Airborne Ascospores Research Articles

Predicting airborne ascospores of Sclerotinia sclerotiorum through machine learning and statistical methods

AbstractA main biological constraint of dry bean (Phaseolus vulgaris) production in Canada is white mould, caused by the fungal pathogen Sclerotinia sclerotiorum. The primary infectious propagules of S. sclerotiorum are airborne ascospores and monitoring the air for inoculum levels could help predict the severity of white mould in bean fields. Daily air samples were collected in commercial dry bean fields in Alberta, Manitoba and Ontario and ascospores were quantified using quantitative PCR. Daily weather data was obtained from in‐field weather stations. The number of ascospores on a given day was modelled using 63 different environmental variables and several modelling methods, both regression and classification approaches, were implemented with machine learning (ML) (random forests, logistic regression and support vector machines) and statistical (generalized linear models) approaches. Across all years and provinces, ascospores were most highly correlated with ascospore release from the previous day (r ranged from 0.15 to 0.6). This variable was also the only variable included in all models and had the greatest weight in all models. Models without this variable had much poorer performance than those with it. Correlations of ascospores with other environmental variables varied by province and sometimes by year. A comparison of ML and statistical models revealed that they both performed similarly, but that the statistical models were easier to interpret. However, the precise relationship between airborne ascospore levels and in‐field disease severity remains unclear, and spore sampling methods will require further development before they can be deployed as a disease management tool.

Modeling the Airborne Inoculum of Polystigma amygdalinum to Optimize Fungicide Programs Against Almond Red Leaf Blotch.

Red leaf blotch (RLB) of almond, caused by the ascomycete Polystigma amygdalinum, is a severe foliar disease endemic in the Mediterranean Basin and Middle East. Airborne ascospores of P. amygdalinum were monitored from 2019 to 2021 in two almond orchards in Lleida, Spain, and a Bayesian beta regression was used to model its seasonal dynamics. The selected model incorporated accumulated degree-days (ADD), ADD considering both vapor pressure deficit and rainfall as fixed effects, and a random effect for the year and location. The performance of the model was evaluated in 2022 to optimize RLB fungicide programs by comparing the use of model predictions and action thresholds with the standard program. Two variants were additionally considered in each program to set the frequency between applications, based on (i) a fixed frequency of 21 days or (ii) specific meteorological criteria (spraying within 7 days after rainfalls greater than 10 mm, with daily mean temperatures between 10 and 20°C, and with a minimum frequency of 21 days between applications). Programs were evaluated in terms of RLB incidence and number of applications. The program based on the model with periodic fungicide applications was similarly effective as the standard program, resulting only in a 2.6% higher RLB incidence but with fewer applications (three to four, compared with seven in the standard program). When setting the frequency between applications by using the meteorological criteria, a higher reduction in the number of applications (two to three) was observed, while RLB incidence increased by roughly 16% in both programs. Therefore, the model developed in this study may represent a valuable tool toward a more sustainable fungicide schedule for the control of almond RLB in northeast Spain.

Airborne inoculum dynamics of Polystigma amygdalinum and progression of almond red leaf blotch disease in Catalonia, NE Spain

AbstractThe dynamics of airborne ascospores and disease progress of red leaf blotch (RLB) of almond, caused by Polystigma amygdalinum, and their correlations with weather variables were studied from 2019 to 2021 in two almond orchards located in Lleida, NE Spain. Airborne ascospores were detected and quantified by real‐time qPCR using species‐specific primers for P. amygdalinum. Ascospores were detected mainly from April to June, with a high variability between the yearly cumulative concentrations. Positive significant correlations were found between the weekly proportion of airborne ascospores and the number of wet and mild days—either combined or separated— accumulated rainfall, number of rainy days, accumulated low temperatures on wet days, and mean and maximum relative humidity. In contrast, several thermal variables (maximum temperature, VPD, and number of warm days) were negatively correlated with ascospore catches. Positive significant correlations were found between the cumulative proportion of ascospores and RLB incidence and severity. Weekly variations in RLB incidence and severity showed significant positive correlations with the number of warm days while negative with the number of mild days. Severity was also positively correlated with several thermal variables (mean, maximum, and minimum temperature, and VPD), and negatively correlated with the number of cold days and wet and mild days. Stronger correlations were generally found with ascospore catches or disease progress when using concurrent weekly weather data. Gompertz, monomolecular, and logistic growth models were evaluated to describe RLB disease progress.

Inoculum and disease dynamics of citrus greasy spot caused by Zasmidium citri‐griseum in sweet orange in Panama

AbstractCitrus greasy spot, caused by Zasmidium citri‐griseum, is a disease characterized by inducing premature defoliation and a reduction in yield in different citrus species. Greasy spot is the most prevalent fungal disease in sweet orange in Panama. Nevertheless, no epidemiological information is available. In this study, the dynamics of the defoliation, inoculum production, airborne inoculum and infection periods of Z. citri‐griseum and their associations with environmental conditions were determined in Panama. The period from December to April was characterized by greater defoliation of trees, with the greatest amount of leaf litter being produced in January and February. The number of days until total leaf decomposition (DLD) was related to the number of rainy days >1 mm (NRD), accumulated rainfall (AR) and average relative humidity (ARH). The number of ascospores released from leaf litter (ASCL) was related to DLD, NRD, AR and average temperature (AT). The greatest amounts of airborne ascospores (AASC) of Z. citri‐griseum occurred during April and May, when the rainy season begins in Panama. Similarly, the highest incidence (INC) of greasy spot in the trap plants coincided with the months of the greatest availability of airborne ascospores. However, infections were also recorded during other times of the year. The AASC or INC data were fitted to Bayesian models including meteorological variables and an autoregressive temporal component, the latter being the most influential. The results obtained in this study will allow the development of more efficient and sustainable fungicide programmes for greasy spot control in Panama.

Sclerotinia species in Serbia and possibilities of their control

Sclerotinia species are economically important, necrotrophic and aggressive plant pathogens with a broad host range and worldwide distribution. They act as airborne or soilborne pathogens, and can be transmitted by seed. These pathogens can affect crops both during the growing season and after harvest. Yield losses due to Sclerotinia diseases in susceptible crops vary and may be as high as 100%. The most common pathogen from the genus Sclerotinia in Serbia is S. sclerotiorum. It occurs regularly on sunflower and its incidence may exceed 50% in some years, thus causing economically important crop losses in Vojvodina. Recently, two new species were detected in Serbia: S. trifolium in alfalfa and S. minor in lettuce plants. Diseases caused by Sclerotinia spp. are difficult to control due to the long-term survival of sclerotia in the soil and development of airborne ascospores. As with many other diseases, there is no single treatment that can completely control these pathogens. Implementation of multiple strategies, such as cultural practices (sanitation, crop rotation and tillage), physical, chemical and biological protection, as well as deployment of resistant cultivars, is necessary for effective disease management.

Indirect Evidence Based on Mating-Type Ratios for the Role of Sexual Reproduction in European and Chinese Populations of Plenodomus biglobosus (Blackleg of Oilseed Rape)

Blackleg (Phoma) disease, caused by the ascomycete fungi Plenodomus biglobosus and P. lingam, threatens oilseed rape (OSR; Brassica napus) crops internationally. In many parts of the world, both species co-occur, but in China only P. biglobosus has so far been reported. Plenodomus biglobosus reproduces asexually (pycnidiospores), but also sexually (pseudothecia-yielding ascospores), via a heterothallic mating system requiring MAT1-1 and MAT1-2 genotypes. However, the roles of airborne ascospore inoculum in driving blackleg disease outbreaks in China are less well understood compared to elsewhere in the world. This is despite the very different agronomic cropping practices in parts of China, in which paddy rice and OSR are often grown in rotation; OSR stubble is often submerged under water for long periods potentially affecting pseudothecial development. Here, we indirectly investigate the potential role of sexual reproduction by developing new polymerase chain reaction (PCR) -based mating-type diagnostics for P. biglobosus and subsequently screening an international collection of 59 European and 157 Chinese isolates. Overall, in both Europe and China, P. biglobosus mating types did not deviate from a 1:1 ratio, such as is generally thought to occur under frequency-dependent selection in sexually reproducing pathogen populations. Both mating types were balanced in all the individual European countries tested (Austria, France, Poland, UK). Conversely, in China, mating types were only balanced in the eastern region; in the northern and southwestern regions there were skewed ratios, more typical of predominantly asexual reproduction, towards MAT1-1 and MAT1-2, respectively. The implications of these findings and future research directions for improved understanding of P. biglobosus epidemiology on OSR, particularly in China, are considered.

Multiple Species of Asteraceae Plants Are Susceptible to Root Infection by the Necrotrophic Fungal Pathogen Sclerotinia sclerotiorum.

The necrotrophic fungal pathogen Sclerotinia sclerotiorum can cause disease on numerous plant species, including many important crops. Most S. sclerotiorum-incited diseases of crop plants are initiated by airborne ascospores produced when fungal sclerotia germinate to form spore-bearing apothecia. However, basal stalk rot of sunflower occurs when S. sclerotiorum sclerotia germinate to form mycelia within the soil, which subsequently invade sunflower roots. To determine whether other plant species in the Asteraceae family are susceptible to root infection by S. sclerotiorum, cultivated sunflower (Helianthus annuus L.) and seven other Asteraceae species were evaluated for S. sclerotiorum root infection by inoculation with either sclerotia or mycelial inoculum. Additionally, root susceptibility of sunflower was compared with that of dry edible bean and canola, two plant species susceptible to S. sclerotiorum but not known to display root-initiated infections. Results indicated that multiple Asteraceae family plants are susceptible to S. sclerotiorum root infection after inoculation with either sclerotia or mycelium. These observations expand the range of plant hosts susceptible to S. sclerotiorum root infection, elucidate differences in root inoculation methodology, and emphasize the importance of soilborne infection to Asteraceae crop and weed species.

Quantitative Trait Locus Mapping for Resistance Against Pyrenopeziza brassicae Derived From a Brassica napus Secondary Gene Pool.

Use of host resistance is the most economical and environmentally safe way to control light leaf spot disease of oilseed rape (Brassica napus). The causal organism of light leaf spot, Pyrenopeziza brassicae, is one of the most economically damaging pathogens of oilseed rape in the United Kingdom and it is considered to have a high potential to evolve due to its mixed reproduction system and airborne ascospores. This necessitates diverse sources of host resistance, which are inadequate at present to minimize yield losses caused by this disease. To address this, we screened a doubled haploid (DH) population of oilseed rape, derived from a secondary gene pool (ancestral genomes) of B. napus for the introgression of resistance against P. brassicae. DH lines were phenotyped using controlled-environment and glasshouse experiments with P. brassicae populations obtained from three different geographic locations in the United Kingdom. Selected DH lines with different levels of resistance were further studied in a controlled-environment experiment using both visual (scanning electron microscope – SEM) and molecular (quantitative PCR) assessment methods to understand the mode/s of host resistance. There was a clear phenotypic variation for resistance against P. brassicae in this DH population. Quantitative trait locus (QTL) analysis identified four QTLs with moderate to large effects, which were located on linkage groups C1, C6, and C9. Of these, the QTL on the linkage group C1 appeared to have a major effect on limiting P. brassicae asexual sporulation. Study of the sub-cuticular growth phase of P. brassicae using qPCR and SEM showed that the pathogen was able to infect and colonise both resistant and susceptible Q DH lines and control B. napus cultivars. However, the rate of increase of pathogen biomass was significantly smaller in resistant lines, suggesting that the resistance segregating in this DH population limits colonisation/sporulation by the pathogen rather than eliminating the pathogen. Resistance QTLs identified in this study provide a useful resource for breeding cultivar resistance for effective control of light leaf spot and form a starting point for functional identification of the genes controlling resistance against P. brassicae that can contribute to our knowledge on mechanisms of partial resistance of crops against pathogens.

Airborne ascospore discharge with co-dispersal of attached epihymenial algae in some foliicolous lichens.

PremiseLichen‐forming fungi that colonize leaf surfaces must find a compatible algal symbiont, establish lichen symbiosis, and reproduce within the limited life span of their substratum. Many produce specialized asexual propagules that appear to be dispersed by rain and runoff currents, but less is known about dispersal of their meiotic ascospores. In some taxa, a layer of algal symbionts covers the hymenial surface of the apothecia, where asci discharge their ascospores. We examined the untested hypothesis that their ascospores are ejected into air currents and carry with them algal symbionts from the epihymenial layer for subsequent lichenization.MethodsLeaves bearing the lichens Calopadia puiggarii, Sporopodium marginatum (Pilocarpaceae), and Gyalectidium viride (Gomphillaceae) were collected in southern Florida. The latter two species have epihymenial algal layers. Leaf fragments with apotheciate thalli were affixed in petri dishes, with glass cover slips attached inside the lid over the thalli. Subsequent discharge of ascospores and any co‐dispersed algae was evaluated with light microscopy.ResultsAll three species discharged ascospores aerially. Discharged ascospores were frequently surrounded by a halo‐like sheath of transparent material. In the two species with an epihymenial algal layer, most dispersing ascospores (>90%) co‐transported algal cells attached to the spore sheath or wall.ConclusionsWhile water may be the usual vector for their asexual propagules, foliicolous lichen‐forming fungi make use of air currents to disperse their ascospores. The epihymenial algal layer represents an adaptation for efficient co‐dispersal of the algal symbiont with the next genetic generation of the fungus.

Dynamics of the ascospore dispersal of Stagonosporopsis citrulli, a causal agent of gummy stem blight of cucurbits

AbstractSexually reproduced, airborne ascospores of Stagonosporopsis citrulli may play a role in its dispersal. S. citrulli causes gummy stem blight (GSB), one of the most important foliar diseases of cucurbits. Four studies were conducted with S. citrulli to investigate for how long ascospores are released and how far they can be dispersed from a source field. In the first study, colonized watermelon debris was sampled during three seasons and samples were assayed for ascospore release. Ascospores were detected 292, 313, and 306 days after inoculation of the source. In the second study, the active release of ascospores from pseudothecia in a Petri dish was monitored for 7 days. The release of ascospores decreased by ≤90% from 1 day after the start of the assay until 7 days after. In the third study, trap plant assays were conducted to measure the dispersal gradient of ascospores up to 366 m from the source. Generally, frequency of pathogen recovery from trap plants decreased with increasing distance from the source. The ascospore dispersal data fitted the exponential model better than the power law model. In the final study, dispersal experiments were conducted under controlled conditions. The incidence of GSB decreased with increasing distance, up to 55 m, from the source. It was concluded that ascospores of S. citrulli can serve as primary inoculum for epidemics and could easily spread among fields. Debris from cucurbit crops can be the source of ascospores for up to 10 months and should be cleared expeditiously.

The Role of Ascospores and Conidia, in Relation to Weather Variables, in the Epidemiology of Stemphylium Leaf Blight of Onion.

Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, is an important foliar disease of onion in northeastern North America. The pathogen produces conidia and ascospores, but the relative contributions of these spore types to epidemics in onion is not known. Determining the abundance of ascospores and conidia during the growing season could contribute to a disease forecasting model. Airborne ascospores and conidia of S. vesicarium were trapped during the growing season of 2015 and 2016 at an onion trial in southern Ontario, Canada, using a Burkard 7-day volumetric sampler. Meteorological data were recorded hourly. Ascospore numbers peaked before the crop was planted and declined rapidly with time and at daily mean air temperatures >15°C. Conidia were present throughout the growing season and appear to be closely related to the development of SLB on onion. Daily spore concentrations were variable, but 59 to 73% of ascospores and approximately 60% of conidia were captured between 0600 and 1200 h. Spore concentrations increased 24 to 72 h after rainfall and precipitation and leaf wetness duration were consistently and positively associated with increases in numbers of conidia and subsequent SLB incidence. The first symptoms of SLB coincided with high numbers of conidia, rainfall, leaf wetness duration ≥8 h, and days with average daily temperature ≥18°C. The number of airborne ascospores was very low by the time SLB symptoms were observed. Ascospores may initiate infection on alternative hosts in early spring, while conidia are the most important inoculum for the epidemic on onions.

Low-Cost Spore Traps: An Efficient Tool to Manage Fusarium Head Blight through Improved Cropping Systems

Fusarium head blight (FHB) is a devastating disease of wheat. Worldwide, Fusarium graminearum is the most dominant FHB-causing species. Its most common toxin, deoxynivalenol (DON), impairs food and feed safety and has an enormous economic impact. Agronomic factors such as crop rotation, soil management and host genotype strongly influence the occurrence of F. graminearum. Infected plant debris from previous crops, on which perithecia and ascospores develop, represent the main source for FHB, and hence, improved cropping systems aim to reduce this inoculum to decrease the infection risk. The best measure to evaluate the disease pressure is spore traps that detect deposited airborne ascospores. Commercial spore traps are expensive and require power sources, thus, they are not suitable for investigations in field experiments with different treatments. In consequence, we developed spore traps containing a Petri dish with Fusarium-selective agar, protected by aluminum dishes and attached on a wooden board. We compared the data of our low-cost trap with those of a commercial high-throughput jet sampler and obtained equivalent results. In field experiments to compare cropping systems, we observed a high correlation between the DON content in wheat grains and the number of colonies from deposited spores. Our spore trap proved to be a highly valuable tool to not only study FHB epidemiology but also to identify innovative cropping systems with a lower risk for FHB and DON contamination.

Analysis of Swiss needle cast disease on Pseudotsuga menziesii var. glauca within a Christmas tree plantation in Mexico

AbstractNothophaeocryptopus gaeumannii causes Swiss needle cast disease in Douglas‐fir forest trees and ornamental plantations. This study analysed disease and pathogen behaviour from summer 2011 to spring 2012. An untreated plot within a Pseudotsuga menziesii var. glauca plantation in Valle de Bravo, Mexico was divided into 12 sub‐plots. At each sampling date, needles were collected from four trees per sub‐plot. The analysis included seven needle age cohorts from branches from two canopy sections, as well as fallen needles from five random trees inside the entire plot. Airborne ascospores were collected weekly in slide traps. Sequencing identified the pathogen from DNA extracted from 18 mycelial colonies (isolates) and ascospores DNA from four selective slide traps. Disease and pathogen characteristics were mostly similar to other reports, except for ascospores inside pseudothecia on the youngest age class of foliage. Defoliation affected lower canopies frequently, while in the middle canopy it ranged from 6% to 98% depending on the needle age. In fallen needles, mature pseudothecia contained ascospores and isolated colonies developed in an agar‐based medium. Ascospore fluctuation peaked in two periods, but the most relevant occurred from 11 February to 17 March 2012, coinciding with bud break. These results contribute further understanding of SNC disease useful in improving the health of Christmas trees and the less‐studied P. menziesii var. glauca.

Distribution of Septoria tritici blotch disease agent Zymoseptoria tritici mating type idiomorphs in Turkey

Zymoseptoria tritici is the pathogen responsible for Septoria tritici blotch (STB) disease in wheat, causing significant yield losses during some years. This disease is observed in wheat production areas in our country as in many regions of the world. In this study, STB diseased wheat leaves were collected from six geographical regions in wheat cultivation areas of Turkey during 2013 and 2014 production seasons. A total of 103 single spore isolates obtained from the collection of Z. tritici population were chosen as representative for testing of their mating type. The isolates were evaluated for mating type (MAT) idiomorph frequency by multiplex PCR using Z. tritici specific MAT primers. The prevalence of mating types was found to be 48.5% (50) and 51.5% (53) for Mat1-1 and Mat1-2, respectively. Chi-square test was used to assess that two mating type scores are at equal frequencies. The study affirms the hypothesis that Z. tritici is reproducing sexually and spread via airborne ascospores aside from rain splash dispersed asexual spores in Turkey. Genetic recombination during sexual mating has a potential to generate more virulent pathotypes of Z. tritici.

Venturia inaequalis trapped: molecular quantification of airborne inoculum using volumetric and rotating arm samplers

Site-specific high throughput monitoring of airborne ascospores of Venturia inaequalis, the causal agent of apple scab, can improve existing warning systems. A new qPCR assay was developed to quantify ascospores collected by a simple rotating-arm spore sampler. The qPCR assay was highly specific and sensitive, with a limit of quantification of 20 ascospores per sample. The new detection system was compared to sampling with a traditional Burkard volumetric spore trap and to microscopic quantification. During controlled ascospore release experiments in a closed environment, strong correlations (ρ: 0.96 to 0.99) were observed between the two types of samplers and the two methods of quantification but significantly larger numbers of spores (log difference: 0.43 to 0.69) were obtained when using the rotating-arm sampler and when using molecular quantification. During comparisons under outdoor conditions over a three-year period, reasonable correlations between the techniques (average ρ = 0.61) were observed. When rotating-arm samplers operate continuously they can get saturated but their counts still correlated better with those from the Burkard sampler than when they only operate during rain and until two hours after. This suggests that ascospores were also captured outside of rain events. Based on these comparisons, molecular quantification of spores captured with the rotating-arm sampler appears to be a sensitive and reliable method to determine airborne ascospores of V. inaequalis and holds promise as a tool to guide targeted fungicide applications in commercial orchards as well as to increase our knowledge of the aerobiology of this pathogen.

Influence of moisture on maturation rate of the Venturia inaequalis (Cooke) Wint. ascospores in central Poland

Influence of moisture on maturation rate of the airborne Venturia inaequalis (Cooke) Wint. ascospores was numerically examined using field data collected from spore traps in Skierniewice, Poland, during 2005–2008 and 2010–2014. In order to avoid a bias resulting from different abundance of airborne ascospores in different seasons, the cumulative proportions of mature ascospores observed in the season were analyzed. The maturation rate was expressed with the probit of cumulative proportion which linearized its sigmoidal shape and made the effect of the weather during the entire season of the same importance. The significant, positive association between the probit slope and the moisture indicators related to precipitation, leaf wetness, and high relative humidity were observed. Additionally, the significant, negative relationships with the average and maximal air temperature were found. The results suggested that rain should be treated as the major factor driving the ascospore development and leaf wetness as the second factor. In conclusion, the main sources of moisture should be taken into account in statistical models of ascospore development, but it seems reasonable to take into account different strength of their association with the development rate.

Antifungal, Plant Growth-Promoting, and Genomic Properties of an Endophytic Actinobacterium Streptomyces sp. NEAU-S7GS2.

Diseases caused by Sclerotinia sclerotiorum have caused severe losses of many economically important crops worldwide. Due to the long-term persistence of sclerotia in soil and the production of air-borne ascospores, synthetic fungicides play limited roles in controlling the diseases. The application of antagonistic microorganisms can effectively reduce the number of sclerotia and eventually eradicate S. sclerotiorum from soil, and therefore considerable interest has been focused on biological control. Streptomyces sp. NEAU-S7GS2 was isolated from the root of Glycine max and its rhizosphere soil. It showed significant inhibitory activity against the mycelial growth of S. sclerotiorum (99.1%) and completely inhibited sclerotia germination. Compared to the control, in the pot experiment the application of NEAU-S7GS2 not only demonstrated excellent potential to control sclerotinia stem rot of soybean with 77 and 38% decrease in disease incidence and disease index, respectively, but could promote the growth of soybean. The light microscopy and scanning electron microscopy showed that co-culture of NEAU-S7GS2 with S. sclerotiorum on potato dextrose agar could lead to contorted and fragmented mycelia of S. sclerotiorum, which was associated with the secretion of hydrolytic glucanase and cellulase and the production of active secondary metabolites by NEAU-S7GS2. The plant growth promoting activity of NEAU-S7GS2 was related to the solubilization of inorganic phosphate, and production of 1-aminocyclopropane-1-carboxylate (ACC) deaminase and indole acetic acid (IAA). To further explore the plant growth promoting and antifungal mechanisms, the complete genome of strain NEAU-S7GS2 was sequenced. Several genes associated with ammonia assimilation, phosphate solubilization and IAA synthesis, together with genes encoding ACC deaminase, glucanase and α-amylase, were identified. AntiSMASH analysis led to the identification of four gene clusters responsible for the biosynthesis of siderophores including desferrioxamine B and enterobactin. Moreover, the biosynthetic gene clusters of lydicamycins, phenazines, and a glycosylated polyol macrolide showing 88% gene similarity to PM100117/PM100118 were identified. These results suggested that strain NEAU-S7GS2 may be a potential biocontrol agent and biofertilizer used in agriculture.

Characterization of Sclerotinia sclerotiorum airborne inoculum, the widespread agent of white mould disease

A means to rationalize the use of fungicides for crop protection and to make agriculture friendlier to environment and human health is to develop disease-risk forecasting systems based on the assessment of airborne inoculum abundance. Sclerotinia sclerotiorum, the pandemic agent of white mould disease, is disseminated via the atmosphere in the form of ascospores. These airborne spores are the primary sources of inoculum initiating the majority of epidemics. However, for witloof chicory (Cichorium intybus var. foliosum L.), there is no data about airborne inoculum of S. sclerotiorum, which makes it difficult to develop a forecasting model. In the present study, we characterized the temporal evolution of the abundance and of the genetic characteristics of S. sclerotiorum inoculum on a witloof chicory field located in Northern France over a 3-year period. To our knowledge, this study provides the first quantification of viable airborne populations of this fungus in witloof chicory crops. Moreover, it provides the first genetic characterization of S. sclerotiorum airborne inoculum. The results show that viable ascospores were present through 80% of the sampling dates. A significant correlation between abundance of airborne ascospores and local relative humidity suggests a local origin of inoculum. However, the existence of a slight genetic differentiation between isolates carried by air masses coming from the West and from the North-West is compatible with the hypothesis of a distant origin of S. sclerotiorum inoculum. We discuss the additional studies that are envisioned to clarify the origin of S. sclerotiorum airborne inoculum in witloof chicory fields.

Immuno-impedimetric Biosensor for Onsite Monitoring of Ascospores and Forecasting of Sclerotinia Stem Rot of Canola

Sclerotinia stem rot, caused by the fungal pathogen Sclerotinia sclerotiorum, is a destructive disease of canola and many other broadleaf crops. The primary inoculum responsible for initiating Sclerotinia epidemics is airborne ascospores released from the apothecia of sclerotia. Timely detection of the presence of airborne ascospores can serve as an early-warning system for forecasting and management of the disease. A major challenge is to develop a portable and automated device which can be deployed onsite to detect and quantify the presence of minute quantities of ascospores in the air and serves as a unit in a network of systems for forecasting of the epidemic. In this communication, we present the development of an impedimetric non-Faradaic biosensor based on anti-S. sclerotiorum polyclonal antibodies as probes to selectively capture the ascospores and sense their binding by an impedance based interdigitated electrode which was found to directly and unambiguously correlate the number of ascospores on sensor surface with the impedance response.

Inoculum dynamics and disease progress of circular leaf spot of persimmon caused by Mycosphaerella nawae in inland Korea

Circular leaf spot is one of the most serious diseases of persimmons in many areas where the crop is grown. This disease is caused by the fungal species Mycosphaerella nawae Hiura & Ikata. Disease cycle of circular leaf spot of persimmon in terms of seasonal pattern of ascospore dispersal and the disease development based on field survey and ascospore potential was studied in the cold inland areas of central Korea (Sangju) for two consecutive years. Based on the number of rain-released and airborne ascospores collected, seasonal release of M. nawae ascospores from pseudothecia occurred from May to September but most ascospores were detected between mid-June and mid-July in both years. There was a strong correlation between the number of water-dispersed ascospores and cumulative rainfall. The first disease symptoms were observed at late August. Disease incidence became maximum at the end of September. Experiments under controlled conditions, including the effects of misting duration, soaking time, and temperature on ascospore release were performed. Over 90% of the total ascospores were released within 2 h of misting. M. nawae ascospores began to be released just after leaf soaking, and the release rate increased exponentially over 8 h. Temperature significantly (P = 0.008) affected the release of M. nawae ascospores from soaked leaf litter during 8 h at controlled condition. The outcome of our experiments suggested that the quantity and temporal distribution of airborne and waterborne M. nawae inoculum is closely related to seasonal rainfall patterns, duration of rainfall, irrigation system, soaking period and temperature.