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.     

Effects of temperature on maturation of pseudothecia of Leptosphaeria maculans and L. biglobosa on oilseed rape stem debris

Effects of temperature on maturation of pseudothecia of Leptosphaeria maculans and L. biglobosa , closely related species which coexist on UK oilseed rape, were investigated. Stages in pseudothecial maturation on naturally infected oilseed rape debris were examined, both in controlled environments (5, 10, 15 or 20°C) under continuous wetness and in natural conditions (debris exposed in September and December 2000, and July, September and November 2002). Pseudothecia sampled weekly were assigned to maturation classes A (asci undifferentiated), B (asci differentiated), C (ascospores differentiated) or D (ascospores mature). Progress in pseudothecial maturation (assessed by time until 50% of pseudothecia reached each class) was similar for L. maculans and L. biglobosa at 15–20°C, but L. biglobosa matured more slowly at < 10°C. Maturation time decreased almost linearly with temperature from 5 to 20°C under continuous wetness but was longer in natural conditions, especially when periods of dry weather occurred. Differences in pseudothecial maturation are likely to contribute to epidemiological differences between L. maculans and L. biglobosa , which may explain their coexistence. It is appropriate to use the degree-day approximation to assess pseudothecial maturation at temperatures between 5 and 20°C, providing debris is wet.     

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.     

Influence of relative humidity and temperature on development of Didymella rabiei on chickpea debris

Didymella rabiei grew saprophytically on pieces of artificially and naturally infected chickpea stem debris under artificial incubation conditions, and formed pseudothecia and pycnidia. The extent of growth was not significantly affected by temperature of incubation within the range 5–25°C, but was significantly reduced as relative humidity (RH) decreased from 100% to 86%, when no growth occurred. Pseudothecia matured at 10°C and constant 100% RH, or at 5 and 10°C and alternating 100%/34% RH. Under these conditions, pseudothecial maturation, assessed by a pseudothecia maturity index, increased over time according to the logistic model. For temperatures higher than 10°C or RH lower than 100%, pseudothecia either did not form ascospores, or ascopores did not mature and their content degenerated. When pseudothecia that initially developed to a given developmental stage were further incubated at a constant 100% RH, temperature became less limiting for complete pseudothecial development as the developmental stage was more advanced. Pycnidia of the fungus developed and formed viable conidia in all environmental conditions studied, except at 86% RH. However, the density of pycnidia formed and the number of viable conidia per pycnidium were significantly influenced by temperature, RH and the type of debris (artificially or naturally infected) used.     

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.     

Differences Between Ascospores and Conidia of Didymella rabiei in Spore Germination and Infection of Chickpea

ABSTRACT Studies were performed to compare the germination and infection of ascospores and conidia of Didymella rabiei under different temperature and moisture conditions. Germination of ascospores and conidia on cover glasses coated with water agar began after 2 h, with maximum germination (>95%) occurring in 6 h at 20 degrees C. No germination occurred at 0 and 35 degrees C. Ascospores germinated more rapidly than conidia at all temperatures. Germination declined rapidly as the water potential varied from 0 to -4 MPa, although some germination occurred at -6 MPa at 20 and 25 degrees C. Ascospores germinated over a wider range of water potentials than conidia and their germ tubes were longer than those of conidia at most water potentials and temperatures. The optimum temperature for infection and disease development by both ascospores and conidia was around 20 degrees C. Disease severity was higher when ascospores were discharged directly onto plant surfaces from naturally infested chickpea debris compared with aqueous suspensions of ascospores and conidia sprayed onto plants Disease severity increased as the length of the wetness period increased. When dry periods of 6 to 48 h occurred immediately after inoculation, disease severity decreased, except for the shorter periods which had the opposite effect. Disease severity was higher with ascospore inoculum when no dry periods occurred after inoculation.     

Survival of Didymella rabiei in chickpea straw debris in Spain

Didymella rabiei grew saprophytically on pieces of infested chickpea stems and pods, and formed pycnidia and pseudothecia. The extent of saprophytic growth and production of viable spores were determined by the incubation conditions. On debris left on the soil surface under natural conditions, the fungus rapidly colonized the tissues, formed abundant pseudothecia and pycnidia, and remained viable throughout the 2 years of the study. When plant debris was buried, D, rabiei was restricted to the original lesions, in which it formed new pycnidia and was viable for 2 to 5 months. Under controlled conditions in the laboratory, D. rabiei extensively colonized plant debris spread over the soil surface. On the other hand, the fungus did not grow on buried debris, or showed only very limited development when the artificially infested debris was buried between two layers of sterilized soil. Incubation temperature was the principal factor associated with the production of conidia and especially ascospores.     

Estimating the dynamics of airborne ascospores of Venturia inaequalis*

A system was elaborated to estimate the dynamics of primary inoculum of Venturia inaequalis in apple orchards. It separates the primary inoculum season into five periods with different risks: absent (ascospores not yet mature); potential (ascospores mature but not yet ready to be discharged); actual (ascospores can be discharged when favourable conditions occur); present (ascospores are airborne); exhausted (all ascospores have been ejected). These periods were determined by two mathematical models, which use meteorological parameters as driving variables. The first model estimates the development stage of the overwintering pseudothecia and then determines when the first pseudothecia contain pigmented and mature ascospores. A threshold of mature ascospores inside pseudothecia defines when the ascospores become ready for discharge. The second model estimates the proportion of the season's ascospores that are airborne on each discharging event, using temperature and leaf wetness, expressed as the degrees accumulated daily in the hours when leaves are wet. Estimates of absent and potential risk were verified by collecting data on the first ascospore discharge in the period 1991/1998 at Bologna and Modena (northern Italy), and they were always found to be accurate. To verify the estimates of actual, present and exhausted risk, the model outputs were compared with data collected by spore samplers at Modena and Bologna in 1997 and 1998: they were sufficiently accurate because the greatest part of the records from the spore sampler fell inside the confidence limits of the model.     

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.     

Development of the teleomorph of Ascochyta rabiei on culture media

Ascochyta blight caused by Didymella rabiei (anamorph: Ascochyta rabiei) is an important foliar disease of chickpea in many countries. The fungus is heterothallic and requires the pairing of two compatible mating types for the teleomorph to develop. In nature, the teleomorph only develops on chickpea debris that overwinters on the soil surface in the presence of both mating types. When natural and synthetic agar media were seeded with conidial suspensions of compatible isolates of D. rabiei from Spain and the United States and incubated under favourable conditions for teleomorph development, the teleomorph only developed on 2?% water agar amended with powdered chickpea stems or hot water extracts of chickpea stems, but not on 14 other natural or synthetic media. Ascospore isolates of D. rabiei from pseudothecia that developed on agar media were indistinguishable in cultural and morphological characteristics from isolates of the fungus from chickpea. Production of pseudothecia and ascospores on the best culture medium was always lower than on stem pieces of chickpea straw used as a control treatment. Ascospores discharged from pseudothecia that developed on powdered chickpea stem media onto chickpea seedlings were pathogenic, inducing symptoms identical to those caused by ascospores from chickpea stem pieces or conidia from a chickpea isolate of the fungus. This is the first report of the teleomorph of D. rabiei developing on culture media.     

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.     

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.     

Ascospore Release and Infection of Apple Leaves by Conidia and Ascospores of Venturia inaequalis at Low Temperatures

ABSTRACT Mills' infection period table describes the number of hours of continuous leaf wetness required at temperatures from 6 to 25 degrees C for infection of apple leaves by ascospores of Venturia inaequalis and reports that conidia require approximately two-thirds the duration of leaf wetness required by ascospores at any given temperature. Mills' table also provides a general guideline that more than 2 days of wetting is required for leaf infection by ascospores below 6 degrees C. Although the table is widely used, infection times shorter than those in the table have been reported in lab and field studies. In 1989 a published revision of the table eliminated a potential source of error, the delay of ascospore release until dawn when rain begins at night, and shortened the times reported by Mills for ascospore infection by 3 h at all temperatures. Data to support the infection times below 6 degrees C were lacking, however. Our objective was to quantify the effects of low temperatures on ascospore discharge, ascospore infection, and infection by conidia. In two of three experiments at 1 degrees C, the initial release of ascospores occurred after 131 and 153 min. In the third experiment at 1 degrees C, no ascospores were detected during the first 6 h. The mean time required to exceed a cumulative catch of 1% was 143 min at 2 degrees C, 67 min at 4 degrees C, 56 min at 6 degrees C, and 40 min at 8 degrees C. At 4, 6, and 8 degrees C, the mean times required to exceed a cumulative catch of 5% were 103, 84, and 53 min, respectively. Infection of potted apple trees by ascospores at 2, 4, 6, and 8 degrees C required 35, 28, 18, and 13 h, respectively; substantially shorter times than previously were reported. In parallel inoculations of potted apple trees, conidia required approximately the same periods of leaf wetness as ascospores at temperatures from 2 to 8 degrees C, rather than the shorter times reported by Mills or the longer times reported in the revision of the Mills table. We propose the following revisions to infection period tables: (i) shorter minimum infection times for ascospores and conidia at or below 8 degrees C, and (ii) because both ascospores and conidia are often present simultaneously during the season of ascospore production and the required minimum infection times appear to be similar for both spore types, the adoption of a uniform set of criteria for ascosporic and conidial infection based on times required for infection by ascospores to be applied during the period prior to the exhaustion of the ascospore supply. Further revisions of infection times for ascospores may be warranted in view of the delay of ascospore discharge and the reduction of airborne ascospore doses at temperatures at or below 2 degrees C.     

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.     

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.     

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.     

Effects of cultivar resistance and fungicide application on stem canker of oilseed rape (Brassica napus) and potential interseasonal transmission of Leptosphaeria spp. inoculum

Phoma stem canker is a damaging disease of oilseed rape (Brassica napus) that causes annual yield losses to UK oilseed rape growers worth approximately £100 million, despite the use of fungicides. In the UK, oilseed rape is sown in August/September and harvested in the following July. The disease epidemics are initiated by ascospores released from Leptosphaeria spp. pseudothecia (ascocarps) on stem stubble in the autumn/winter. Control of this disease is reliant on the use of cultivars with “field resistance” and azole fungicides. This study investigated the effects of cultivar resistance and application of the fungicide prothioconazole on the severity of stem canker before harvest and the subsequent production of pseudothecia on the infected stubble under natural conditions in the 2017/2018, 2018/2019, and 2019/2020 cropping seasons. The application of prothioconazole and cultivar resistance decreased the severity of phoma stem canker before harvest, and the subsequent production of Leptosphaeria spp. pseudothecia on stubble in terms of pseudothecial density. Results showed that stems with less severe stem cankers produced fewer mature pseudothecia of Leptosphaeria spp. on the infected stubble. This investigation suggests that the most sustainable and effective integrated control strategy for phoma stem canker in seasons with low quantities of inoculum is to use cultivars with medium or good field resistance and apply only one spray of prothioconazole when required.     

Dead Spot Severity, Pseudothecia Development, and Overwintering of Ophiosphaerella agrostis in Creeping Bentgrass

ABSTRACT Dead spot (Ophiosphaerella agrostis) is a damaging disease of young 相似文献   

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

Sexually 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.     

Relationship between severity of blackleg (Leptosphaeria maculans/L. biglobosa species complex) and subsequent primary inoculum production on oilseed rape stubble

The relationship between severity of blackleg, or phoma stem canker ( Leptosphaeria maculans/L. biglobosa ), and subsequent primary inoculum production on oilseed rape ( Brassica napus ) stubble was investigated at two sites in France over 3 years. The quantity of primary inoculum produced in the following year increased with canker severity, from 1·9 to 10·8 pseudothecia cm⁻² on stubble with the least and most severe cankers, respectively. Stubble incubated at Le Rheu (cooler, more rain) had 1·7 times more pseudothecia than stubble incubated at Grignon. Stubble collected at Grignon had 2·7 times more pseudothecia than that collected at Le Rheu. The use of Darmor, a cultivar with a good level of quantitative resistance, reduced the severity of canker in the field, but not the subsequent inoculum production for stubble of the same canker severity class. At both sites, maturation of pseudothecia occurred after 63–75 days of incubation and increased with canker severity with a mean of 0·5 and 3% mature pseudothecia appearing per favourable day, on stubble with the least and most severe cankers, respectively. A simplified procedure for pseudothecial quantification proved satisfactory: for all three observers, most (91–96%) of the fructifications counted as pseudothecia were real pseudothecia. Only a few (4–14%) of the fructifications considered as non-pseudothecia were in fact pseudothecia of L. maculans . The total area occupied by pseudothecia, which was simpler and faster to evaluate, was correlated (coefficient of determination, R ² = 71%) with the number of counted pseudothecia. The results presented here make it possible to forecast the quantity of available primary inoculum for a given disease severity.