The Epidemiology of Purple Leaf Blotch on Leeks in Victoria, Australia

The incidence of purple leaf blotch disease was investigated on seven successive commercial leek crops grown at Cranbourne, Victoria between 1996 and 1997. First symptoms occurred on older leaves, 54–69 days after transplanting. Lesions with typical symptoms were colonised by either Alternaria porri (6%), Stemphylium vesicarium (42%) or mixtures of both pathogens (52%). Purple leaf blotch was caused by a disease complex and was endemic at nobreak Cranbourne due to the continuous cropping of leeks. Disease incidence in all monitored crops increased as plants matured (123–158 days after transplanting) until harvest but never exceeded 11% due to fortnightly applications of mancozeb. Disease levels showed no significant correlation with weekly temperature, precipitation, relative humidity or leaf wetness duration. Disease levels were significantly (P < 0.05) higher on autumn/winter (May/June) 1997 crops when 38 periods of leaf wetness 8 h because of dew and low temperatures (10–13 °C). The weekly rate of increase of disease incidence was significantly (P < 0.01) correlated with days after transplantation. nobreak Concentrations of airborne A. porri and S. vesicarium conidia within leek crops showed a diurnal periodicity and maximum numbers were trapped between 11:00 and 15:00 h. The concentration of airborne S. vesicarium conidia was three to six times the concentration of airborne A. porri conidia. Conidia were more abundant during spring/summer (September–February). Ascospores of Pleospora allii were found during May–September. The greater concentrations of airborne S. vesicarium conidia suggest that it may be the dominant pathogen in the purple leaf blight complex. Fungicide sprays were unnecessary until 8–10 weeks after transplanting, and regular protectant sprays curtailed but did not eradicate purple leaf blight. The results indicated that predictive models, based on temperature and the frequency of leaf wetness periods 8 h, will assist in reducing fungicide inputs as plants mature and, in southern Victoria, fungicide applications on leeks should be timed for autumn/winter when infection periods occur.     

田间空气中小麦白粉菌分生孢子的动态监测研究

2012和2013两年度采用Burkard定容式孢子捕捉器,对田间空气中小麦白粉病菌分生孢子的监测结果表明,小麦冠层内、外白粉菌分生孢子浓度存在显著的正相关性,冠层内的白粉菌分生孢子浓度明显高于冠层外;田间空气中分生孢子的浓度逐渐升高,到小麦灌浆期达到最大值之后逐渐降低。时间序列分析结果表明,两年度田间空气中白粉菌分生孢子浓度均符合ARIMA(1,1,0)模型且与温度有显著的相关性,建立了基于温度的白粉菌分生孢子浓度预测模型,模型回归效果均达到了显著水平。研究结果发现,田间白粉病病情与空气中病菌分生孢子和关键气象因子具有显著相关性,并在此基础上分别建立了基于空气中分生孢子浓度,以及基于分生孢子浓度和气象因子的田间白粉病病情预测模型,其中基于分生孢子浓度的预测模型普适性要优于基于分生孢子浓度和气象因子的预测模型,可以用来预测田间小麦白粉病的发生流行程度。     

Horizontal and vertical distribution of airborne conidia of Botrytis cinerea in a gerbera crop grown under glass

The horizontal and vertical distribution of airborne conidia ofBotrytis cinerea in a gerbera crop in two glasshouses (100 m² and 350 m²) was studied during 18 months in 1988 and 1989. Conidia ofB. cinerea were caught in simple spore traps consisting of agar in Petri dishes placed in a regular pattern at three different heights in the glasshouse and counted as colonies, after incubation. Lesions due to conidial infection were counted on gerbera petals. The horizontal and vertical distribution of conidia ofB. cinerea in a gerbera crop grown under glass was fairly uniform in both distinct glass-houses. Conidia ofB. cinerea trapped in a glasshouse can originate from sources inside and outside the glasshouse. No significant interaction was found between location and time for the colony counts and for the log transformed (ln(N+1)) lesion counts. The results of this study suggest that spore trapping at one height and at a limited number of locations and dates is sufficient for efficient monitoring ofB. cinerea in a glasshouse.     

Molecular methods for the detection and quantification of Pestalotiopsis and Neopestalotiopsis inoculum associated with macadamia

Pestalotiopsis blight and leaf spot in macadamia are caused by species of Pestalotiopsis and Neopestalotiopsis. Conidia produced by these pathogens are mostly dispersed by wind and rain splash and are the primary infecting propagules. Rapid detection and quantification of airborne conidia are essential to determine the seasonal dynamics of inoculum and critical infection periods in macadamia orchards. A quantitative PCR (qPCR) using a new primer pair and a hydrolysis probe was developed to detect and quantify airborne conidia of Pestalotiopsis and Neopestalotiopsis species. The amplification efficiency of the qPCR was 96.4% and the assay had a limit of detection of 400 fg of Pestalotiopsis/Neopestalotiopsis gDNA, which corresponds to approximately 10 conidia. The qPCR assay coupled with spore trapping was used to monitor airborne conidia dispersal under field conditions. The results showed that the assay is specific and sensitive for detecting and quantifying Pestalotiopsis/Neopestalotiopsis conidia in macadamia orchards. The detection and quantification of the pathogen inoculum will improve our understanding of disease epidemiology and the ability to manage these diseases in macadamia.     

Comparison of the epidemiology of ascochyta blights on grain legumes

Asochyta blights of grain legumes are caused by fungal pathogens in the genus Ascochyta. Different species infect the different legume species, and in pea three species including Phoma medicaginis var. pinodella have been implicated in ascochyta blight. The impact of the diseases varies between crops, countries, seasons and cropping systems, and yield loss data collected under well-defined conditions is scarce. However, ascochyta blights are considered major diseases in many areas where legumes are grown. Symptoms appear on all aerial parts of the plant, and lesions are similar for most of the species, except for M. pinodes and P. medicaginis var. pinodella. Infected seed, stubble and/or air-borne ascospores are major sources of primary inoculum. Their importance varies between species and also between regions. All Ascochyta spp. produce rain-splashed conidia during the cropping season which are responsible for the spread of the disease within the crop canopy. Only in pea are ascospores involved in secondary disease spread. Limited data suggests that Ascochyta spp. may be hemibiotrophs; however, toxins characteristic for necrotrophs have been isolated from some of the species. Modelling of ascochyta blights is still in the developmental stage and implementation of such models for disease forecasting is the exception.     

Effect of plant roots on the germination of microsclerotia ofVerticillium dahliae

Induction of germination of microsclerotia by exudates from plant roots may be important for the control ofV. dahliae. Laboratory experiments with root observation boxes were carried out to assess the influence of root tips of seven crop species and cultivars on the germination of microsclerotia ofVerticillium dahliae in soil under controlled conditions. The root density of crops was measured in a field experiment. The results of the laboratory experiments and the field experiment were combined to estimate the total effect of crops on the population of microsclerotia in the field. Germination of microsclerotia was stimulated by all crops compared to a control without a crop. Among crops, roots of potato cvs Element and Astarte had a larger stimulation effect on microsclerotia than that of potato Ostara, pea, flax, sugar beet or onion. The number of hyphae per microsclerotium decreased with distance from the root surface regardless of the crop species or cultivar. Differences in root densities, in the affected root zones and in the stimulation effect on germination of microsclerotia caused large differences among crops in the effect on the population of microsclerotia in the soil. However, growing a rop with the special purpose to reduce the level ofV. dahliae inoculum in the soil is an inefficient control measure, because only a small part of the total soil volume is affected by roots and the number of hyphae per microscleroium affected is too low.Abbreviations MS microsclerotia, microsclerotium     

Conidial density of <Emphasis Type="Italic">Monilinia</Emphasis> spp. on peach fruit surfaces in relation to the incidences of latent infections and brown rot

To evaluate the effect of conidial density of Monilinia spp. on the fruit surface on the incidence of latent infection and brown rot in peaches, eleven field surveys were performed in commercial orchards located in Cataluña, Spain over four growing seasons from 2002 to 2005, and nine surveys were conducted to determine the sources of overwintered Monilinia spp. inoculum. There was a significant positive relationship (r?=?0.69) between the numbers of conidia of Monilinia spp. on the fruit surface and the incidence of latent infections, but not with brown rot at harvest. Although mummified fruit, twigs and pits have been identified as being able to carry the pathogen from year to year in peaches grown in Spanish orchards, no relationships between any of these sources and the numbers of conidia on the fruit surface, or incidence of latent infection or brown rot were found. The effect of temperature (T), solar radiation (SR), rainfall (R) and wind speed (WS) on the area under the number of conidia of Monilinia spp. curve (AUncC) on peach surfaces was analysed. Regression analysis revealed that T, SR, R, and WS could account for 99% of the total variation in the area of the AUncC on peach surfaces. Thus, in order to reduce the incidence of latent infection and brown rot it is essential not only to remove the sources of primary inoculum but also to reduce the number of Monilinia spp. conidia on the fruit surface. Furthermore, the sources of airborne conidia of Monilinia spp. should be taken into consideration in disease management programmes in Spain.     

Effects of inoculum density,leaf wetness duration and nitrate concentration on the occurrence of lettuce leaf spot

In Ehime Prefecture, Japan, lettuce leaf spot (Septoria lactucae) caused huge losses in marketable lettuce yields. To explore potential measures to control disease outbreaks, the effects of inoculum density, leaf wetness duration and nitrate concentration on the development of leaf spot on lettuce (Lactuca sativa) were evaluated. Conidia were collected from diseased plants in an infested field by single-spore isolation and were used to inoculate potted lettuce plants with different conidial concentrations. Lesions developed on inoculated lettuce plants at inoculum concentrations from 10⁰ to 10⁶ conidia/ml. The disease was more severe when the inoculum exceeded 10² conidia/ml, and severity increased with increasing concentrations. Assessment of the relationship between disease development and the duration of postinoculation leaf wetness revealed that symptoms appeared when the inoculated plants remained wet for 12 h or longer. The number of lesions and total nitrogen content in the lettuce leaves both increased when nitrate was applied.     

Horizontal spread of beet necrotic yellow vein virus in soil

Horizontal dispersal of beet necrotic yellow vein virus (BNYVV) by means of viruliferous zoospores ofPolymyxa betae was studied in greenhouse experiments. BNYVV was not detected in roots of sugar beet plants grown in silver sand for 4 weeks at a root-free distance of 5 cm from eitherP. betae- and BNYVV-infected plants or BNYVV-infested soil. Spread of BNYVV from inoculum sources in the field was studied in the absence and presence of tillage practices. Active dispersal in combination with root growth from and towards point sources of inoculum contributed only little to horizontal dispersal of viruliferous inoculum and spread of disease during the season, as determined for one soil type, two different years and in the absence of tillage and tread. In the second beet crop after application of inoculum to whole field plots, more BNYVV-infected plants were detected at 2 m than at 8 m distance from the infested plots in the tillage direction. In the third year, disease incidence at 8 m was high and equivalent to that at 2 m.     

The role of seeds and airborne inoculum in the initiation of leaf blotch (Rhynchosporium secalis) epidemics in winter barley

Both airborne spores of Rhynchosporium secalis and seed infection have been implied as major sources of primary inoculum for barley leaf blotch (scald) epidemics in fields without previous history of barley cropping. However, little is known about their relative importance in the onset of disease. Results from both quantitative real‐time PCR and visual assessments indicated that seed infection was the main source of inoculum in the field trial conducted in this study. Glasshouse studies established that the pathogen can be transmitted from infected seeds into roots, shoots and leaves without causing symptoms. Plants in the field trial remained symptomless for approximately four months before symptoms were observed in the crop. Covering the crop during part of the growing season was shown to prevent pathogen growth, despite the use of infected seed, indicating that changes in the physiological condition of the plant and/or environmental conditions may trigger disease development. However, once the disease appeared in the field it quickly became uniform throughout the cropping area. Only small amounts of R. secalis DNA were measured in 24 h spore‐trap tape samples using PCR. Inoculum levels equivalent to spore concentrations between 30 and 60 spores per m³ of air were only detected on three occasions during the growing season. The temporal pattern and level of detection of R. secalis DNA in spore tape samples indicated that airborne inoculum was limited and most likely represented rain‐splashed conidia rather than putative ascospores.     

Sources and seasonal dynamics of inoculum for brown spot disease of pear

The dynamics of the production of Stemphylium vesicarium conidia and Pleospora allii ascospores from different inoculum sources on the ground were compared in a model system of a wildflower meadow mainly composed of yellow foxtail, creeping cinquefoil and white clover. The meadow was either inoculated (each October) or not inoculated with a virulent strain of S. vesicarium, and either covered or not covered with a litter of inoculated pear leaves. Spore traps positioned a few centimetres above the ground were exposed for 170 7-day periods between October 2003 and December 2006. Ascospores and conidia were trapped in 46 and 25% of samples, respectively. Ascospore numbers trapped from the pear leaf litter were about five times higher than those from the meadow, while conidial numbers were similar from the different inoculum sources. The ascosporic season was very long, with two main trapping periods: December–April, and August–October; the former was most important for the leaf litter, the latter for the meadow. The conidial season lasted from April to November, with 92% of conidia caught between July and September. The fungus persistently colonized the meadow: the meadow inoculated in early October 2003 produced spores until autumn 2006. The present work demonstrates that orchard ground is an important source of inoculum for brown spot of pear. Thus, it is important to reduce inoculum by managing the orchard ground all year long.     

Use of sclerotia of<Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> for efficient production of conidia of<Emphasis Type="Italic">Coniothyrium minitans</Emphasis> in liquid culture

A study was conducted to determine the feasibility of using sclerotia ofSclerotinia sclerotiorum for producing conidia ofConiothyrium minitans in liquid culture. The medium (SST) was made of water containing 2.0, 1.5, 1.0 or 0.5% (w/v) ground sclerotia ofS. sclerotiorum and 100 μgl ⁻¹ thiamine hydrochloride (HCl). One ml of conidial suspension (2 × 10⁷ conidia ml⁻¹) ofC. minitans LRC 2534 was inoculated into 100 ml of SST medium or control (thiamine HCl in water) and incubated at 20 ± 2°C on a shaker at 200 rpm. Subsamples were removed periodically and examined under a compound microscope. Conidia in the SST media germinated within 24 h, developed into branched hyphae within 48 h, produced pycnidia after 3–4 days, and the pycnidia released mature conidia after 7 days. Production of conidia varied with the concentration of sclerotia in the SST medium. It was lower (3.6 × 10⁶ conidia ml⁻¹) at 0.5% but higher (1.2 × 10⁸ conidia ml⁻¹) at 2%. The new conidia were viable and the colonies developing from them showed the original morphological characteristics. It was concluded that using SST liquid medium as a substrate for mass production of conidia ofC. minitans has potential for use in commercial development of this mycoparasite as a biocontrol product. http:www.phytoparasitica.org posting Jan. 23, 2007.     

Monitoring conidial density of <Emphasis Type="Italic">Monilinia fructigena</Emphasis> in the air in relation to brown rot development in integrated and organic apple orchards

In a three-year Hungarian study, conidial density of Monilinia fructigena in the air determined from mid-May until harvest was related to brown rot disease progress in integrated and organic apple orchards. Conidia of M. fructigena were first trapped in late May in both orchards in all years. Number of conidial density greatly increased after the appearance of first infected fruit, from early July in the organic and from early August in the integrated orchard. Conidial number continuously increased until harvest in both orchards. Final brown rot incidence reached 4.3–6.6% and 19.8–24.5% in the integrated and organic orchards, respectively. Disease incidence showed a significant relationship with corresponding cumulative numbers of trapped conidia both in integrated and organic orchards, and was described by separate three-parameter Gompertz functions for the two orchards. Time series analyses, using autoregressive integrated moving average (ARIMA) models, revealed that the temporal patterns of the number of airborne conidia was similar in all years in both integrated and organic orchards. Conidia caught over a 24-h period showed distinct diurnal periodicity, with peak spore density occurring in the afternoon between 13.00 and 18.00. Percent viability of M. fructigena conidia ranged from 48.8 to 70.1% with lower viability in dry compared to wet days in both orchards and all years. Temperature and relative humidity correlated best with mean hourly conidial catches in both integrated and organic apple orchards in each year. Correlations between aerial spore density and wind speed were significant only in the organic orchard over the 3-year period. Mean hourly rainfall was negatively but poorly correlated with mean hourly conidial catches. Results were compared and discussed with previous observations.     

The Distribution and Spread of Sorghum Downy Mildew in Sorghum and Maize Fields in Nigeria and Zimbabwe

Sorghum downy mildew (Peronosclerospora sorghi, SDM) is a damaging disease of sorghum and maize crops in Africa. Runs analysis was used to study the distribution of systemically infected sorghum and maize plants in Nigeria and Zimbabwe. The temporal and spatial development of local lesions of SDM on sorghum in Zimbabwe was investigated by assessing the local lesion symptoms caused by conidia in plots with a single point source of inoculum. With ordinary runs analysis, there was evidence of clustering of disease in some fields in the humid areas of Nigeria and the semi-arid areas of Nigeria and Zimbabwe. Clustering was found in two of the eight runs analyses performed on maize in the humid south of Nigeria, and in only one of the eight runs in Zimbabwe, which was interpreted as a predominance of random infection at the time of assessment and at the spatial scales assessed. Symptoms of local lesions of SDM developed rapidly across plots from an introduced point source of infection. After 9 days-exposure to the source of inoculum, the incidence of diseased leaves was 1.2%, and after 50 days it was 74.5%. A disease gradient which initially developed flattened as the plot became uniformly diseased. The predominant wind direction was NNE, and most rapid spread of disease was towards the SSW and WSW. In conclusion, local lesions can spread rapidly in sorghum crops, suggesting that they may be an important source of conidial inoculum for further local and systemic infections during the growing season.     

Forecasting the spread of aerially transmitted crop diseases with a binary classifier for inoculum survival

The risk of between‐field spread of disease is typically omitted from crop disease warning systems, as it is difficult to know the number and location of inoculum sources and thus predict the abundance of inoculum arriving at healthy crops. This study explores the utility of a simple approach to predicting risk of between‐field spread, based on the estimated probability that inoculum will survive the transportation process. Using potato late blight as a case study, the effect of solar radiation on the viability of detached Phytophthora infestans sporangia was assessed. A model to estimate the probability of spore survival was derived using a binomial generalized linear mixed model (GLMM), and receiver operating characteristic (ROC) curve analysis and cross‐validation were used to evaluate the global performance of the model as a binary classifier for discriminating between viable and nonviable sporangia. The model yielded an area under the ROC curve of 0.92 (95% CI = 0.90–0.93), signifying an excellent classification algorithm. Inspection of the curve provided a number of suitable decision threshold (or cut‐off) probabilities for discriminating between viable and nonviable sporangia. The classifier was tested as a forecasting system for potato late blight outbreaks using 10 years of outbreak data from across Great Britain. There was a marked differentiation among the cut‐offs, but the best prediction outcome was an accuracy of 89% with an alert frequency of 1 in 7 days. This model can be easily modified or the methodology replicated for other pathosystems characterized by airborne inoculum.     

Studies on inoculum sources of angular leaf spot of beans caused by Phaeoisariopsis griseola in Uganda

Abstract

Angular leaf spot of beans caused by Phaeoisariopsis griseola is a major problem on this crop in Eastern Africa. The sources of inoculum for this disease were investigated. The causal fungus was confirmed as seedborne in all the cultivars tested. The fungus caused seed discolouration but not all infected seeds were discoloured. Seed to seedling transmission was low. The fungus survived in infected crop debris for a maximum of nine and four to six months under indoor and outside conditions respectively. Under soil, the fungus survived for only two months. Infected offseason crops and volunteer plants were present at the time of planting the seasons’ crops and were an obvious source of the inoculum. It is concluded that the seed, crop debris, off‐season crops and volunteer plants are all possible sources of P. griseola infection under the local conditions.     

Conidial dispersal by Alternaria brassicicola on Chinese cabbage (Brassica pekinensis) in the field and under simulated conditions

This study investigated conidial dispersal in the field, and effects of simulated wind and rain on the dispersal of A. brassicicola on Chinese cabbage ( Brassica pekinensis ). Spores were sampled using a Burkard volumetric spore sampler and rotorod samplers in a Chinese cabbage crop. Disease incidence in the field was well fitted by a Gompertz curve with an adjusted r ² of >0·99. Conidia of A. brassicicola were trapped in the field throughout the growing season. Peaks of high spore concentrations were usually associated with dry days, shortly after rain, high temperature or high wind speed. Diurnal periodicity of spore dispersal showed a peak of conidia trapped around 10·00 h. The number of conidia trapped at a height of 25 cm above ground level was greater than that at 50, 75 and 100 cm. Conidial dispersal was also studied under simulated conditions in a wind tunnel and a rain simulator. Generalized linear models were used to model these data. The number of conidia caught increased significantly at higher wind speeds and at higher rain intensities. Under simulated wind conditions, the number of conidia dispersed from source plants with wet leaves was only 22% of that for plants with dry leaves. Linear relationships were found between the number of conidia caught and the degree of infection of trap plants.     

Detection of Pseudomonas syringae pv. aptata in Irrigation Water Retention Basins by Immunofluorescence Colony-staining

Bacterial blight of cantaloupe (Cucumis melo) caused by Pseudomonas syringae pv. aptata was first observed in south-western France and has since spread to all cantaloupe-growing areas of this country. Use of pesticides registered for this disease has proved ineffective and no commercial cultivars of cantaloupe are resistant to this blight. To develop control strategies for this disease, the principal sources of inoculum were investigated. Among the different sources of inoculum studied, we report the isolation of P. syringae pv. aptata from irrigation water retention basins in south-western France using the immunofluorescence colony-staining (IFC) method. In this study, the pathogen was detected at a low concentration (12 and 70cful^–1) in two different retention basins. These results suggest that P. syringae pv. aptata can survive in water used to irrigate cantaloupe crops and could be a source of inoculum for epidemics of bacterial blight. To develop control strategies for this bacterial disease, the importance of water retention basins as sources of inoculum for bacterial blight of cantaloupe needs to be evaluated relative to other potential sources such as seeds, plants from nurseries and plant debris in the soil.     

Coffee Berry Disease

Abstract

Coffee Berry disease is caused by a mutant pathogenic strain of Colletotrichum coffeanum Noack which has spread throughout tropical Africa from its point of origin in Kenya. Fruit is attacked in all stages, and total destruction of the crop can be caused. The disease was confined for many years to the higher altitudes, because suitable conditions of temperature and humidity were infrequent in the lower ones. Following the inception of a wetter and cooler climatic phase in E. Africa in 1961, coffee in the lower altitudes is now attacked.

Infection of the fruit is initially by infective conidia produced on the maturing wood of the twigs, where the pathogen “overwinters”. When one crop only is on the tree, this inoculum is more important than that from lesioned fruit; but where a series of over-lapping crops is present, the reverse is the case. In the former situation control can be based on reduction of inoculum from the twigs: in the latter, however, this is ineffective, and control can only be achieved by a series of protective sprays.

Varieties of coffee vary greatly in susceptibility to berry disease. No commercially acceptable variety is, however, completely resistant, although genetic material embodying resistance is available.     

Influence of environmental conditions in a glasshouse on conidia ofBotrytis cinerea and on post-harvest infection of rose flowers

Quantification and horizontal distribution of air-borne inoculum ofBotrytis cinerea in a rose crop in a glasshouse of 300 m² was studied in 1991 and 1992. Conidia ofB. cinerea were caught in spore traps consisting of an agar medium selective forB. cinerea in Petri dishes placed within the crop, at flower height 1 m above the ground. Spore catches were counted as colonies, after incubation. Lesions due to conidial infection were counted on petals of rose flowers, also after incubation. Relative humidity (RH) and temperature within the glasshouse and global radiation and windspeed outside were recorded during the experiments. The horizontal distribution ofB. cinerea in a rose crop grown under glass was fairly uniform in both years. In 1991 a clear seasonal pattern in the number of colonies could not be found. In 1992 the number of colonies were high in August, September and October. The number of lesions on rose flowers showed a distinct pattern in both years. In August, September and October many lesions were counted whereas in the other months few lesions appeared. In linear regression analysis, variation in numbers of colonies (spore catches) could not be explained by environmental factors recorded during the experiments. Linear regression accounted for 76 and 63% of the variation in the number of lesions on rose flowers in 1991 and 1992, in terms of relative humidity (positively correlated), global radiation outside the glasshouse (negatively correlated) and, numbers of colonies on spore traps (positively correlated). The results in the rose crop suggest that RH, global radiation and spore density in glasshouses are important variables in regulating the numbers of lesions during storage and transport. The numbers of spores in glasshouses are dependent on the production system. A glasshouse with a system resulting in wet dead tissue on the ground give higher amount of spores in the glasshouse air and through that high numbers of lesions on flowers. On roses outside the glasshouses very high numbers of lesions were counted sometimes, mostly during and after rain showers, as a result of rain-deposition of spores onto the flowers.