin and pathogenesis-related (PR) proteins. Chitosan, an N-acetyl- and d-glucosamine co-polymer, is a plant stimulator that induces defense genes and the reactive oxygen species scavenging system [33]. Chitosan has been tested to control multiple diseases in many crops before and after harvest and has a positive effect on enriching rhizosphere biodiversity, which is especially important in organic farming [34]. In potato, some studies have shown that chitosan application can inhibit several diseases, such as late blight, and this was confirmed in our tests and results presented in Table 4 and Table 5 [17]; the literature also endorses the promising effect of chitosan in controlling potato viruses [35] and other fungal diseases [36]. Rabea et al. [37] confirmed that late blight can be delayed after eight sprays of 0.1% chitosan and provided 60% protection against late blight by mixing 4% chitosan with a plant elicitor [38]. In our strategies, chitosan was applied at a 2% concentration and confirmed its ability to protect plants and extend their vegetation period; mixing chitosan with copper was also effective (Table 4 and Table 5). Some late blight reducing potential for 0.4% chitosan was found in field tests performed on the cultivars Nicola and Ditta. Chitosan (0.4%) and the copper fungicide were tested earlier, and chitosan (0.4%) accompanied by horsetail and licorice products seemed to be able to cause some degree of disease reduction, even under an extremely late infection regime [39]. Good results with a low-level copper formulation (copper sulfate pentahydrate), together with chitosan as an adhesive substance to increase rainfastness, were also obtained by Hadwiger and McBride [40]. In other study, applications of chitosan were tested against P. infestans in outdoor conditions [41]. This experiment showed that chitosan is very effective against P. infestans, and our results also showed a positive effect. A single application of a 0.4% solution of chitosan provided an inhibitory effect of 37%; after four applications, an inhibitory effect of up to 99.3% was obtained, and it also confirmed that the four applications used in our strategies are needed.

Recent studies indicate that other basic substances, including lecithins and horsetail macerate [42], can provide levels of protection against late blight comparable to those achieved with copper-based treatments.

5. Conclusions

Preventive treatments contributed to increased yields compared with untreated potato plants. Applications of emulsified sunflower oil at 10% or copper at 2 kg/ha resulted in yield improvement in some potato cultivars, with similar effects achieved when copper was combined with chitosan, chitosan was applied alone, or onion extract was used at 25%. The most critical factor influencing treatment effectiveness was the susceptibility of the potato variety to P. infestans. For less-susceptible cultivars, the use of basic substances alone may be sufficient to protect plants, extend the vegetation period, and maintain yield. In contrast, for highly susceptible varieties, combining copper with basic substances is recommended. For cultivars with a higher susceptibility to P. infestans, a combination of copper and basic substances is recommended, and it is possible to carry out BS treatments after copper treatments or, alternately, with at least six treatments 7–10 days apart. Research has confirmed the inhibitory effect of basic substances on pathogen growth. However, under field conditions, the yield obtained is the result of a combination of several factors, including cultivar susceptibility, pathogen pressure, and the pathogen’s sensitivity to the substances used.