Effects of environmental factors on discharge and germination of ascospores of Venturia nashicola

Experiments were conducted under controlled environmental conditions to study the effects of temperature, duration of wetness, relative humidity (RH) and light on the discharge and germination of ascospores of Venturia nashicola , the causal agent of pear scab in China. Discharge of ascospores from pseudothecia required free water or 100% RH. A period of soaking in water as short as 10 s was sufficient to initiate the discharge of ascospores. Temperatures from 10 to 30°C did not significantly affect the temporal trend of ascospore discharge. A greater proportion of ascospores was discharged under light than in the dark. However, a period of light as short as 10 min, either during the initial wetting of pseudothecia or interrupting the darkness, was sufficient to reduce the inhibitory effect of darkness on ascospore discharge. Ascospores were discharged within 10 min after pseudothecia were wetted and most ascospores ( c. 80%) were discharged within the first hour. The temporal pattern of ascospore discharge could be well described by a logistic model, which estimated that 50% of ascospores were discharged within half an hour of wetting. Ascospores germinated over a wide range of temperatures from 5 to 30°C, with an optimum at c . 20°C. Temporal dynamics of ascospore germination at six temperatures (5, 10, 15, 20, 25 and 30°C) were satisfactorily described by logistic models.     

Phenology of Didymella rabiei Development on Chickpea Debris Under Field Conditions in Spain

ABSTRACT The development of Didymella rabiei on debris of naturally infected chickpea was investigated in four chickpea-growing areas with different climatic conditions in Spain during 1987 to 1992. D. rabiei extensively colonized chickpea debris and formed pseudothecia and pycnidia. Differentiation of pseudothecial initials occurred regularly across experimental locations by November, 1 month after placement of debris on the soil. Ascospore maturation occurred mainly from late January to late March, depending on location and year. Maximum ascospore discharge from sampled debris pieces placed under suitable environmental conditions occurred 2 to 4 weeks after ascospore maturation, after which ascospore release decreased sharply. Pseudothecia were exhausted, due to ascospore discharge, by the beginning of summer. New asci did not develop in empty pseudothecia and no pseudothecia formed in tissues after the first season. Ascospore maturation and liberation in cooler locations were more uniform and occurred later compared to maturation in warmer locations. Also, production of asci and ascospores per pseudothecium was much higher in cooler than in warmer locations. A similar relationship was found for density of pseudothecia and pycnidia and conidia production per pycnidium. The percentage of mature pseudothecia increased according to the logistic model, with the cumulative number of Celsius degree days calculated by computing the mean of the maximum and minimum daily air temperatures on rainy days from the date of debris placement on the soil. There were significant differences among model parameter estimates between cooler and warmer locations, but minor differences were found among parameters for locations with similar environmental conditions. There was an inverse linear relationship between the average temperature during the period of pseudothecia maturation and the number of asci produced per pseudothecium.     

Epidemiology of Blackleg (Leptosphaeria maculans) of Canola (Brassica napus) in Relation to Maturation of Pseudothecia and Discharge of Ascospores in Western Australia

ABSTRACT The timing of maturation of pseudothecia and discharge of ascospores of the blackleg fungus (Leptosphaeria maculans) is critical in relation to infection early in the cropping season of canola. During 1998 to 2000, development of pseudothecia was investigated on residues of the previous year's canola crop collected from four agroclimatically different locations: Mount Barker (southern high rainfall), Wongan Hills (central medium rainfall), Merredin (central low rainfall), and East Chapman (northern low rainfall) in Western Australia. The pseudothecia matured on residues at different times after harvest in various regions. In general, pseudothecia maturity occurred earlier in the high-rainfall areas than in medium- and low-rainfall areas. An ascospore discharge pattern was investigated from residues of crop from the previous year (6-month-old residues) at three locations-Mount Barker, Wongan Hills, and East Chapman in Western Australia-and from 18-month-old residues that were burnt and raked in the previous year at Mount Barker and East Chapman. Ascospore discharge commenced earlier in high-rainfall (>450 mm) areas (Mount Barker) and late in northern low-rainfall (<325 mm) areas (East Chapman). The major ascospore showers took place during May (late autumn) and June (early winter) at Mount Barker and during July and August (mid- to late winter) at East Chapman. The number of ascospores discharged was extremely low at East Chapman compared with Mount Barker. At both locations, the number of ascospores discharged from 18-month-old residues that were raked and burnt in the previous year were only approximately 10% of those discharged from previous year's residues left undisturbed. The discharge of ascospores on any given day was negatively correlated with accumulated temperatures, maximum temperature, evaporation, minimum and maximum soil temperatures, and solar radiation and was positively correlated with the minimum temperature, rain, and minimum relative humidity. This is the first report describing how pseudothecia mature on residues in different rainfall areas in Western Australia, and it potentially can be used in developing a forecasting system to avoid the synchronization of major ascospore showers with the maximum susceptibility period of canola seedlings.     

Real-time PCR quantification and spatio-temporal distribution of airborne inoculum of Mycosphaerella graminicola in Belgium

Two kinds of propagules play a role in Mycosphaerella graminicola dissemination: splash-dispersed pycnidiospores and airborne sexual ascospores. A method based on real-time polymerase chain reaction (PCR) assay and using Burkard spore traps was developed to quantify M. graminicola airborne inoculum. The method was tested for its reliability and applied in a spore trap network over a 2-year period in order to investigate the spatio-temporal distribution of airborne inoculum in Belgium. At four experimental sites, airborne inoculum was detected in both years. A seasonal distribution was observed, with the highest mean daily quantities (up to 351.0 cDNA) trapped in July and with clusters detected from September to April. The first year of trapping, a mean daily quantity of 15.7 cDNA of M. graminicola airborne inoculum was also detected in the air above a building in a city where the spatio-temporal distribution showed a similar pattern to that in the field. Mean daily quantities of up to 60.7 cDNA of airborne inoculum were measured during the cereal stem elongation and flowering stages, suggesting that it contributes to the infection of upper leaves later in the season. Most detection, however, tended to occur between flowering and harvest, suggesting significant production of pseudothecia during that period. Variations in mean daily quantities from 1.0 to 48.2 cDNA were observed between sites and between years in the patterns of airborne inoculum. After stem elongation, the quantities detected at a site were positively correlated with the disease pressure in the field. Quantities trapped at beginning of the growing season were also well correlated with the disease level the previous year. Multiple regressions revealed that some factors partly explain the daily variations of airborne inoculum.     

Environmental Factors Affecting Pseudothecial Development and Ascospore Production of Mycosphaerella citri, the Cause of Citrus Greasy Spot

ABSTRACT Mycosphaerella citri, the cause of citrus greasy spot, produces pseudothecia and ascospores in decomposing leaf litter on the grove floor. In laboratory studies, the effect of wetting and drying and temperature on the formation, maturation, and production of pseudothecia and ascospores was evaluated on mature, detached grapefruit leaves. Production of pseudothecia was most rapid when leaves were soaked five times per week for 2 h per day, but pseudothecial density and total ascospore production were greatest when leaves were soaked three times per week for 2 h per day. In duration of wetting studies, 3 h per day, 3 days per week brought about the most rapid production, but 10 to 30 min per day resulted in production of the most pseudothecia and ascospores. Pseudothecia and ascospore production were greatest at 28 degrees C and declined rapidly at lower and higher temperatures. Maturation of pseudothecia was slow at 20 and 24 degrees C, but production was high at 24 degrees C; at 32 degrees C, pseudothecia matured rapidly, but degenerated quickly. No mature pseudothecia were produced on leaves maintained continuously under wet conditions. In field studies, leaves were placed on the grove floor monthly from April 2000 to September 2001. Pseudothecia production was rapid during the summer rainy season from June to September. Pseudothecia produced on leaves placed in the grove from October to May developed and matured more slowly but were produced in much larger numbers than in summer. The number of days to first pseudothecial initials, 50% maturation, first discharge of ascospores, leaf decomposition, as well as pseudothecial density and incidence, were negatively related to average temperature. Total ascospore production was unrelated to temperature.     

Dynamics of ascospore maturation and discharge in Erysiphe necator, the causal agent of grape powdery mildew

Dynamics of ascocarp development, ascospore maturation, and dispersal in Erysiphe necator were studied over a 4-year period, from the time of ascocarp formation to the end of the ascosporic season at the end of June in the following spring. Naturally dispersed chasmothecia were collected from mid-August to late November (when leaf fall was complete); the different collections were used to form three to five cohorts of chasmothecia per year, with each cohort containing ascocarps formed in different periods. Chasmothecia were exposed to natural conditions in a vineyard and periodically sampled. Ascocarps were categorized as containing mature or immature ascospores, or as empty; mature ascospores inside chasmothecia were enumerated starting from late February. Ascospore discharge was determined using silicone-coated slides that were placed 3 to 4 cm from sections of the vine trunk holding the chasmothecia. Before complete leaf fall, 34% of the chasmothecia had mature ascospores, 48% had immature ascospores, and 18% were empty; in the same period, the trapped ascospores represented 56% of the total ascospores trapped in an ascosporic season (i.e., from late summer until the next spring or early summer). The number of viable chasmothecia diminished over time; 11 and 5% of chasmothecia had mature ascospores between complete leaf fall and bud break and after bud break, respectively. These ascocarps discharged ≈2 and 42% of the total ascospores, respectively. All the ascocarp cohorts released ascospores in autumn, survived the winter, and discharged viable ascospores in spring; neither ascospore numbers nor their pattern of temporal release was influenced by the time when chasmothecia were collected and exposed in the vineyard. Abundance of mature ascospores in chasmothecia was expressed as a function of degree-days (DD) (base 10°C) accumulated before and after bud break through a Gompertz equation (R2 = 0.92). Based on this equation, 90% of the ascospores were mature when 153 DD (confidence interval, 100 to 210 DD) had accumulated after bud break. Most ascospores were trapped in periods with >2 mm of rain; however, a few ascospores were airborne with <2 mm of rain and, occasionally, in wet periods of ≥3.5 h not initiated by rain.     

The life cycle of <Emphasis Type="Italic">Stemphylium vesicarium</Emphasis>, the causal agent of Welsh onion leaf blight

Symptoms of Welsh onion leaf blight, caused by Stemphylium vesicarium, are divided into two types, i.e., brown oval lesions and yellow mottle lesions. Yellow mottle lesions exert considerable economic damage on Welsh onion in northern Japan. In this study, we investigated the life cycle of the pathogen in terms of seasonal fluctuation of spore dispersal and its relationship with development of disease, formation period of pseudothecia and overwintering of the pathogen based on field surveys, spore trapping and fungal isolation. Conidia were trapped throughout the cropping season except before mid June, when no ascospores were trapped. Brown oval lesions, which contained a large number of conidia, usually occurred in July followed by yellow mottle lesions with an increasing number of conidia trapped. These observations suggest that conidia released from brown oval lesions play an important role as a secondary inoculum source of the disease, leading to the development of yellow mottle lesions. Pseudothecia on leaves were first observed at the end of the cropping season or immediately after harvest (late October). The pathogen overwintered in the form of pseudothecia produced on leaves with or without symptoms. Ascospores failed to be trap in the field during the interval between before and beginning of the cropping season in April–May. However, pot experiments demonstrated that ascospores were released from leaf debris in November and rapidly increased in number after snow melt. From this circumstantial evidence, we hypothesize that ascospores are the primary inoculum source of Welsh onion leaf blight.     

The effect of winter weather conditions on the ability of pseudothecia of Leptosphaeria maculans and L. biglobosa to release ascospores

Leptosphaeria maculans and L. biglobosa are damaging pathogens of oilseed rape. The infection of plants occurs predominantly in early autumn or spring by spores produced in pseudothecia. The aim of this study was to investigate whether pseudothecia formed in the autumn are still viable in the spring and to what extend they are destroyed by winter frosts. The studies presented here demonstrated that winter frosts can render pseudothecia unable to release spores. Nevertheless, ascospores present in pseudothecia unable to discharge ascospores, were fully capable of germination, regardless of the incubation temperature. No significant differences were found between the studied Leptosphaeria species in their response to frost. A multiple regression equation has been elaborated to forecast the ability of pseudothecia to release ascospores, based on winter temperatures. Considerable correlation was found between the ascospore release in the autumn and the ability of pseudothecia to release ascospores over the winter period and the subsequent symptoms of stem canker before harvest. We have demonstrated that the potential and the survival of inoculum can have a large impact on the success of the pathogen. This may be particularly important in the light of forecasted climate change. Higher winter temperatures may increase the ability of pseudothecia to release ascospores and the discharge of ascospores of L. maculans and L. biglobosa into the air, and cause early plant infections. This in turn will increase the number of infected plants, the disease incidence at harvest, and reduce the yield of oilseed rape.     

A method to monitor airborne Venturia inaequalis ascospores using volumetric spore traps and quantitative PCR

Apple scab caused by the fungus Venturia inaequalis can result in significant crop losses if not managed effectively. Sanitation as part of an integrated management strategy aims to significantly reduce primary inoculum to lower the disease pressure. This study evaluates the possibility of molecular detection and quantification of ascospore discharge and the use of this method to test for efficacy of orchard sanitation treatments. A method to detect and quantify airborne ascospores was developed using volumetric spore traps (VSTs). V. inaequalis specific primers were tested on daily VST samples from two orchard sections (leaf litter removed compared to leaf litter left) during spring. A molecular method to detect and quantify ascospores was tested by amplifying genomic regions of the mitochondrial CYP51A1 gene, and the ITS region using SYBR® green. Timing of ascospore discharge was compared to predicted infection risk and a degree day model using weather data. The average spore detection limit was estimated to be at levels of 1 pg μl^?1 DNA (approximately 37 ascospores) per daily spore trap reading using CYP51A1 primers. Using the CYP51A1 primer pair, primary inoculum was estimated to be 51 % lower in the orchard sections where leaves had been removed, indicating that this method could be used to evaluate the efficacy of alternative control strategies such as leaf removal to reduce potential ascospore dose. This is the first report of combining VSTs and quantitative PCR to monitor airborne V. inaequalis ascospores.     

Detection and quantification of airborne inoculum of Sclerotinia sclerotiorum using quantitative PCR

This paper reports the development of a new specific diagnostic technique to accurately quantify airborne inoculum of Sclerotinia sclerotiorum and discusses its potential use in disease-forecasting schemes, using examples of three contrasting epidemic seasons: 2007, when there was a severe epidemic of sclerotinia stem rot (SSR) in England and high numbers of airborne ascospores were trapped at Rothamsted, and, in contrast, 2003 and 2004, when the incidence of SSR in England was low and low numbers of airborne ascospores were trapped at Rothamsted. DNA was extracted from wax-coated plastic tapes, such as those used in Burkard (Hirst-type) spore traps and rotating-arm traps. A SYBR-green quantitative PCR (qPCR) method produced a linear relationship between ascospore numbers and S. sclerotiorum DNA (mean 0·35 pg DNA per spore) and was able to detect DNA representing as few as two ascospores. The technique was insensitive to DNA of the host plant, Brassica napus , and other plant pathogens, including Sclerotinia minor , S. trifoliorum and Botrytis cinerea , and common airborne fungal genera such as Cladosporium and Penicillium . There was no relationship between rainfall and numbers of airborne ascospores of S. sclerotiorum present at Rothamsted during the period of infection in the severe SSR season (2007).     

Is the onset of septoria tritici blotch epidemics related to the local pool of ascospores?

To elucidate the early epidemic stages of septoria tritici blotch, especially the relationship between the onset of epidemics, the local availability of primary inoculum, and the presence of wheat debris, the early disease dynamics and airborne concentration in Zymoseptoria tritici ascospores were concomitantly assessed at a small spatiotemporal scale and over two years, using spore traps coupled with a qPCR assay. One plot, with the crop debris left, provided a local source of primary inoculum, while the other plot, without debris, lacked any. According to the assay's limits of detection, daily spore trap samples were classified as not detectable or not quantifiable, detectable, and quantifiable. The proportions of samples assigned to the different classes and numbers of spores in samples classified as quantifiable were significantly different between years, time periods (from November to March) and spore trap location (field with or without debris). The effect of year on the airborne ascospore concentration was high: 22 daily peaks with more than 230 ascospores m^?3 day^?1 were identified in the autumn of 2012/13, but none in the autumn of 2011/12. The local presence of wheat debris had no obvious effect on the amount of airborne ascospores or on the earliness of the two epidemics, especially in the year with high inoculum pressure (2012/13). These results suggest that the amount of primary airborne inoculum available in a wheat crop is not a limiting factor for the onset of an epidemic.     

Environmental Factors Affecting the Release and Dispersal of Ascospores of Mycosphaerella citri

ABSTRACT Greasy spot, caused by Mycosphaerella citri, produces a leaf spot disease affecting all citrus species in Florida and the Caribbean Basin. M. citri produces pseudothecia and ascospores, which are considered the principal source of inoculum, in decomposing leaves on the grove floor. In studies using a computer-controlled environmental chamber, a single rain event triggered release of most mature ascospores beginning 30 to 60 min after the rain event. Additional rain events did not bring about further release. High relative humidity without rain triggered release of low numbers of ascospores, but vibration and red/infrared irradiation had little or no effect on ascospore release. After three to four cycles of wetting and drying of leaves, all pseudothecia had matured and released their ascospores. In the field, ascospores were detectable starting about 2 h after the beginning of a rain or irrigation and most ascospores were released within 16 h. Ascospore release was greatest following rain events and somewhat less following irrigations, and low numbers of ascospores were detectable on days without precipitation. Ascospore numbers declined linearly with horizontal distance from the source and as a function of the logarithm of ascospore numbers with vertical distance. Low numbers of ascospores were detected 7.5 m above the ground and 90 m downwind from the grove. Ascospore release can be advanced by irrigating frequently during dry, nonconducive conditions to stimulate ascospore release when environmental conditions are unfavorable for infection, but the eventual effects on disease severity are uncertain.     

Ascospores of the Japanese pear scab fungus (<Emphasis Type="Italic">Venturia nashicola</Emphasis> Tanaka &; Yamamoto) are discharged during the day

We investigated the diurnal pattern of ascospore discharge of the Japanese pear scab fungus (Venturia nashicola Tanaka & Yamamoto) in an orchard. Ascospores of V. nashicola were mainly discharged during the day. Most ascospores were discharged from 7:00 to 19:00: 99.6% in 2001, 99.3% in 2002, and 93.8% in 2005. Because the ascospores were discharged only when the fallen diseased leaves were wet from precipitation, the wetness of these leaves is probably imperative for spore discharge. Ascospore discharge began immediately after precipitation in the daytime. When it rained at night, however, ascospore discharge did not begin until the following morning and never began immediately after precipitation. We also investigated other meteorological factors. When fallen diseased leaves were wet, the percentage of ascospore discharge was positively correlated with the amount of solar radiation and atmospheric temperature and negatively correlated with relative humidity. Ascospore discharge was interrupted by a decrease in solar radiation and atmospheric temperature and by increased relative humidity at night. This report is the first that V. nashicola discharges ascospores primarily during the day.     

Airborne ascospores ofDidymella rabiei as a major primary inoculum for Ascochyta blight epidemics in chickpea crops in southern Spain

The incidence and severity of Ascochyta blight in potted chickpea trap plants exposed for 1-wk periods near infested chickpea debris in Córdoba, Spain, or in chickpea trap crops at least 100 m from infested chickpea debris in several locations in southern Spain were correlated with pseudothecial maturity and ascospore production ofDidymella rabiei from nearby chickpea debris. The period of ascospore availability varied from January to May and depended on rain and maturity of pseudothecia. The airborne concentration of ascospores ofD. rabiei was also monitored in 1988. Ascospores were trapped mostly from the beginning of January to late February; this period coincided with that of maturity of pseudothecia on the chickpea debris. Most ascospores were trapped on rainy days during daylight and 70% were trapped between 12.00 and 18.00 h. Autumn-winter sowings of chickpea were exposed longer to ascospore inoculum than the more traditional spring sowings because the autumn-winter sowings were exposed to the entire period of ascospore production on infested chickpea debris lying on the soil surface.     

The full life cycle of Leptosphaeria maculans completed on inoculated oilseed rape incubated under controlled conditions

Because epidemics of successive cropping seasons are not independent, epidemiological studies need to encompass the processes occurring during the transmission of epidemics from one season to the next. With Leptosphaeria maculans, infected stubble allows carry‐over of the fungus. Generation experiments using recurrent selection on field plots are a useful means of comparing the effects of selection pressures. However, the full life cycle of the fungus, from plant infection to the next generation of ascospores, has not yet been achieved under controlled conditions. Studies were undertaken to achieve an experimental set‐up with sexual reproduction under controlled conditions. Cankered oilseed rape stems were produced under controlled conditions, after inoculation with a mixture of 12 isolates across both mating types. Stems were cut longitudinally and attached to styropore plates. Stem halves were incubated outside or in climate chambers regularly soaked in tap water to ensure maturation. Incubation was stopped when mature pseudothecia were observed. In all three independent experiments, more stem halves had pseudothecia when incubated under controlled conditions (30–100%) than incubated outside (0–80%). To the authors’ knowledge, this is the first study achieving the full life cycle of the fungus under controlled conditions, from infection of the plant to mature pseudothecia. This opens up the prospect of running experiments year‐round to better understand inoculum production, to compare fungal fitness, or to run generation experiments with exotic pathogen populations.     

A Model of the Effect of Pseudothecia on Genetic Recombination and Epidemic Development in Populations of Mycosphaerella graminicola

ABSTRACT It is generally agreed that ascospores are the origin of primary infections for the disease septoria tritici blotch of wheat caused by the fungus Mycosphaerella graminicola (anamorph Septoria tritici). The epidemic during the growing season was previously ascribed to the asexual pycni-diospores dispersed over short distances by rain splash, but recent observations suggest that the airborne ascospores also may play a role. As a consequence, the composition of the pathogen population over the growing season may change through genetic recombination. In an attempt to resolve the relative importance of the two spore types to the epidemic over the growing season, a model simulating disease caused by both types of spores was constructed and analyzed. The conclusion from the analysis of this model is that sexual recombination will affect the genetic composition of the population during a growing season. A considerable proportion of spores released at the end of the growing season may be sexual descendants of the initial population. However, ascospores are unlikely to affect the severity of the epidemic during the growing season. This is due to the much longer latent period for pseudothecia compared with pycnidia, resulting in ascospores being produced too late to influence the epidemic.     

Maturation of pseudothecia and discharge of ascospores of <Emphasis Type="Italic">Leptosphaeria maculans</Emphasis> on oilseed rape stubble

The effects of temperature, wetness and darkness on formation of pseudothecia and the effect of temperature on the release of ascospores of L. maculans on oilseed rape stubble were studied in a controlled environment in South Australia. Pseudothecia of L. maculans developed at 5–20°C and the time taken to reach maturity and discharge ascospores decreased from 58 days at 5°C to 22.2 days at 15°C. The optimum temperature of those tested for pseudothecium maturation was between 15°C and 20°C but fewer pseudothecia were observed at 20°C than at 15°C. Exposure to a 12 h photoperiod enhanced pseudothecium formation on the stubble compared with continuous darkness. No pseudothecia formed on stubble moistened once a day at 15°C, whereas three sprays of water per day decreased maturation time in comparison with two sprays per day. More ascospores were released for a longer duration at 20°C than at 5–15°C, although peak sporulation occurred earlier at 5–10°C than at 20°C. These findings highlight the importance of moisture, temperature and light for production and release of inoculum from stubble. This information, combined with field data, may help to predict the onset of inoculum release.     

The Occurrence of Mycosphaerella graminicola and its Anamorph Septoria tritici in Winter Wheat during the Growing Season

The disease septoria tritici blotch of wheat is initiated by ascospores of the teleomorph Mycosphaerella graminicola or pycnidiospores of the anamorph Septoria tritici. We report for the first time the presence of the teleomorph, M. graminicola, in Denmark. With the objective of elucidating the importance of the teleomorph for the development of septoria tritici blotch, data on the occurrence of fruit bodies of the anamorph (pycnidia) and the teleomorph (pseudothecia) stages were collected over three growing seasons. Pseudothecia were present in the springs, however, high numbers of pseudothecia compared to pycnidia were not observed until July, too late to influence the epidemic. On an individual leaf layer, pycnidia were observed well before pseudothecia. As the leaves aged, progressively higher proportions of fruit bodies were observed to be pseudothecia. The period from the appearance of pycnidia to detection of pseudothecia was estimated as 29–53 days. At harvest, high proportions of sporulating fruit bodies in the crop were pseudothecia, suggesting that the primary source of inoculum for new emerging wheat crops in autumn is likely to be ascospores.     

Environmental Influences on the Release of Ophiosphaerella agrostis Ascospores Under Controlled and Field Conditions

ABSTRACT Ophiosphaerella agrostis, the causal agent of dead spot of creeping bentgrass (Agrostis stolonifera), can produce prodigious numbers of pseudothecia and ascospores throughout the summer. The environmental conditions and seasonal timings associated with O. agrostis ascospore release are unknown. The objectives of this research were to (i) determine the influence of light and relative humidity on ascospore release in a controlled environment, (ii) document the seasonal and daily discharge patterns of ascospores in the field, and (iii) elucidate environmental conditions that promote ascospore release under field conditions. In a growth chamber, a sharp decrease (100 to approximately 50%; 25 degrees C) in relative humidity resulted in a rapid (1- to 3-h) discharge of ascospores, regardless of whether pseudothecia were incubated in constant light or dark. In the field, daily ascospore release increased between 1900 and 2300 h and again between 0700 and 1000 h local time. The release of ascospores occurred primarily during the early morning hours when relative humidity was decreasing and the canopy began to dry, or during evening hours when relative humidity was low and dew began to form. Few ascospores were released between 1100 and 1800 h when the bentgrass canopy was dry. The release of ascospores also was triggered by precipitation. Of the ascospores collected during precipitation events, 87% occurred within 10 h of the beginning of each event.     

Phenology of development of Didymella rabiei pseudothecia and modelling of ascospores discharge in Tunisia

Didymella rabiei development was investigated on naturally blight-infested debris in six Tunisian locations during two consecutive seasons. Pseudothecia initiated their development 2 months after their incubation on the soil surface. They reached their maturity earlier in the locations of Korba, Beja, Bizerte and Bousalem (March–April) than in Morneg and Tunis (June). After ascospore discharge, new asci and ascospores did not develop in empty pseudothecia, and the fungus survived during the second season by forming pycnidia. Pseudothecia maturity index (PMI) was determined based on internal developmental stages and a logistic model was proposed to establish an eventual correlation between PMI and Celsius degree days (CDD) cumulated over time during rainy days (for rain ≥1 mm). Results showed that PMI increased significantly over cumulative number of CDD in the six locations. In Korba, Beja, Bizerte and Bousalem locations, maximum rate of pseudothecia maturity occurred during an acceleration phase between 150 and 350 cumulated CDD during which pseudothecia reached the mature stage. The maturity rate of pseudothecia in Morneg and Tunis locations was slower as compared to the latter four locations and mature pseudothecia were observed at 450–500 cumulated CDD. The onset of pseudothecial maturity was also estimated by applying a pre-established model previously developed for Ascochyta disease management. Based on this model, which considers a combination of daily mean temperature and daily total rainfall, the number of suitable events required for pseudothecial maturity was determined at each location.