Factors Affecting the Release of Ascospores of Anisogramma anomala

ABSTRACT Relationships between environmental factors and release of ascospores of Anisogramma anomala, the causal agent of eastern filbert blight, were examined in four European hazelnut (Corylus avellana) orchards during a 2-year period. In each orchard, Burkhard volumetric spore traps and automated weather-monitoring equipment were deployed for 12-week periods beginning at budbreak, when hazelnut becomes susceptible to infection. Ascospores of A. anomala were released when stromata on the surface of hazelnut branches were wet from rain but not from dew. Release of ascospores ceased after branch surfaces dried. The duration of free moisture on branch surfaces regulated the initiation and rate of ascospore release, but no significant effects of temperature, relative humidity, wind, or light on ascospore release were apparent. Most (>90%) ascospores were captured during precipitation events that exceeded 20 h in duration, which represented about 10% of the total precipitation events each season. Quantitative relationships between the hourly capture of A. anomala ascospores and hours since the beginning of a precipitation event were developed. With the onset of precipitation, the hourly rate of ascospore capture increased until the fifth hour of rain, remained relatively constant between the fifth and twelfth hours, and then declined gradually. During the 12-week spore-trapping periods, the likelihood and rates of ascospore release associated with precipitation were highest at budbreak and then declined through April and May until early June, when the reserve of ascospores in the perithecia was depleted. Large numbers of ascospores were captured in the volumetric spore traps, indicating that ascospores may be commonly dispersed long distances on air currents as well as locally by splash dispersal within the canopy, as reported previously. The results indicate that monitoring seasonal precipitation patterns may be useful for estimating the quantity and temporal distribution of airborne inoculum during the period that the host is susceptible to infection.     

大豆茎溃疡病菌子囊壳诱导及孢子萌发方法探索

大豆茎溃疡病是为害大豆的重要病害,世界上十大大豆生产国中已有7个国家发现大豆茎溃疡病菌的分布,对当地农业造成严重威胁[1]。该病是我国进境植物检疫性有害生物[2],目前没有发生为害的报道。大豆茎溃疡病菌以菌丝和子囊壳在大豆植株及其残体上越冬,子囊壳在干燥35 d     

Detection and quantification of airborne inoculum of Hymenoscyphus pseudoalbidus using real‐time PCR assays

A method based on real‐time polymerase chain reaction (PCR) and the use of rotating‐arm spore traps was developed for quantifying airborne Hymenoscyphus pseudoalbidus ascospores. The method was sensitive and reproducible, and the collection efficiency was 10% of the spores present in the air. The temporal ascospore dispersal pattern was studied over 3 years by collecting spores every 15 days for a 24 h air‐sampling period during the ash‐growing season. The highest production was detected from the end of June to the beginning of September. The overall ascospore production did not differ significantly among stands within a specific year but there were differences from year to year. There was a positive correlation between air temperature and the number of ascospores trapped, with most of the positive samples being observed at temperatures above 12°C. The vertical profile of ascospore dispersal showed a strong decrease in ascospore density within a height of 3 m, regardless of date of collection. An analysis of the spore traps installed at increasing distances from an infected stand showed that most of the ascospores were deposited downwind within 50 m of the stand. These data are discussed in context of the epidemiology of the disease.     

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.     

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.     

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.     

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.     

Heterogeneity of the aerial concentration and deposition of ascospores of <Emphasis Type="Italic">Venturia inaequalis</Emphasis> within a tree canopy during the rain

Scab is an important disease of apple and its control depends almost exclusively on frequent use of fungicides. Primary scab infection in the spring assumes several steps: ascospore maturation, liberation of ascospores that become airborne, deposition on susceptible tissues, and infection. However, the spatial heterogeneity of ascospores within the tree canopy is unknown. Aerial concentration of ascospore (ACA), ascospore concentration in rain water (ACR) and ascospore deposition (AD) were therefore measured at six heights (20–257 cm from the ground) with rotating-arm air samplers, funnels, and greased glass slides, respectively, during five rain events in 2001 and in 2002. In addition, ACR and AD were measured at eight locations within tree canopy at 196 cm height. Apple scab was assessed at the end of the primary infection period in each sampling location within the apple tree. A similar experimental design was used in 2003 to study the spatial heterogeneity of both AD and primary scab lesions. ACA and AD decreased with increasing height, while ACR increased with increasing height. Based on both variance to mean ratio and the power law relationship in both years, the ACR was heterogeneous, while AD was heterogeneous only during the peaks of ascospore release. The ACR was significantly higher at the centre of the trees and the AD was significantly higher at the centre and at the western edge of the trees. Only the cumulative AD was significantly correlated with apple scab lesions at the same location (r = 0.83). In 2003, a similar pattern of spatial heterogeneity within the tree canopy was observed for AD and primary scab lesion counts and there was a linear relationship (R ² = 0.84) between these two variables. It was concluded that ACR and AD within the tree canopy are not randomly distributed at least during peaks of ascospore release and that AD is a good estimate of primary scab lesion development. This spatial heterogeneity should be considered when estimating ascospore deposition using mathematical models or when quantifying ascosporic inoculum using spore samplers.     

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.     

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.     

Production and immunodetection of ascospores of Mycosphaerella brassicicola: ringspot of vegetable crucifers

Pseudothecia containing abundant ascospores of Mycosphaerella brassicicola were produced in vitro on Brussels sprout decoction agar at 15°C under a 16-hour photoperiod of different light regimes. Spermogonia containing spermatia were also produced on the decoction agar. Ascospores were released when cultures were misted with SDW and placed under continuous light. Germination of ascospores was highest between 20°C and 25°C and spores remained viable at relative humidities above 93.5%. Exposure of ascospores to 55% relative humidity for 24 h reduced their germination to 75%. A polyclonal antiserum raised against whole ascospores was used to detect, by immunofluorescence, the ascospore and mycelial wall of M . brassicicola , following reaction with anti-rabbit IgG FITC conjugate. Autofluorescence of spore and mycelial components of other fungal species could be eliminated using the counterstains Evan's blue and eriochrome black at 0.2% and 0.5%, respectively, in phosphate buffered saline (pH 7.2). A procedure was developed to detect, by immunofluorescence, ascospores of M . brassicicola on artificially inoculated Melinex spore tape. Coating of the spore tape with bovine serum albumin provided a suitable support medium and blocking agent for detection of ascospores in the field. The potential use of the system for selective detection of ascospores of M . brassicicola in infected crops of vegetable brassicas in the presence of other ascosporic fungi is discussed. Keywords : ascospores, immunofluorescence, Mycosphaerella brassicicola , spore production, spore trapping .     

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.     

A mechanistic model simulating ascosporic infections by Erysiphe necator,the powdery mildew fungus of grapevine

A new dynamic model for Erysiphe necator ascosporic infections on grapevine was developed. Between budbreak of vines and the time when the pool of ascospores is depleted, the model uses weather data for calculating, at daily intervals: curve of ascospore maturation; ascospore discharge events and relative proportion of the discharged ascospores; infection periods and their relative infection severity; and progress of latency period and time when secondary infections should begin. The model was validated over a 4‐year period (2005–2008) in 26 vineyards in Italy by comparing model predictions with actual observations of the first seasonal symptoms of powdery mildew. The model showed high sensitivity, specificity and accuracy. Proportions of true and false positive predictions were TPP = 0·94 and FPP = 0·26, respectively. Because a proportion of predicted infection periods did not result in actual disease onset, confidence was higher for prediction of non‐infections than for prediction of infections. Most of the false positive predictions occurred in the earlier growth stages of the host, when the surface area of susceptible tissue may be very small so that the probability that ejected ascospores land on susceptible tissue is low. An equation was then developed to describe the probability that a predicted infection period results in disease onset as a function of the growth stage of vines at the time of prediction. The new model should improve early season powdery mildew management by helping vineyard managers schedule fungicide sprays or schedule the scouting of the vineyard for detection of first disease signs.     

Ascospore release by <Emphasis Type="Italic">Venturia inaequalis</Emphasis> during periods of extended daylight and low temperature at Nordic latitudes

Darkness suppresses ascospore release in Venturia inaequalis, but the impact of light levels during the extended twilight and dusk that typify Nordic spring conditions is poorly understood. Volumetric spore traps were operated at two different locations in Norway over several years. During the season of asocspore release (approximately 1 April to 30 June), on 25 occasions when rain started during night (after 23:00 h and before 04:00 h) and leaves remained wet until at least midnight the following day, the cumulative percentage of spores trapped at sunrise did not exceed 1%, irrespective of temperature. Three hours after sunrise, cumulative ascospore release reached 0.8%, 3.0%, and 8.1% at temperatures of 0 to 5°C, 5 to 10°C, and >10°C, respectively, and 50% release occurred at 11, 9, and 8 h after sunrise. Additional field and laboratory studies indicated that the protracted dawn and dusk of Nordic latitudes, either alone or in combination with low temperatures, does not substantially alter previously reported patterns of ascospore release.     

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.     

Effects of temperature,water regime and irrigation system on the release of ascospores of Mycosphaerella nawae,causal agent of circular leaf spot of persimmon

Experiments were conducted under controlled conditions to quantify the effects of temperature, water regime and irrigation system on the release of Mycosphaerella nawae ascospores from leaf litter in Spanish persimmon orchards. The effect of temperature on ascospore release was best described by a Gompertz model. The end of the lag phase of ascospore release occurred at 9·75°C, and the end of the exponential phase at 15·75°C. Few ascospores were discharged from dry leaves wetted with 0·1 or 0·5 mm water, but significant amounts were recovered with 1–50 mm water. About half of the total ascospores were released after three wetting and drying cycles, but 32 cycles were necessary for a complete discharge. No significant difference in ascospore release was detected when the leaf litter was wetted by flood and drip irrigation. However, considering the proportion of soil area wetted in both systems, inoculum release was significantly reduced by drip irrigation. The potential of drip irrigation as a cultural control measure should be investigated.     

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.     

Comparative pathogenicity of sexual and asexual spores of Zymoseptoria tritici (septoria tritici blotch) on wheat leaves

Zymoseptoria tritici ascospores and pycnidiospores are considered the main forms of primary and secondary inoculum, respectively, in septoria tritici blotch epidemics. The pathogenicity of the two types of spores of the same genotypic origin were compared through a two‐stage inoculation procedure in controlled conditions. Adult wheat leaves were inoculated with ascospores collected from field sources, yielding 119 lesions; pycnidiospores collected from 12 lesions resulting from these ascospore infections were then used for inoculation. Lesion development was assessed for 5 weeks; latent period, lesion size, and pycnidium density were estimated for different isolates. The latent period was calculated as the maximum likely time elapsed between inoculation and either the appearance of the majority of the sporulating lesions (leaf scale) or the appearance of the first pycnidia (lesion scale). The latent period was significantly longer (c. 60 degree‐days, i.e. 3–4 days) after infection with ascospores than with pycnidiospores. No difference was established for lesion size and density of pycnidia. A comparison with other ascomycete fungi suggested that the difference in latent period might be related to the volume of spores and their ability to cause infection. Fungal growth before the appearance of lesions may be slower after inoculation with an ascospore than with a pycnidiospore. The mean latent period during the very beginning of epidemics, when first lesions are mainly caused by ascospores, may be longer than during spring, when secondary infections are caused by pycnidiospores. Disease models would be improved if these differences were considered.     

Influence of Light, Relative Humidity, and Maturity of Populations on Discharge of Ascospores of Venturia inaequalis

ABSTRACT Ascospore release in 20 populations of Venturia inaequalis was generally suppressed in wind tunnel tests during darkness and simulated rain, but the following relieved this suppression: (i) exposure to low relative humidity during simulated rain and (ii) protracted incubation of leaf samples and the consequent senescence of the pathogen population. No counterpart to (i) was observed under orchard conditions. Although V. inaequalis also released a high percentage of ascospores during darkness in field studies under simulated rain late in the season of ascospore release, this phenomenon has not been reported for natural rain events. A threshold value of 0.5 muW/cm(2) at 725 nm was identified as the minimum stimulatory light intensity. Ascospore release increased with increasing light intensity from 0.5 to 5.2 muW/cm(2) at 725 nm. There was also an intrinsic increase in ascospore release as duration of rain increased. In orchards, the combined impact of both processes is probably responsible for a delay in reaching peak ascospore release at several hours after sunrise. Ascospore release during darkness will generally constitute a small proportion of the total available supply of primary inoculum. Significant ascospore release, and therefore infection periods, can be assumed to begin shortly after sunrise, when rain begins at night in orchards with low potential ascospore dose (PAD). A PAD level of 1,000 ascospores per m(2) of orchard floor per season is suggested as a threshold, above which the night-released ascospores should not be ignored.     

Modelling black Sigatoka epidemics with seasonal dispersal of Mycosphaerella fijiensis ascospores over a banana plantation in the Ribeira Valley,São Paulo,Brazil

Knowing the patterns of Black Sigatoka development is essential to propose adequate disease management practices and evaluate their effects, which can be achieved through temporal analysis by integrating the evolving interactions of the pathosystem components, expressed by data on cumulative incidence and severity, and summarizing these data in a disease progress curve. Airborne spores are essential components for the progression of an epidemic in the context of a specific pathosystem. In this perspective, the spore count is an essential approach to assess and model an epidemic. This study evaluated the temporal dynamics of Black Sigatoka in a banana plantation in the Ribeira Valley, state of São Paulo, Brazil, while simultaneously performing a year-long evaluation of fungal spore aerobiology. The disease was intense during the rainy season, but the leaf emergence rate was high enough for quickly inverting the severity peak (between 169 and 197 days of evaluation). After that, the disease severity raised until reach the higher rates (around the score 7 out of 8). The disease progress curve of Black Sigatoka showed peaks of extreme severity, one in the rainy and another in the dry season, with high levels of ascospores in the air. The ascospore concentration and the severity of the disease correlated significantly on the same day of sampling and 15 days after ascospore sampling, corresponding to the average latency period of the disease in the region. The patterns of the disease progress curve in both peaks fitted the monomolecular model, with higher rates of disease increase in the rainy season.