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Article

The Extent of Lecanosticta acicola Spread Along the Polish Baltic Coastline

by
Piotr Boroń
1,*,
Klaudia Bulanda
1,*,
Marzena Kaźmierczak
2,
Bartłomiej Grad
1,
Anna Majewska
1 and
Anna Lenart-Boroń
3
1
Department of Forest Ecosystem Protection, Faculty of Forestry, University of Agriculture in Kraków, 31-425 Krakow, Poland
2
Department of Ecology and Silviculture, Faculty of Forestry, University of Agriculture in Kraków, 31-425 Krakow, Poland
3
Department of Microbiology and Biomonitoring, Faculty of Agriculture and Economics, University of Agriculture in Kraków, 31-120 Krakow, Poland
*
Authors to whom correspondence should be addressed.
Forests 2025, 16(12), 1830; https://doi.org/10.3390/f16121830 (registering DOI)
Submission received: 22 October 2025 / Revised: 1 December 2025 / Accepted: 2 December 2025 / Published: 6 December 2025
(This article belongs to the Section Forest Health)
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Abstract

This paper describes a study conducted to investigate the spread of Lecanosticta acicola, the cause of brown spot needle blight (BSNB), in Pinus mugo dune forests in the Polish Baltic region. Between 2023 and 2025, 22 sites were surveyed, including coastal forests and some ornamental plantings. Characteristic BSNB symptoms were recorded in 21 of the 22 locations, and the pathogen’s presence was confirmed through culture isolation and species-specific PCRs. The disease was most severe in compact, monospecific P. mugo thickets, where defoliation exceeded 30%, while mixed stands with P. sylvestris or occasionally P. nigra exhibited lower infection rates. A degree of infection of P. sylvestris was observed in proximity to heavily infected P. mugo stands, confirming interspecific transmission under high inoculum pressure. We observed nearly ubiquitous occurrence of L. acicola along the coast, suggesting an advanced stage of establishment. However, the very recent detection of the pathogen at the westernmost sites indicates ongoing westward spread. These findings highlight the vulnerability of coastal P. mugo populations and underline the need for genetic diversity data that would allow us to trace the origins and pathways of L. acicola spread in the south-eastern Baltic region. The potential adaptation of the pathogen to P. sylvestris in the future would pose a serious risk for Polish forestry.

1. Introduction

Brown spot needle blight (BSNB) caused by an ascomycete foliar pathogen, Lecanosticta acicola (Thüm.) Syd., is one of the serious emerging threats for pines in Europe [1]. The pathogen completes its lifecycle on needles, resulting in their necrosis and premature loss. Potentially, severe defoliation may lead to stunted development of young trees and reduced growth. Tree mortality due to L. acicola infections has been reported multiple times for several pine species see [1]. The same as below. The disease has been known since at least the late 19th century from the south-eastern USA [2], and from the early 20th century onwards it has been recognized as a serious threat to native pines in that region [3,4].
Since then, the pathogen has spread to numerous locations worldwide, causing varying levels of damage. In short, the current range of L. acicola includes Europe, eastern North America, Central America and Colombia, and east Asia. The current disease reports can be readily tracked using a dedicated geo-database, available at http://www.portalofforestpathology.com [1,5,6], which is the most comprehensive up-to-date source for L. acicola occurrence.
The first reliable record of BSNB in Europe is from P. radiata in Spain and dates back to the 1940s [7]. Initially, relatively slow expansion of the pathogen was observed, resulting in new disease reports from Austria, France, Germany, Italy, Spain, Switzerland, and Croatia see [1]. The same as below. The situation changed, however, during the last two decades, when numerous new BSNB outbreaks occurred throughout the continent, resulting in the pathogen being present in 24 European countries [1].
The pathogen itself originates from North/Central America where numerous Lecanosticta species occur [1,8]. Of these L. acicola is the most widespread and the only one occurring outside its native region. It is a genetically diverse species encompassing three main lineages [8,9,10], two of which are present in Europe: these are the northern lineage (originally from the eastern USA and Canada) and the southern lineage (originally from the southern USA). Thus, the results of genetic analyses show that the current occurrence of L. acicola in Europe is a result of multiple independent introduction events [9,10].
Typical BSNB symptoms start with the development of yellow, or sometimes light grey-green or reddish brown, spots surrounding infection points on needles [8]. As the disease develops, these spots turn brown, often surrounded by a yellow halo, and for some host species are also resin-soaked [11]. These symptoms are followed by dieback of the entire needle, starting from the tip, and its premature shedding. The disease is usually much more prevalent on the previous years’ needles, leaving the new growth temporarily unaffected, and the damage starts from the lower part of the crown [1,8]. The effect is much less pronounced in Pinus mugo thickets, which retain a relatively low and compact canopy even at advanced age. These disease symptoms are similar, especially at the early stage, to those caused by Dothistroma spp. This has led to numerous instances of misidentification in the past; thus, molecular identification methods are now required for BSNB reporting.
As of today, Tubby et al. [1] provide the most comprehensive list of species, almost exclusively pines, for which susceptibility to L. acicola infection has been established. There are three susceptibility levels recognized in this review, i.e., low, medium, and high. Interestingly, numerous species are listed in more than one category. This is the case for some very important central European pines, for instance, P. sylvestris, P. mugo, and P. nigra, which have been determined to be either less, moderately, or highly susceptible depending on the region and growth conditions.
The first record of BSNB in Poland dates back to 2011 [12,13] from the Karkonosze range (Karkonosze National Park). However, this symptom-based report lacks molecular verification and should, therefore, be treated with caution. The first confirmed case of BSNB from Poland comes from 2017 [14]. This time the pathogen was detected on the coast of the Baltic Sea; however, some confusion exists pertaining to the exact site. Whereas the location is reported as the Ustka environs, the coordinates point to Białogóra, which is more than 80 km east along the coast. Nevertheless, four new locations have been added since then to the L. acicola geo-database, all from P. mugo from sites along the Baltic coastline: two forest locations and two urban greenery/ornamental plantings. Such a distribution shows that L. acicola occurrence is linked to P. mugo as the main host and to the Baltic coastal region.
This region is notorious for relatively frequent occurrence of P. mugo, either in coastal forests or as ornamentals. The history of its presence here starts approximately in the 1820s on the Curonian Spit where it was introduced, using Danish stock material, to stop dramatic dune shifts [15]. The species is particularly well suited for dune stabilization due to its pioneer ecological strategy, wind firmness, and sandy substrata and salt-spray tolerance [16,17]. These very proprieties made the species ideal as a fore-crop and for afforestation of most exposed dune ridges [17,18]. The example of the success on the Curonian Spit led to wide introduction of P. mugo throughout the region as well as in the wider Baltic area, including in Denmark, Norway, Finland, Sweden, Estonia, Lithuania, and Poland [16,19].
Although P. mugo subsp. mugo (referred to throughout this paper as P. mugo or dwarf mountain pine) is a native species in Poland, its natural occurrence is restricted to several mountain locations in southern Poland. There it forms its own mountain belt, respectively above and below Norway’s spruce upper mountain forest and alpine meadow belts [20]. Thus, for the rest of the country, including the Baltic coastal region, it is actually an exotic species occurring outside of its natural habitat. According to the BDL database [19] there are 177 variously sized (average size 3.6 ha) forest subcompartments on the Polish coast with a dominant share of P. mugo. However, these are not evenly distributed as there are three major mountain pine reach areas: the Wiślana Spit, Hel peninsula (around the Hel town itself, but not along the entire Hel Spit), and the open coast region stretching from Karwia to the east to Rowy to the west, including Słowiński National Park. Such an occurrence pattern most strongly resembles the situation found on the Curonian Spit, Lithuania. Only nine P. mugo-dominated subcompartments occur outside these areas, five at the base of the Gdańsk Bay and four far to the west of Rowy (see Figure 1). Considering reports on the occurrence of L. acicola in multiple sites on the Polish coast [14] and its reported fast spread in the neighboring countries [21], one should assume the Baltic population of P. mugo in Poland is highly threatened by damage caused by BSNB.
Therefore, the aims of this study were (i) to assess and document the extent of BSNB distribution in P. mugo thickets on the Polish coast of the Baltic Sea established for dune stabilization, and (ii) to determine and document instances of L. acicola occurrence within the region outside P. mugo dune forests, including on P. sylvestris and on P. mugo ornamentals.

2. Materials and Methods

The observations in the field and collection of needle samples were carried out in three major surveys: in summer 2023 (6 July to 11 July), winter 2023 (18 December to 21 December), and summer 2024 (4 July to 9 July). Some locations were revisited in summer 2025 to confirm the continued presence of the disease and the prevalence of symptoms. The exact list of visited locations is presented in Table 1, whereas Figure 1 shows their geographic distribution. Most of the sites comprise dense patches of P. mugo dwarf forest planted on shifting sand dunes for the exact purpose of their stabilization and afforestation. Sites of this type were selected in a systematic manner using the BDL database [19]. Using the database, we identified all forest subcompartments with P. mugo as a dominant species along the entire length of the Polish Baltic coast. As the subcompartments proved to be unevenly distributed along the coastline, we adopted the following rule for selection of survey sites: in areas where P. mugo-dominated stands were relatively common we visited subcompartments at arbitrary intervals of ca. 5 km; wherever P. mugo subcompartments occurred more than 5 km from each other, we visited all of them. Using this approach, the entire Polish coast of the Baltic Sea was mapped and examined for the presence of BSNB in P. mugo-dominated stands. The only exceptions were stands within Słowiński National Park, where access to locations outside touristic trails is restricted.
The surveys conducted at each site included a search for characteristic BSNB symptoms on needles within P. mugo stands, as well as on the needles of P. sylvestris and/or P. nigra individuals if present within or in direct proximity to the examined stand. For every forest location (indicated as dune forests in Table 1), general observations of disease incidence were recorded. For this, we used the same binary high/low incidence scale with an arbitrary 30% cutoff as previously used by Boroń et al. [22] for a similar pathogen, Dothistroma septosporum. Disease incidence was considered high if the visually assessed proportion of symptomatic needles exceeded 30% of the overall amount of still unshed needles on 25 trees selected at random throughout the entire stand; any estimation below 30% of infected needles was considered low.
When found, symptomatic needles were collected in paper envelopes from 3 to 6 point locations selected at random across the forest subcompartment or group of ornamentals. At each point location a pooled needle sample was collected, comprising an undetermined number of needles (up to ca. 100) originating from a single tree or a cluster of up to ten closely spaced trees located no more than 10 m from each other. When no typical BSNB symptoms were observed on needles within the examined stand, a single sample (up to ca. 100 needles) was collected, comprising needles from various trees with unspecific symptoms of any kind visible on the needles, usually consisting of necroses or discolorations. In this form the needle samples were stored at −20 °C for further processing.
Table 1. Locations of surveyed sites, parameters of Pinus mugo stands, disease incidence estimation, and results of pathogen identification. The sites are numbered in west to east order. Disease incidence estimates: NO—not observed; L—low incidence (<30%—infected needles); H—high incidence (>30% of infected needles); NA—not analyzed. The number of isolates/PCR in planta positives includes either the number of PCR-confirmed L. acicola isolates generated from needle samples collected at the listed dates or the number of L. acicola-positive PCR results per number of tested DNA extracts.
Table 1. Locations of surveyed sites, parameters of Pinus mugo stands, disease incidence estimation, and results of pathogen identification. The sites are numbered in west to east order. Disease incidence estimates: NO—not observed; L—low incidence (<30%—infected needles); H—high incidence (>30% of infected needles); NA—not analyzed. The number of isolates/PCR in planta positives includes either the number of PCR-confirmed L. acicola isolates generated from needle samples collected at the listed dates or the number of L. acicola-positive PCR results per number of tested DNA extracts.
#LocationCoordinatesGrowth
Conditions		Host and AgeL. acicola
Observed in:		Disease
Incidence		Detection MethodNumber of
Isolates/PCR in Planta Positives
1Dźwirzyno54.163438, 15.421252dune forestP. mugo; 15winter 2023
summer 2024
summer 2025		NO
L
L		isolations + PCR42
NA
NA
2Mielno54.255583, 16.047972ornamentalP. mugo; 30winter 2023NAin planta PCR8/10
3Łazy54.284133, 16.133913dune forest 1P. mugo; 45summer 2023
winter 2023
summer 2024		NO
NO
NO		in planta PCR0/10
0/10
0/10
4Dąbki54.355256, 16.272687dune forestP. mugo 2; 30summer 2024Hisolations + PCR24
5Rusinowo54.511056, 16.509722ornamentalP. mugo; 30summer 2023
summer 2025		NA
NA		in planta PCR10/10
NA
6Łeba center54.748306, 17.556361ornamentalP. mugo; 20summer 2023NAisolations + PCR5
7Łeba
(Sarbska Spit)		54.776814, 17.637164dune forestP. mugo; 97summer 2023
summer 2024
summer 2025		H
H
H		isolations + PCR44
NA
NA
8Dymnica54.786219, 17.697160dune forestP. mugo; 81summer 2023Hisolations + PCR38
9Sasinko54.790673, 17.751834dune forestP. mugo; 91
P. sylvestris; 25		summer 2023H 3isolations + PCR56
6
10Kopalino54.806926, 17.831828dune forestP. mugo; 96summer 2023Hisolations + PCR49
11Lubiatowo54.822468, 17.903625dune forestP. mugo; 81summer 2023Hisolations + PCR48
12Białogóra54.827512, 17.999713dune forestP. mugo; 96summer 2023Hisolations + PCR49
13Dębki54.831578, 18.052842dune forestP. mugo; 86summer 2023Hisolations + PCR41
14Widowo54.831801, 18.143029dune forestP. mugo; 90summer 2023Hisolations + PCR50
15Hel 354.633967, 18.809438dune forestP. mugo; 95summer 2023Hisolations + PCR26
16Hel 254.606510, 18.818236dune forest 4P. sylvestris; 23summer 2023NAisolations + PCR20
17Hel 154.601663, 18.816047dune forestP. mugo; 105summer 2023Hisolations + PCR34
18Sobieszewska
Island		54.350285, 18.846477dune forest 1P. mugo; 89summer 2023
summer 2024		L
L		isolations + PCR41
NA
19Sztutowo54.350235, 19.184413dune forest 1P. mugo; 93winter 2023Lisolations + PCR18
20Kąty Rybackie54.352060, 19.226277dune forest 1P. mugo; 88summer 2023
winter 2023		L
L		isolations + PCR3
15
21Wiślana Spit 154.404688, 19.512823dune forestP. mugo; 88winter 2023Hisolations + PCR43
22Wiślana Spit 254.453418, 19.632085dune forest 1P. mugo; 63winter 2023Lisolations + PCR31
1 P. mugo stand overgrown by P. sylvestris and/or P. nigra. 2 Mistakenly identified as P. banksiana in BDL database. 3 Disease incidence estimated only for P. mugo. 4 Small P. sylvestris patch surrounded by heavily infected P. mugo stand.
For most of the sites, the presence of L. acicola was confirmed by two-step isolation of pure cultures of the pathogen; these locations are indicated in Table 1 by a number of isolates grown from needle samples collected at that site. Prior to isolation, the needles were placed in moist chambers (25 cm wide Petri plates with wet paper towels) for 2–3 days to stimulate production of conidia. Moist-chamber-incubated needles were then examined under a stereomicroscope (mag. 30×) for the presence of conidia, visible as an olive-green conidial mass erupting from fruiting bodies. The isolation itself started with dispersing the conidial mass, each time from a separate needle, in a droplet of sterile water placed directly on the fruiting body with a Pasteur pipette. Then, water carrying dispersed spores was sucked back into the same pipette and transferred to a Petri plate with MEA+TH medium [Malt Extract, 20 g/L + LAB-AGAR 15 g/L (Biomaxima, Lublin, Poland) + tetracycline hydrochloride 0.2 mg/mL (Glentham Life Sciences, Corsham, UK)], where it was spread over the medium with a Drigalski spatula. The plates were incubated at 20 °C in the dark for 3–6 days until 0.1–0.5 mm diameter colonies of L. acicola emerged. The single L. acicola colonies were then transferred, along with a 1–2 mm3 agar block, onto a new MEA+TH plate, paying particular attention to selecting only colonies which were well separated from each other and located as far as possible from contaminating yeast or mold colonies. The resulting L. acicola isolates were incubated at 20 °C in the dark until they reached ca. 1 cm diameter.
All acquired isolates (each isolate originating from a separate needle) were tested using species-specific PCR primers designed by Ioos et al. with a reported analytical sensitivity of 2 pg of pure pathogen DNA [23]. For two locations with ornamental P. mugo plantings (Mielno and Rusinowo), as well as for two forest locations without visible BSNB symptoms (Dźwirzyno—winter 2023 survey and Łazy—all surveys), the L. acicola occurrence was tested directly in 10 independent needle samples with the same PCR approach (see Supplementary Figure S1). The procedure started with DNA extraction, either from MEA+TH grown mycelium or from needle tissue. Mycelium fragments for isolation (10–30 mg) were collected with a sterile scalpel directly from the colonies, placed in a 2 mL microcentrifuge tube with two stainless steel beads (⌀ 3 mm), and homogenized in an oscillation mill (MM 200; Retsch, Haan, Germany) for 2 min at 25 Hz. Needle fragments (ca. 15-mm long) carrying symptoms, either typical BSNB symptoms and fruiting bodies or any unspecific symptoms, were dissected and pre-comminuted with sterile scissors. Then, the samples were transferred to 2 mL microcentrifuge tubes with three stainless steel beads (⌀ 3 mm) and homogenized using the same oscillation mill (6 min at 25 Hz). Total genomic DNA for both types of samples was extracted using the GeneJET Genomic DNA Purification Kit (ThermoFisher, Waltham, MA, USA) according to the manufacturer’s protocol, and stored at −80 °C for further analysis. The identification test itself relies on the use of L. acicola species-specific primers LAtef-F and LAtef-R targeting the translation elongation factor 1-α region [23]. Appropriate positive and negative (molecular grade water) controls were included in all runs. Reaction mixtures comprised 1× PCR Mix Plus Green PCR ready mix (A&A Biotechnology, Gdynia, Poland), 8 mmol MgCl2, and 0.08 μm of each primer in a total volume of 25 μL. Amplifications were run on a T100 Thermal Cycler (Bio-Rad, Hercules, CA, USA) using the following cycling profile: initial denaturation at 94 °C for 5 min, 25 touchdown cycles consisting of 30 s of denaturation at 94 °C, 30 s of annealing at decreasing temperatures (0.5°/cycle from 67.5 °C in the 1st to 55 °C in the 25th cycle), and 1 min of elongation at 72 °C; for the next 20 cycles a constant annealing temperature of 55 °C was used, followed by a final elongation step at 72 °C for 10 min. Our previous experience shows that such cycling conditions work well for DNA extracts originating from pine needles, allowing for reliable and specific amplification for a variety of primer pairs covering a wide range of melting temperatures. The reaction results were verified using agarose gel electrophoresis [electric field intensity 8 V/cm, 30 min., 1% agarose gel supplemented with SimplySafe DNA stain (0.001%, EURx, Gdańsk, Poland)]. Positive identifications were indicated by the successful amplification of the target product of 237 bp.

3. Results

In this study, the BSNB occurrence in P. mugo stands along the Polish Baltic coast was investigated. Most of the observations were conducted during three surveys in summer 2023, winter 2023, and summer 2024; however, some locations were visited multiple times. During the surveys, characteristic symptoms of the disease (see Figure 2) were observed in the vast majority of forest locations, which were the main focus of this study, as well as in several ornamental plantings (Table 1). These include Białogóra, Choczewo Forest District, which is the very site where the disease was observed for the first time on the Polish coast of the Baltic Sea. For all these sites the presence of BSNB pathogen L. acicola was confirmed by the isolation of pure cultures and/or by a species-specific PCR assay. The only site where BSNB symptoms were not observed during any visit was Łazy (Karniszewice Forest District). The BSNB-free status of this site was further indicated by the negative results of a PCR test for L. acicola conducted after each survey based on needle samples carrying unspecific symptoms. Thus, in total, the pathogen was detected in 21 out of 22 surveyed locations (Table 1), with multiple sites yielding numerous isolates originating from all pooled samples of symptomatic needles (see Supplementary Table S2). This indicates an established and widespread occurrence of L. acicola in coastal P. mugo thickets.
However, the situation for the two westernmost forest sites is more complicated. These include Dźwirzyno (Gościno Forest District) and Łazy (Karniszewice Forest District). Both of the sites proved to be BSNB-free either in summer 2023 (Łazy) or in winter 2023 (Dźwirzyno). There were also no L. acicola detections from these sites using the PCR assay. The only location in this area where BSNB symptoms were observed as early as winter 2023 is Mielno, where the disease was spotted on a group of ornamentals. During the next survey in summer 2024 the Łazy site was still BSNB-free, but Dźwirzyno showed typical BSNB symptoms, including readily visible L. acicola fruiting bodies on needles; based on the needle samples from 2024 a total of 42 L. acicola isolates were obtained here.
While BSNB symptoms were observed in almost all examined P. mugo thickets planted along the Polish coastline, the disease incidence varied between locations. The main differences in the disease incidence reflected the stage of succession of pine species within a given dune forest. Wherever P. mugo individuals grew in monospecific compact thickets the incidence of the disease was high: the incidence of diseased needles easily exceeded 30%, including in current-year needles, resulting in high defoliation levels. The only exception to this rule is once again the westernmost Dźwirzyno site where disease incidence was low despite compact monospecific composition. The disease occurrence pattern changed when stands became overgrown by other pine species, usually by P. sylvestris but sometimes also by P. nigra. This process involves gradual decline of dwarf mountain pine due to reduced access to sunlight, resulting in heavily reduced foliage and eventually dieback of P. mugo individuals. In such conditions, however, the BSNB incidence decreases significantly, rarely exceeding 30%. Succession to other pine species is of course gradual and our casual observations across this type of location indicate that the more overgrown the P. mugo patch becomes, the lower the observed BSNB incidence becomes.
Despite the fact that the vast majority of BSNB cases reported in this study originate from P. mugo, either forest stands or ornamental plantings, the disease was also observed on P. sylvestris. Indeed, a total of 26 L. acicola isolates originate from P. sylvestris needles collected at two sites (see Table 1). However, in each case the Scots pine individuals on which BSNB symptoms were observed grew within or in direct proximity to heavily infected P. mugo thickets. The disease incidence on them was always lower compared to that on mountain pine. Besides this, no readily visible stand-alone cases of BSNB on pines other than P. mugo were observed. Moreover, readily noticeable variation in disease incidence on P. sylvestris individuals was frequently observed; whereas some trees displayed varying levels of infection, others were completely disease-free (Figure 2c) despite heavy inoculum pressure resulting from close proximity to infected P. mugo.

4. Discussion

The mass introduction of P. mugo for dune stabilization on the Polish coast dates back to at least the early 20th century. According to the BDL database [19], the oldest still existing mountain pine stands are 114 years old (as of 2025). Out of 177 identified subcompartments, 156 were established prior to World War II’s end in 1945, and only 3 of them are younger than 40 years old (see Supplementary Table S1). This is particularly evident in the eastern part of the Polish coast, where the average age of P. mugo is 91 years. This means there have been no new predominantly mountain pine plantings for dune stabilization east of Rowy (54°40′07.8″ N 17°03′20.0″ E, Słowiński National Park western margins) for 59 years, which is the age of the youngest of P. mugo stands in this part of the coast. The advanced age of these stands results in a situation where they undergo succession to other pine species, predominantly P. sylvestris. Thus, it can be argued based on these data that introduction of P. mugo in this part of the coast, that is, as a fore-crop, has served its purpose, as most of the shifting dunes have been successfully afforested and the remaining few are protected for touristic and ecological reasons [24,25]. Only four P. mugo-dominant subcompartments exist on the Polish coast to the west of Rowy, and these are much younger (average age of 36) and much more widely separated from each other. The Dźwirzyno site is unique in this regard as it is the westernmost of all the subcompartments and the only new planting of P. mugo (22 years old) for dune stabilization along the entire Polish coast [19]. That being said, there are a number of forest stands along the coast with a minority occurrence of mountain pine. This is particularly true for the Ustka surroundings and the area of the adjutant Ustka-Wicko Morskie Central Air Force Training Range, where the BDL database lists 42 forest subcompartments (see Supplementary Table S1), with P. mugo as the most common species within the understory layer [19]. Apart from this, numerous ornamental plantings have been established throughout the Baltic regions using P. mugo for the last few decades [22].
Such a distribution pattern of P. mugo stands strongly favors natural spread of L. acicola in the eastern part of the Polish coastline. The primary dispersal mode of L. acicola is asexual and involves water-splashed conidia [26,27,28]. This is a very short-distance method where infection success diminishes rapidly just 1.5 to 3 m from the source [26], although much longer travels of conidia of 60 m have been demonstrated using spore traps [29]. This short-distance effect is most probably the reason for the greatly reduced BSNB incidence at sites where P. mugo stands became overgrown by other pine species. Such a situation inevitably leads to increased distances between P. mugo individuals and, thus, reduced infection rates. Another factor favoring L. acicola spread in thickets composed mostly of P. mugo is its dwarf habit. Most pine species when matured acquire a degree of resistance to foliar diseases [30,31]. In large part, this is due to higher canopies and greater spacing between trees sustained in older stands. This reduces infection rates from shed needles on the forest floor and allows for lower overall humidity at the canopy level [32]. These effects are less pronounced in dwarf P. mugo. The long-distance natural spread in L. acicola is achieved via wind-disseminated ascospores [27]. Completing the sexual cycle in L. acicola requires both mating types to be present and, indeed, both MAT1-1 and MAT1-2 idiomorphs have been reported to occur in Poland, the Baltics, and most of Europe, however usually not in proportions suggesting frequent sexual recombination [10]. Thus, the potential for natural ascospore-mediated dispersal along the Polish coast exists even between areas with less frequent occurrence of P. mugo, which is its main host in Poland and in northern and central Europe [8].
However, human-mediated dispersal is also a very important factor in L. acicola‘s worldwide expansion [10]. As the planting of P. mugo as a dune stabilization method on the Polish coast has been phased out for the most part, the pathogen’s dispersal with forestry planting material does not seem to be significant. On the contrary, transport of P. mugo seedlings for very popular ornamental plantings poses a greater threat. However, activities related to touristic traffic could also play a significant role in L. acicola dispersal along the Baltic coast in Poland, as the region is visited by millions of tourists each summer season. This spread pathway has been previously suggested for L. acicola in Slovenia [1]. However, the relative importance of touristic-traffic-related factors requires further verification with additional data.
Lecanosticta acicola spread in Europe is relatively well documented [1]. For this study, the most relevant events pertain to the spread of the BSNB pathogen in the Baltic Sea region, where it was detected for the first time in Estonia in 2008 [33]. It was then detected in 2010 on the Curonian Spit, Lithuania [34], in 2012 in the Latvian National Botanical Garden in Salaspils near Riga [35], and finally in 2017 on the Polish coast [14]. However, the exact chronology of its dispersal in the region has not been established. The literature lacks any reports of systematic surveys on pine needle pathogens in P. mugo dune populations in Poland in at least the past two decades. The study of Boroń et al. 2016 [22], focusing on detection of Dothistroma spp. in Poland, does include two coastal sites where P. nigra stands were examined in 2013; however, no P. mugo stands on the coast were visited then. Therefore, it is currently impossible to establish for how long L. acicola was present in northern Poland prior to 2022 [14].
Our results show nearly ubiquitous occurrence of L. acicola and its associated BSNB symptoms in P. mugo stands along the Polish coast of the Baltic Sea, revealing a much worse epidemiological situation than previously recognized based on Raitelaitytė et al.’s [14,36] reports. The pathogen universally occurred in all examined sites throughout all three major areas of high P. mugo concentration. It also occurred in all examined sites located at the base of the Gdańsk Bay. Based on this result, it is safe to assume that virtually all mountain pine-dominated stands throughout this part of the coast are infected by L. acicola. Here, we also observed numerous instances of Scots pine, a species native to Poland, being infected if present in direct proximity to heavily infected P. mugo individuals. These were always relatively young trees (up to ca. 30 years old), and the incidence of BSNB symptoms was always lower compared to mountain pine, but the incidence of symptoms on Scots pine varied significantly between individuals. In this regard, this situation is very similar to that observed for Dothistroma septosporum, whose infection of exotic pine species, including P. mugo, results in much higher disease incidence and usually more severe damage compared to infections of native P. sylvestris [22]. Thus, the main concern resulting from the spread of these pathogens on exotic pines is the risk of emergence of P. sylvestris-adapted genotypes, whose economic impact on Scots pine-dominated forestry in Poland would be potentially orders of magnitude greater.
A similar situation, although not completely the same, occurs in the western part of the Polish coast, where pure or nearly pure P. mugo forest subcompartments are much less frequent. Initially, only two such sites were identified here and both proved to be L. acicola-free in 2023 (Dźwirzyno, winter 2023; Łazy, summer and winter 2023). However, the pathogen was clearly present in the area, detected on ornamentals (Mielno, winter 2023). Already during the summer 2024 survey, the Dźwirzyno site showed clear signs of infection (confirmed later molecularly), as did another newly identified site in the area (Dąbki); however, the Łazy site once again remained L. acicola-free.
Such results suggest the very recent dispersal of L. acicola to the area, as we believe we caught the pathogen’s introduction to Dźwirzyno site within a six-month window between July and December 2023. However, this could also be due to chance as the site is occupied by the youngest P. mugo stand among all identified on the coast. Thus, it is possible that the L. acicola population already established in the area, e.g., on ornamental P. mugo plantings, did not have time to reach this particular site. This question, as well as the broader history of L. acicola dispersal along the Polish Baltic coast, can be relatively easily resolved in a genetic diversity analysis.
This paper does not provide genetic diversity data for the Polish coastal population of L. acicola; thus, we refrain from speculating on the population’s genetic makeup. However, two recent papers, a preliminary investigation of local L. acicola diversity in Lithuania [36] and a much more comprehensive worldwide genetic diversity study [10], do include isolates from a single Polish site. Both these analyses suggest that the Polish coastal population is related to wider eastern Baltic stock, but not to the L. acicola genetic cluster dominating the Curonian Spit. This, of course, suggests that the pathogen spread from the Baltics to the Polish coastal regions, either naturally or via human-mediated transport, intentional or otherwise, of plant material [36]. The alternative explanation, which is that L. acicola dispersal is from southern Poland, where BSNB was previously reported (but without molecular detection of the pathogen), is much less likely, as L. acicola has not been detected in two NGS-based studies of fungi occurring in pine needles in Poland [37,38].
The main limitation of our study is the lack of more detailed disease incidence and genetic diversity data enabling robust statistical interpretation of infection and dispersal trends. The extensive collection of L. acicola isolates generated in this study from multiple sites representing nearly all areas of the Polish coast where P. mugo occurs opens a unique opportunity to track the pathogen’s dispersal using molecular markers and phylogeographic methods. Thus, molecular diversity analysis is the necessary next logical step required to understand the history of L. acicola dispersal in the region. Such data will allow us to verify our working hypothesis assuming that the situation currently observed represents the very initial stages of L. acicola expansion on the Polish coast. The work on such a genetic analysis is currently underway by our team.

5. Conclusions

What is evident from our study is that all areas of major P. mugo concentrations present on the Polish coast of the Baltic Sea are heavily affected by BSNB. The disease is the most damaging in dense mountain pine thickets that produce monoculture conditions. Mixed P. mugo/P. sylvestris stands were less affected; however, under such conditions dwarf mountain pine tends to be outcompeted and gradually declines. A degree of disease spread can be observed among P. sylvestris trees in the coastal dune regions; however, it was always lower, even under heavy inoculum loads, compared to that observed on P. mugo. A potential shift in L. acicola behavior, resulting in increased virulence toward P. sylvestris, would represent a significant threat to Polish forestry in the Baltic region.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/f16121830/s1. Supplementary Figure S1. Representative results of Lecanosticta acicola species-specific PCR assay. Please note the size of the expected target product of ~237 bp. Positive results: 1-3 and 6-10; negative results: 4 and 5; M—DNA ladder, PC—positive control, NC—negative control. Supplementary Table S1. List of all Pinus mugo dominated forest subcompartments identified along the Polish section of the Baltic Sea coastline. Supplementary Table S2. List of confirmed Lecanosticta acicola isolates generated in in the study. All the isolates were identified molecularly using species-specific PCR.

Author Contributions

Conceptualization, P.B. and A.L.-B.; methodology, P.B. and K.B.; formal analysis, P.B. and A.M.; investigation, P.B., K.B., M.K., B.G., A.M. and A.L.-B.; writing—original draft preparation, P.B.; writing—review and editing, K.B., M.K., B.G., A.M. and A.L.-B.; visualization, P.B. and A.M.; supervision, P.B.; funding acquisition, A.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Science and Higher Education of the Republic of Poland (SUB/040013–D019). Field surveys received additional funding under the student activity program of the University of Agriculture in Krakow as internal donations for organization of student scientific camps (Student Research Circle of Foresters, Faculty of Forestry, 2023 and 2024).

Data Availability Statement

All data supporting the findings of this study are available within the paper. Raw data can be made available upon request to either of the corresponding authors.

Acknowledgments

We would like to sincerely thank all the students who participated in scientific camps organized in 2023 and 2024 by the Section of Forest Mycology and Phytopathology, Student Research Circle of Foresters, Faculty of Forestry, University of Agriculture in Kraków, during which most of the field observations were conducted. Without you, this work would not have been so much fun.

Conflicts of Interest

The authors declare no conflicts of interest.

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Forests 16 01830 g001
Figure 1. Locations of surveyed sites along the Polish section of the Baltic Sea coastline; numbers refer to named sites in Table 1. Please note the uneven distribution of P. mugo-dominated stands, the vast majority of which are located within areas outlined in dashed green (see Supplementary Table S1 for more detailed geo-data information).
Figure 1. Locations of surveyed sites along the Polish section of the Baltic Sea coastline; numbers refer to named sites in Table 1. Please note the uneven distribution of P. mugo-dominated stands, the vast majority of which are located within areas outlined in dashed green (see Supplementary Table S1 for more detailed geo-data information).
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Forests 16 01830 g002
Figure 2. Pinus mugo dune forests in the Polish Baltic Sea coastal region and BSNB symptoms. Health condition of trees in 80+-year-old single-species P. mugo stands (a,b), and mixed P. mugo/P. sylvestris stands (c); please note heavy damage caused by L. acicola and readily visible difference in BSNB susceptibility between dwarf mountain pine, 100+ years old (c, white arrows), and Scots pine, 5–15 years old (c, black arrows); (d,e) typical disease symptoms on P. mugo needles in early July, including current-year needles (e); L. acicola isolates on MEA (f). Scale bars in subpanels b and c represent approximate dimensions at the foreground.
Figure 2. Pinus mugo dune forests in the Polish Baltic Sea coastal region and BSNB symptoms. Health condition of trees in 80+-year-old single-species P. mugo stands (a,b), and mixed P. mugo/P. sylvestris stands (c); please note heavy damage caused by L. acicola and readily visible difference in BSNB susceptibility between dwarf mountain pine, 100+ years old (c, white arrows), and Scots pine, 5–15 years old (c, black arrows); (d,e) typical disease symptoms on P. mugo needles in early July, including current-year needles (e); L. acicola isolates on MEA (f). Scale bars in subpanels b and c represent approximate dimensions at the foreground.
Forests 16 01830 g002
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Boroń, P.; Bulanda, K.; Kaźmierczak, M.; Grad, B.; Majewska, A.; Lenart-Boroń, A. The Extent of Lecanosticta acicola Spread Along the Polish Baltic Coastline. Forests 2025, 16, 1830. https://doi.org/10.3390/f16121830

AMA Style

Boroń P, Bulanda K, Kaźmierczak M, Grad B, Majewska A, Lenart-Boroń A. The Extent of Lecanosticta acicola Spread Along the Polish Baltic Coastline. Forests. 2025; 16(12):1830. https://doi.org/10.3390/f16121830

Chicago/Turabian Style

Boroń, Piotr, Klaudia Bulanda, Marzena Kaźmierczak, Bartłomiej Grad, Anna Majewska, and Anna Lenart-Boroń. 2025. "The Extent of Lecanosticta acicola Spread Along the Polish Baltic Coastline" Forests 16, no. 12: 1830. https://doi.org/10.3390/f16121830

APA Style

Boroń, P., Bulanda, K., Kaźmierczak, M., Grad, B., Majewska, A., & Lenart-Boroń, A. (2025). The Extent of Lecanosticta acicola Spread Along the Polish Baltic Coastline. Forests, 16(12), 1830. https://doi.org/10.3390/f16121830

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