A Comparative Study of the Pathogenicity of Fusarium circinatum and other Fusarium Species in Polish Provenances of P . sylvestris L .

The fungal pathogen Fusarium circinatum is the causal agent of Pine Pitch Canker (PPC), a disease which seriously affects different species of pine in forests and nurseries worldwide. In Europe, the fungus affects pines in northern Spain and Portugal, and it has also been detected in France and Italy. Here, we report the findings of the first trial investigating the susceptibility of Polish provenances of Scots pine, Pinus sylvestris L., to infection by F. circinatum. In a greenhouse experiment, 16 Polish provenances of Scots pine were artificially inoculated with F. circinatum and with six other Fusarium species known to infect pine seedlings in nurseries. All pines proved highly susceptible to PPC and displayed different levels of susceptibility to the other Fusarium spp. tested. The findings obtained indicate the potentially strong threat of establishment of an invasive pathogen such as F. circinatum following unintentional introduction into Poland.


Introduction
A substantial increase in the number of newly recognised invasive forest pathogens and of new destructive diseases has recently been noted in forest stands in Europe in the last few decades [1].Among these, the fungal pathogen Fusarium circinatum Nirenberg & O'Donnell may pose a serious threat to the ecological and economic sustainability of forest ecosystems.In addition, climate change is likely to predispose trees to attack from pathogens and will probably encourage the spread of pathogens into new areas [2,3].
The ascomycete fungus Fusarium circinatum infects a wide range of pine Pinus species and can cause pitch pine canker (PPC).Although Pinus radiata D.Don seems to be the most susceptible species [4], up to 60 pine species have been reported to be susceptible to PPC, including Mediterranean species such as Aleppo pine (P.halepensis Miller) and Maritime pine (P.pinaster Aiton), as well as European species such as Scots pine (P.sylvestris L.) and also various North American and Asian species planted in Europe, such as Lodgepole pine (P.contorta Douglas), white pine (P.strobus L.), Japanese red pine (P.densiflora Siebold & Zucc.) and Japanese black pine (P.thunbergii Parl.) [5,6].The pathogen is widely distributed and causes dieback and death of susceptible pine species.Fusarium circinatum was first found in North America, and to date the disease has been reported in Central and South America, South Africa, Asia, USA, Mexico, Haiti, Japan, Chile [7] and, more recently, in Europe.Nowadays, the disease is present in forests in Spain [8] and Portugal [9] and it has also been reported in parts of France and Italy [10,11], although it has probably subsequently eradicated from both of the latter countries [12,13].The disease threatens the sustainability of pine forests in Spain, Portugal, France, Italy and Greece, among other countries.These areas are at risk, as confirmed by climatic data and host distribution [2,3].However, other European countries are also at risk of infection [4][5][6].An increase in imports of live plants and other material has led to a high risk of the pathogen being introduced through infested goods.However, considering the high level of susceptibility of young pines to the disease, and the ease of infection via seeds or seedlings, the threat of F. circinatum to European nurseries is particularly serious.
Scots pine (Pinus sylvestris) is a common and economically important timber species in Europe.The widespread distribution of an alien invasive pathogen such as Fusarium circinatum thus poses a serious threat to the biodiversity of forest ecosystems, especially Scots pine stands.In 2007, the European Commission adopted provisional emergency measures to prevent the introduction and spread of Gibberella circinata Nirenberg and O'Donnell (the sexual stage of the pitch canker pathogen) within the Community (Commission Decision 2007/433/EC of 18 June 2007) [6].The presence of F. circinatum in forest plantations and nurseries has led to large reductions in crop yields as well as loss of revenue due to the high costs of monitoring and control of the pathogen and to bans on planting susceptible species (Pinus spp.and Pseudotsuga menziesii (Mirb.)Franco) in infected areas in Spain and on the export of timber and other products (Spanish Royal Decree 637/2006 and 65/2010).Nurseries in which plants were infected by F. circinatum were typically the first point of entry of the disease and still constitute a reservoir for the transmission of the PPC to forest plantations.Moreover, various Fusarium spp.affect the viability of seeds and young pine plants, causing damping-off disease and resulting in losses due to non-viable seeds and weakening or death of seedlings.Fusarium spp.are reported to play an important role in the development of damping-off disease in plantations and forest nurseries, mainly when seedlings are grown in containers with peat moss, due to the acidic nature of this substrate [14].Fusarium oxysporum is a soil-borne fungus species that can act as a saprophyte as well as an aggressive root and seed pathogen in a wide variety of hosts including Scots pine seedlings and many economically important crops [15][16][17].In Poland, most forest tree nurseries have experienced serious problems with damping-off rot caused by Rhizoctonia and Fusarium spp.[18,19].Some seed-borne Fusarium spp.have been recognized to be aggressive pathogens of pine seedlings and crops.The establishment of pine plantations on former farmland in Poland may lead to infection of pine seeds and seedlings by different Fusarium spp.remaining in the soil.
The aim of this research was to test the pathogenicity of F. circinatum and other Fusarium species in order to evaluate the threat posed by these fungi, which may potentially affect seedlings of Polish provenances of P. sylvestris.
With this aim, we compared the development of pitch canker symptoms in Polish provenances of Scots pine seedlings to establish whether the susceptibility or tolerance of plants to F. circinatum is related to their geographical origin.Moreover, we also assessed the potential effects of six isolates of other Fusarium spp., including species typically affecting agricultural crops.

Fungal Isolates
An inoculum of Fusarium circinatum was produced by growing the fungus on potato dextrose agar (PDA) for seven days in Petri dishes at 25 • C under laboratory conditions.In addition, isolates of Fusarium poae, F. oxysporum, F. graminearum (A and D isolates), F. tricinctum and F. culmorum of Polish origin were selected for comparison of virulence (Table 1).For the in vitro virulence test, we followed the method of James et al. (1997), as briefly described below [21].

In Vitro Virulence Test
Stratified seeds of three different Polish provenances of P. sylvestris (Klosnowo, Lasowice and Dębno, Table 2) were placed inside a mesh bag and soaked in tap water.The seeds were germinated on moistened, sterile Whatman No. 3 filter paper in sterile Petri dishes and incubated at about 22 • C.After 15 days, the pine seedlings were transferred to Petri dishes containing the fungal inocula.Thus, for each fungal isolate, 12 Petri dishes each with two of the seedlings were prepared (i.e., 24 replicates).The Petri dishes were incubated at approximately 23 • C under artificial lighting (160 Watts per m 2 ), and sterile water was added as required.Disease symptoms (root rot, damping-off, and disease free, respectively) were monitored over a 21-day period.After 21 days, all germlings were harvested and examined for disease symptoms before being re-isolating the fungi onto malt extract agar (MEA).Seedling survival was scored as follows: 1 point was awarded for each day the seedlings survived (maximum 15 points); another 1, 2 or 5 points were awarded depending on the type of disease (root rot, damping-off, and disease free, respectively); and 5 addition points were awarded when the root doubled or more in size.The scores ranged from 5-25, with higher values reflecting lower virulence of the isolate.The scores were converted to a reciprocal score of 0-100, with virulence ratings of 0 indicating no fungal infection, and 100 indicating that the seedlings died within three days [21,22].
Based on the findings of previous studies [19,21,22], virulence scores >60 indicated moderate-high virulence, scores between 40 and 59 indicated low virulence, and scores below 39 indicated lack of virulence.

Greenhouse Pathogenicity Tests
Preliminary assessment of the pathogenicity of F. circinatum and other Fusarium species for Polish provenances of pine seedlings was carried out at the Research Centre of Quarantine, Invasive and Genetically Modified Organisms, Institute of Plant Protection-NRI (Poznan, Poland).The F. circinatum and other Fusarium isolates were cultivated on PDA at 24.5 • C for seven days (Table 1).
Two-month-old seedlings of 12 Polish provenances (37-42 seedlings per provenance) and 72 one-year-old seedlings of another Polish provenances of Pinus sylvestris (Jablonna) were used in preliminary pathogenicity tests (Table 2).
Inoculums of F. circinatum were added to pots in which individual two-month-old pine seedlings were growing, by pouring spore suspensions (ca. 1 × 10 6 CFU/mL water) on to the soil (approximately 1 × 10 6 CFU per seedling).
For each of the 7 Fusarium species tested, the one-year-old pine seedlings were inoculated by making a wound on the stem with the aid of a scalpel and pipetted 10 µL of Fusarium spore suspension (1 million spores mL −1 ) onto the wound [20].The wound was then covered with a strip of Parafilm to prevent desiccation.Control seedlings were inoculated in the same way, with sterilized water, and the wounds were also covered with a strip of Parafilm.
The morphological condition of all pine seedlings was first evaluated on a weekly, and then a monthly basis as visually healthy, weakened or dead plants subject to typical symptoms of pitch canker or damping-off.Disease susceptibility was estimated at 60 and 120 days post-inoculation (dpi).
Samples of seedlings, 5 mm length segments, were placed in Eppendorf tubes.Whole samples of the two-month-old seedlings were analyzed, while two segments of length 5 mm were cut from the stem of each one-year-old seedling.The samples were homogenized by cutting them into pieces with scissors and shaking the pieces in tubes with two glass beads.Subsamples (500-700 uL) of all homogenized samples were transferred to 2 mL screw cap tubes.The tubes were then placed overnight in a freeze-drier with the lids loosely screwed on (covered with Parafilm to keep them in place).All samples were analyzed using conventional PCR tests for detection/reisolation of F. circinatum with species-specific primers (CIR1 and CIR4 described below).

DNA Extraction, Purification and PCR Amplification
Fresh or frozen samples were ground to a fine powder with liquid nitrogen in a mortar and pestle before DNA extraction.Ground material (500 to 700 mg) was transferred to a sterile microfuge tube.The DNA was extracted and purified using the NucleoSpin ® Plant II Midi kit (MACHEREY-NAGEL product, Düren, Germany).The DNA of the samples was then quantified along with standard reference samples in a NanoDrop ND-1000 spectrophotometer (Wilmington, DE, USA).The DNA in individual samples was diluted to 5-10 ng/µL.
The diluted DNA samples were analyzed by conventional PCR with species-specific primers to detect the presence/absence of F. circinatum.The specific forward primers CIRC1A (TCG ATG TGT CGT CTC TGG AC) and reverse primers CIRC4A (CGA TCC TCA AAT CGA CCA AGA) were used to amplify the IGS rDNA region [23].The PCR reaction mixture included 1× PCR buffer supplied with the DNA polymerase, 0.25 mM each dNTP, 2 mM MgCl 2 , 0.5 µM of each CIRC1A and CIRC4A primer, 0.05 U/µL DNA polymerase and 6.0 µL of DNA.
The PCR reaction was carried out in a thermocycler Veriti 96 Thermal Cycler (Life Technologies™, Life Technologies™, Camarillo, CA, USA) and involved initial denaturation at 95 • C for 3 min, followed by 40 cycles consisting of denaturation at 95 • C for 30 s, annealing at 64 • C for 55 s and elongation at 72 • C for 50 s.The final elongation step was carried out at 72 • C for 12 min.The PCR products were visualized by electrophoresis in a 1% agarose gel.
For identification of the Fusarium species, the internal transcribed spacer (ITS) region was analyzed.In addition, the β-tubulin gene and the partial elongation factor 1-alpha (EF1-a) gene were amplified and sequenced for fungi from the genera.The β-tubulin gene was amplified using Bt2a and Bt2b primers [24] and the EF1-a gene was amplified using the EF1F and EF2R primers, following the protocols reported by Kristensen et al. (2005) for the identification of Fusarium spp.[25].
For PCR amplification of the ITS regions of fungal ribosomal RNA (ITS rRNA), the primers ITS1F [26] and ITS4 [27] were used.Each PCR reaction contained 200 µM deoxyribonucleotide triphosphates, 0.2 µM of each primer, 0.03 U/µL Thermo Green Taq polymerase with reaction buffer, and 2.75 mM MgCl 2 (final concentration).The initial denaturation step at 95 • C for 5 min was followed by 35 amplification cycles consisting of denaturation at 95 • C for 30 s, annealing at 55 • C for 30 s, and extension at 72 • C for 30 s.The thermal cycling was ended by a final extension step at 72 • C for 7 min.PCR products were size separated on 1% agarose gels and visualized under UV light.PCR conditions for EF1-a amplification were the same as those for ITS except for the annealing temperature, which was 60 • C. The thermal cycling condition for the β-tubulin gene was 2 min at 95 • C followed by 35 cycles of 95 • C for 30 s, 58 • C for 45 s, and 72 • C for 45 s, with a final extension of 72 • C for 5 min.
The raw sequence data were analyzed using the SeqMan Pro version 10.0 software included in the DNASTAR package (DNASTAR, Madison, WI, USA).The identity of the ITS rRNA sequences was determined by comparison with GenBank databases [28].The following criteria were used for the identification: sequence coverage >80%; similarity to taxon level 98%-100%; similarity to genus level 92%-97%.

Statistical Analysis
All data were analyzed using Kolmogorov-Smirnov test to check for adherence to the normal distribution and Bartlett's test to check the homogeneity of variance.Data on lesion length of Scots pine seedlings inoculated with F. circinatum or other Fusarium species were subjected to analysis of variance (ANOVA) by using the general linear model (GLM) module in Statistica software STATISTICA ® 7.0 (StatSoft, Inc., Tulsa, OK, USA).When the ANOVA indicated significant treatment effects (p < 0.05), a post hoc HSD Tukey test was used to compare the means.For all fungal isolates, Chi-square tests were used to determine any differences in plant mortality.Means were compared using a post hoc HSD Tukey test with a significance level of p < 0.05.Confidence intervals were determined to distinguish between treatments representing different seedling provenances and Fusarium species.Following the ANOVA, significant differences between treatments were further evaluated by a post hoc HSD Tukey test.Survival analysis was performed to test the mortality until the end of the experiment by using the nonparametric Kaplan-Meier estimator with the "Survival" package [29] implemented in an R software environment (R Foundation for Statistical Computing, Vienna, Austria).

Effect of Fusarium Species on Pine Seedling
The growth of pine seedlings inoculated with different species of Fusarium varied considerably (Table 3).Fusarium circinatum killed almost all the pine seedlings in this assay, but all other isolates of Fusarium tested, i.e., F. poae, F. graminearum (A and D), F. tricinctum and F. culmorum, had little impact on seedling health (Table 3).Only F. oxysporum strongly affected the plant development, resulting in mortality of 62.5%-66.7% of the seedlings by day 21.
Both the mortality rate and root growth inhibition were almost equal in all provenances of Scots pine seedlings tested (F = 2.6356, p = 0.104 and F = 1.8089, p = 0.5291).F. oxysporum caused the highest mortality rates (62.5%-66.7%,F = 9.3473, p = 0.001053)), while F. poae, F. graminearum (A and D) and for F. culmorum caused low morality rates (0%-4.2%).F. tricinctum caused intermediate mortality rates of 12.5%-20.8%.F. oxysporum also significantly reduced plant growth (32% of plants weakened).No dead or weakened seedlings were observed in the control treatments.In other variants the mortality did not exceed 5% and the number of weakened seedlings varied from 0% to 4%, except for F. tricinctum (6.1%-8.3% of seedlings weakened and mortality rate, 12.7%-20.5%).Root growth was strongly inhibited by F. oxysporum (79.4%-88.7%)extensive vascular discoloration of the root tissue and affecting seedling viability (p = 0.03 for root growth inhibition and p = 0.00124 for mortality).F. tricinctum also inhibited root growth (58.2%-64.5%,p-value = 0.0152), while Fusarium poae, F. graminearum (A and D) and F. culmorum caused low levels of root growth inhibition of respectively 14.4%-16.2%,17.8%-19.8%,18.5%-20.5% and 18.5%-20.4%(Table 3), The virulence scores for Fusarium poae, F. graminearum (A and D) F. tricinctum and F. culmorum were on average, 11.3, 13.3 and 11.7, 11.3 and 33.7 (data not shown, rate from 0 to 100) indicating that these species are non-pathogenic.Although F. culmorum was awarded significantly higher scores (p-value = 0.014), this isolate may also be included in the group of isolates displaying low virulence for pine seedlings.Only F. oxysporum was awarded high scores for all three provenances of seedlings tested (average score 52) and can be considered moderately-highly virulent.Among tested Fusarium spp., Fusarium circinatum is the highest level mortality for pine seedlings and is the most virulent pathogen.

Greenhouse Pathogenicity Tests
Artificial inoculation of seedlings with F. circinatum led to the formation of necrotic lesions, resin flow of and mortality of 90% of one-year-old seedlings of one of the Scots pine provenances from Poland (Jablonna) (Table 4) during the fourth month of the assay.During the first two months of the trial, small necrotic lesions of 1.6-2.8mm were observed in 30% of the seedlings, with no clear symptoms of pitch canker, while necrotic lesions 5.3-6.7 mm in length were observed in 40% of seedlings, which also showed typical symptoms of disease such as basal needle dieback, wilting and side shoot dieback.The longest necrotic lesions (8.4-8.7 mm) were observed in 30% of dead seedlings two months after inoculation.The lesions were generally covered with resin and extended vertically in both directions from the point of inoculation.Dead and declining seedlings showed symptoms of stem necrosis, discoloration and loss of needles.No dieback or decline was observed in the control seedlings.
Inoculation of seedlings with the other six Fusarium spp.commonly led to the formation of small necrotic wounds, which did not differ significantly in size from those observed in the control seedlings (Table 4).Only F. oxysporum demonstrated very low virulence causing discoloration of needles around the inoculation point; the other Fusarium species did not cause any symptoms of weakening, shoot decline or seedling mortality (Table 4).Moreover, there were no significant differences in the size of the lesion or seedling condition in relation to the six Fusarium spp.One-year-old seedlings inoculated with the Fusarium species other than F. circinatum showed no symptoms of weakening or damping off (p < 0.01).
The inoculated Fusarium spp.(including F. circinatum) were successfully re-isolated from 100% of the plants.The identity of samples was 99%-100% consistent with F. circinatum strain FCC4880 28S-18S, GenBank: KC147556.1.All re-isolated fungal species and wood samples from the inoculated and control seedlings used for direct amplification and sequencing of the fungal IGS rDNA region yielded successful amplification.The presence of F. circinatum was only confirmed in the seedlings that had been inoculated with this pathogen, and it was not detected in the control or the other groups of seedlings.
Two-month-old infected pine seedlings showed typical symptoms of damping-off, with up to 32% mortality (depending on the provenance considered) in the two months after inoculation.Survival analysis demonstrated the susceptibility of P. sylvestris to F. circinatum over a period of four months (Figure 1).During the first two months of the trial, small necrotic lesions of 1.6-2.8mm were observed in 30% of the seedlings, with no clear symptoms of pitch canker, while necrotic lesions 5.3-6.7 mm in length were observed in 40% of seedlings, which also showed typical symptoms of disease such as basal needle dieback, wilting and side shoot dieback.The longest necrotic lesions (8.4-8.7 mm) were observed in 30% of dead seedlings two months after inoculation.The lesions were generally covered with resin and extended vertically in both directions from the point of inoculation.Dead and declining seedlings showed symptoms of stem necrosis, discoloration and loss of needles.No dieback or decline was observed in the control seedlings.
Inoculation of seedlings with the other six Fusarium spp.commonly led to the formation of small necrotic wounds, which did not differ significantly in size from those observed in the control seedlings (Table 4).Only F. oxysporum demonstrated very low virulence causing discoloration of needles around the inoculation point; the other Fusarium species did not cause any symptoms of weakening, shoot decline or seedling mortality (Table 4).Moreover, there were no significant differences in the size of the lesion or seedling condition in relation to the six Fusarium spp.One-year-old seedlings inoculated with the Fusarium species other than F. circinatum showed no symptoms of weakening or damping off (p < 0.01).
The inoculated Fusarium spp.(including F. circinatum) were successfully re-isolated from 100% of the plants.The identity of samples was 99-100% consistent with F. circinatum strain FCC4880 28S-18S, GenBank: KC147556.1.All re-isolated fungal species and wood samples from the inoculated and control seedlings used for direct amplification and sequencing of the fungal IGS rDNA region yielded successful amplification.The presence of F. circinatum was only confirmed in the seedlings that had been inoculated with this pathogen, and it was not detected in the control or the other groups of seedlings.
Two-month-old infected pine seedlings showed typical symptoms of damping-off, with up to 32% mortality (depending on the provenance considered) in the two months after inoculation.Survival analysis demonstrated the susceptibility of P. sylvestris to F. circinatum over a period of four months (Figure 1).The final inspection of the 12 provenances, four months after inoculation, generally revealed severe plant mortality (up to 100%) in ten provenances, while two provenances (Kwidzyń and Czaplinek) displayed a high survival rate (viability rating from 81.8-86.3%)(Figure 1).Cumulative The final inspection of the 12 provenances, four months after inoculation, generally revealed severe plant mortality (up to 100%) in ten provenances, while two provenances (Kwidzy ń and Czaplinek) displayed a high survival rate (viability rating from 81.8%-86.3%)(Figure 1).Cumulative proportions of survival, compared using the Kaplan-Meier estimate, differed significantly between provenances (F = 4.4445, p-value = 0.0027).Survival of control seedlings was 100% at the end of the experiment and was significantly higher than the mean survival of all the inoculated groups (Figure 1).The mortality rate of seedlings of the two provenances with the highest survival rates was significantly higher than in the control group (p = 0.000179) and the other ten provenances (p = 0.00002).

Preliminary Assessment of Pathogenicity of Fusarium Circinatum to Polish Pine Provenances
Previous studies carried out in Europe, the US and Asia have shown considerable differences in the susceptibility of different pine species to F. circinatum [30][31][32].Most studies have reported that seedlings and older plants of P. sylvestris are moderately to highly susceptible to the pathogen [33][34][35].Some research conducted in infested nurseries [7] revealed that the pathogen was only isolated from P. radiata and P. pinaster, while Pinus nigra J.F. Arnold, P. sylvestris and Pseudotsuga menziesii seedlings did not show any symptoms of pitch canker.The results of the present study indicated that if F. circinatum appears in Poland, it will be an important pathogen of P. sylvestris and could affect and kill pine seedlings within a short period of time.Studies of this type generally rely on inoculation data from greenhouse or field results for the rapid, accurate assessment of the risk the pathogen poses a serious threat to plants.
Previous studies [36,37] on the resistance of Monterey pine (Pinus radiata) and bishop pine (Pinus muricata D.Don) to F. circinatum showed significant differences in the length of lesions between trees.In the present study, inoculation of one-year-old seedlings in greenhouse trials led to formation of lesions of different lengths.After two months, 30% of seedlings formed lesions shorter than 5 mm, and 40% of symptomatic seedlings had necrotic lesions longer than 5 mm, and the longest lesions were observed in the dead seedlings (data not shown).The inoculations were performed under controlled conditions in a greenhouse to prevent the influence of stress factors such as drought/flood, insect pests or other pathogens on the results.Seedlings with the smallest lesions were considered more or less tolerant to the pathogen at this stage.However, some recent studies have revealed that F. circinatum may occur in a latent form [38], suggesting that PPC symptoms may develop up until death of the seedlings.
During the four-month trial period, only 10% of the trees that survived had lesions shorter than 5 mm.The scores for seedling tolerance were therefore similar during the initial period after inoculation.Four months after inoculation, the mean lesion length did not differ significantly between individual seedlings.The comparison showed that F. circinatum caused significantly longer necrotic lesions than the inoculated seedlings in the control group (p < 0.05).
By contrast, during the greenhouse trial, significant differences were observed in the susceptibility of two-month-old seedlings of 12 Polish provenances of Scots pine to F. circinatum.The absence of symptoms on the seedlings of the Kwidzy ń and Czaplinek provenances during the entire study period (compared with the other ten provenances, which were severely affected) indicates that selection for genetic resistance to F. circinatum may be conducted to improve resistance to pine pitch canker [39].Assessment of the development of the symptoms in the Polish provenances of P. sylvestris show that all except the Kwidzy ń and Czaplinek provenances are highly susceptible to pitch canker.This may be because the plants were too young to develop effective physiological mechanisms of plant resistance.Genetic variability in populations of P. sylvestris growing in eight locations Poland has been described on the basis of cone morphology [40].The populations were found to form two geographic groups, in the northeastern and southwestern areas [40].However, no differences in the genetic variability of Polish provenances of Scots pine have been identified in relation to several genetic markers [41].The variation in the susceptibility to pitch canker observed in the present study may drive the implementation of genetic breeding programs aimed at management of this disease in the future.However, the significant differences in the tolerance of two-month-old seedlings of 12 Polish provenances to F. circinatum and the high viability of seedlings observed in the four months after inoculation suggest that further studies should first be conducted to obtain conclusive results.Survival analysis of live seedlings was similar to that conducted with live trees and may distort the results [42], with delayed mortality of plants being interpreted as tolerance to the pathogen.Seedlings that survived after inoculation with F. circinatum were excluded from the Kaplan-Meier estimate as the time to death was not known [42].However, some studies demonstrate that infection by F. circinatum can occur during the latent phase.Moreover, latent infection influences the susceptibility of Monterey pine seedlings to pitch canker [43]; further observation of inoculated seedlings over a longer period is planned for a more detailed evaluation of the susceptibility of seedlings to pitch canker.
Nevertheless, the high level of susceptibility of P. sylvestris indicates a potential risk to this important tree species in the event that F. circinatum is introduced into Poland, at least in nursery facilities where F. circinatum could spread worldwide.

Virulence of Other Fusarium Species
Damping off is the most common and serious disease affecting seedlings in forest nurseries in many countries.Many species of Fusarium are found even in very poor and degraded soil, resulting in financial losses, mostly in nurseries and forest plantations.Fusarium spp.are commonly found to affect the roots of coniferous seedlings, causing damping-off and wilt in pine seedlings [44,45].Our findings demonstrate that all of the Fusarium species tested successfully infected pine seedlings.All six Fusarium species caused damping-off disease or weakening in 15-day-old pine seedlings.F. oxysporum is the most common pathogen in Polish forest nurseries and was found to be the most virulent to seedlings.F. tricinctum displayed lower virulence than F. oxysporum but much higher virulence than the other Fusarium species.The findings are consistent with those of other studies Polish nurseries where serious problems exist with damping-off rot caused by the Fusarium species [17,18].Apart from F. circinatum, the Fusarium species tested are better known as pathogens of agricultural crops; Fusarium poae, F. graminearum (A and D) and F. culmorum were particularly less virulent to P. sylvestris seedlings than F. oxysporum and F. tricinctum, which displayed much more variable levels of virulence.These species may be the first potential invaders to transfer from agricultural to forest lands.However, the greenhouse trial and measurement of the mean length of lesions demonstrated that one-year-old seedlings of P. sylvestris were not significantly susceptible to the Polish isolates of Fusarium spp.tested.No symptoms of damping-off or root necrosis were observed during the 4-month trial period.These results suggest that one-year-old seedlings of P. sylvestris appeared to be resistant to F. oxysporum, F. tricinctum, F. poae, F. graminearum (A and D) and F. culmorum.In a previous study, preliminary assessment of the pathogenicity of pine seedlings in Petri dishes proved to be an effective method of determining the pathogenic capacity of Fusarium species prior to greenhouse trials [45].As expected, the fifteen-day-old pine seedlings were more sensitive to the most of Fusarium species tested than one-year-old pine seedlings, in which root growth inhibition was less important.
The study findings demonstrated that F. oxysporum may act as a serious root rot pathogen in pine seedlings, while the virulence of F. tricinctum is moderate or low.At the same time, all the Fusarium species tested did not appear to pose an important risk to one-year-old pine seedlings, which is good news for forest nursery owners.These findings may be of interest to private and state owners of agricultural land destined for afforestation, as such land may be colonized by infectious seed-borne Fusarium, originating from agricultural crops.The study findings also show that the risk of pine seedlings planted in former farmland in Poland being infected with Fusarium spp. is rather low, despite the presence in the soil of Fusarium species known to affect agricultural crops.
The study findings indicate that Polish provenances of Scots pine seedlings are susceptible to damping-off caused by F. circinatum and an invasion of this pathogen represents a serious threat to Polish forestry.

Conclusions
The results of the present study indicate that if the fungus F. circinatum reaches Scots pine in Poland nurseries, it will be very damaging and probably kill pine seedlings quite rapidly.Greenhouse trials involving inoculation of two-month-old and one-year-old seedlings demonstrated the high susceptibility of Scots pine seedlings to pitch canker disease, although two provenances showed signs of higher tolerance.This illustrates the potential usefulness of investigating the genetic resistance of individual clones to F. circinatum (for potential breeding programs) to enable control of this disease if it becomes established in forest ecosystems.
The pine shoot beetle (Tomicus piniperda) and other bark beetles are considered potential vectors of F. circinatum as they could transmit the pathogen during maturation feeding on the shoots of healthy pine trees [46,47].As T. piniperda is one of the most common bark beetles in Poland, it could increase the potential risk of spread of the pathogen and disease throughout the country.
The correlation between the susceptibility of provenances and individuals to F. circinatum and possibility of the pathogen being transferred by bark beetles may require studies in field trees and nursery plants [8,40,47].However, field research on F. circinatum in Poland is restricted by quarantine measures and therefore experiments are generally limited to greenhouses in the Research Centre of Quarantine Organisms.Nonetheless, further studies will be conducted with inoculated and non-inoculated seedlings to determine which genotypes are most resistant to pitch canker by focusing on individual resistance of seedlings.
Moreover, our findings indicate that other species of Fusarium (F.poae, F. graminearum and F. culmorum) are significantly less virulent to P. sylvestris seedlings than F. oxysporum and F. tricinctum.However, only F. oxysporum had moderate virulence, while F. poae, F. graminearum and F. culmorum proved to be weakly virulent or non-virulent.
In summary, the present study confirms the importance of carrying out susceptibility tests with Polish provenances of Scots pine with the aim of facilitating a breeding program for this tree species, as well as the need to explore the individual resistance of pine genotypes in preparation to deal with the problem of the probable appearance and establishment of Fusarium circinatum in Poland.

Figure 1 .
Figure 1.Plot of survival probability determined using the Kaplan-Meier test estimates the survival function for 12 provenances of Pinus sylvestris seedlings infected with Fusarium circinatum.

Figure 1 .
Figure 1.Plot of survival probability determined using the Kaplan-Meier test estimates the survival function for 12 provenances of Pinus sylvestris seedlings infected with Fusarium circinatum.

Table 1 .
Origin of isolates of Fusarium species used in the study.

Table 3 .
Effects of different Fusarium isolates on Pinus sylvestris growth and root development.
With columns, values followed by different letters are significantly different (HSD Tukey post hoc test, α = 0.05).

Table 4 .
Level of mortality and lesion length (mm) in one-year-old P. sylvestris seedlings inoculated with different species of Fusarium.
With columns, variables followed by the same letter are not significantly different (p = 0.05: post hoc HSD Tukey test).