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.     

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.     

Effect of Rain Distribution Alteration on Splash Dispersal of Colletotrichum acutatum

ABSTRACT Splash dispersal of Colletotrichum acutatum conidia from infected strawberry fruit was assessed using a rain simulator to determine the properties of rain (e.g., intensity [millimeters/hour] or drop size distribution) most related to dissemination. Dispersal with a simulated rain corresponding to a natural rain of about 11 mm/h was compared with dispersal of three other simulated rains that had larger and smaller drop sizes, on average, than idealized natural rains. Splash droplets were collected in sheltered petri plates with a selective medium for Colletotrichum, and colonies formed from conidia entrained in the droplets were counted and used as the measure of dispersal. Colonies were mostly confined to a 27-cm radius from the source, and density of colonies decreased exponentially with the distance squared, as indicated by the fit of a diffusion-type model to the data. Splash dispersal was more affected by drop size distribution than rain intensity or other properties of the generated rains. That is, there was a direct positive relationship between total colonies over 61 min of rain for a circular area with a 72-cm radius (Sigma) and the mass (volume) median diameter of impacting drops (D(0)') for four rain-simulation treatments. In a separate study, strawberry fruit were exposed to the same four simulated rains at two distances from a point source and for two rain durations. Although the proportion of infected fruit (y) increased with time and decreased with distance, rain treatment did not significantly affect y, as predicted based on past work with a wide range of intensities of simulated rains.     

Rain-induced dispersal in Puccinia arachidis,studied by means of a rainfall simulator

A rainfall simulator was used on groundnut (Arachis hypogaea) plots artificially infected withPuccinia arachidis in order to study urediniospore dispersal caused by various amounts of rainfall. Several trapping method were used to compare dry, splash and drip dispersal caused by rain. The spore liberation mechanisms activated by rain and the flows of spores which they generate can be studied at the infected canopy or at the sporulating lesion scales. Two systems referring to these scales were considered to discuss the results. Dry dispersal seemed preponderant. The spore content of the lesions in the canopy was strongly reduced by rain. The results indicated that light rain showers may promote disease dispersal, whereas it may be hampered by heavy showers.Samenvatting Een regensimulator, een krachtig beregeningsapparaat, werd ingezet boven een aantal aardnoot-(Arachis hypogaea) veldjes geïnoculeerd met de aardnootroest (Puccinia arachidis) om de uredosporenverspreiding te bestuderen bij verschillende hoeveelheden regenval. Diverse vangtechnieken werden toegepast om droge verspreiding, spatverspreiding en druipverspreiding ten gevolge van regen te vergelijken. De sporeninhoud van de sporenhoopjes op het gewas werd door regen sterk verminderd. De resultaten, verkregen in twee verschillende, maar elkaar aanvullende analyses, geven aan dat lichte regenbuien de ziekteverspreiding bevorderen, terwijl zware buien deze zouden afremmen.     

Rain Splash Dispersal of Gibberella zeae Within Wheat Canopies in Ohio

ABSTRACT Rain splash dispersal of Gibberella zeae, causal agent of Fusarium head blight of wheat, was investigated in field studies in Ohio between 2001 and 2003. Samplers placed at 0, 30, and 100 cm above the soil surface were used to collect rain splash in wheat fields with maize residue on the surface and fields with G. zeae-infested maize kernels. Rain splash was collected during separate rain episodes throughout the wheat-growing seasons. Aliquots of splashed rain were transferred to petri dishes containing Komada's selective medium, and G. zeae was identified based on colony and spore morphology. Dispersed spores were measured in CFU/ml. Intensity of splashed rain was highest at 100 cm and ranged from 0.2 to 10.2 mm h(-1), depending on incident rain intensity and sampler height. Spores were recovered from splash samples at all heights in both locations for all sampled rain events. Both macroconidia and ascospores were found based on microscopic examination of random samples of splashed rain. Spore density and spore flux density per rain episode ranged from 0.4 to 40.9 CFU cm(-2) and 0.4 to 84.8 CFU cm(-2) h(-1), respectively. Spore flux density was higher in fields with G. zeae-infested maize kernels than in fields with maize debris, and generally was higher at 0 and 30 cm than at 100 cm at both locations. However, on average, spore flux density was only 30% lower at 100 cm (height of wheat spikes) than at the other heights. The log of spore flux density was linearly related to the log of splashed rain intensity and the log of incident rain intensity. The regression slopes were not significantly affected by year, location, height, and their interactions, but the intercepts were significantly affected by both sampler height and location. Thus, our results show that spores of G. zeae were consistently splash dispersed to spike heights within wheat canopies, and splashed rain intensity and spore flux density could be predicted based on incident rain intensity in order to estimate inoculum dispersal within the wheat canopy.     

Effects of a Cover Crop on Splash Dispersal of Colletotrichum acutatum Conidia

ABSTRACT A rain simulator, with generated rains of 11 and 30 mm/h, was used to determine the effect of a cover crop or intercrop on the splash dispersal of Colletotrichum acutatum conidia. Dispersal through sudangrass, which can be used as a 'living mulch', was tested at two planting densities (140 or 280 kg/ha) and two heights (5 and 20 cm) and compared with a control consisting of a bare soil. Dispersal of C. acutatum conidia was assessed by counting colonies formed from spore-bearing splash droplets deposited in sheltered petri plates containing a selective medium. Both a cover crop and rain intensity significantly affected splash dispersal as measured by the interpolated total number of colonies (denoted by Sigma) from 0 to 72 cm from the inoculum source and in a time span of 61 min of generated rain (P < 0.001). However, there was no significant interaction of cover crop and intensity (P > 0.90). Dispersal with a 30-mm/h rain was higher than dispersal with a 11-mm/h rain, and presence of a cover crop significantly reduced dispersal compared with bare soil (P < 0.001). Of the treatments with sudangrass, cover crop planting density did not affect dispersal overall, but there was greater spore dispersal with the taller sudangrass at the higher planting density, due in part to the higher rate of water splashing with the tall grass compared with the short grass. Spore deposition in the petri plates could be functionally related to distance and time using a diffusion-type model, and parameter estimates could be used to explain the effects of cover crop on Sigma. Although the relationship between cover crop properties and splash dispersal is complex, results show the potential beneficial effects of the cover crop on disease management.     

The role of rain in dispersal of the primary inoculum of Plasmopara viticola

Although primary infection of grapevines by Plasmopara viticola requires splash dispersal of inoculum from soil to leaves, little is known about the role of rain in primary inoculum dispersal. Distribution of rain splashes from soil to grapevine canopy was evaluated over 20 rain periods (0.2 to 64.2 mm of rain) with splash samplers placed within the canopy. Samplers at 40, 80, and 140 cm above the soil caught 4.4, 0.03, and 0.003 drops/cm(2) of sampler area, respectively. Drops caught at 40 and 80 cm (1.5 cm in diameter) were larger than drops at 140 cm (1.3 cm). Leaf coverage by splashed drops, total drop number, and drop size increased with an increase in the maximum intensity of rain (mm/h) during any rain period. Any rainfall led to infection in potted grapevines placed outside on leaf litter containing oospores, if the litter contained germinated oospores at the time of rain; infection severity was unrelated to rain amount or intensity. Results from vineyards also indicate that any rain can carry P. viticola inoculum from soil to leaves and should be considered a splash event in disease prediction systems. Sampling for early disease detection should focus on the lower canopy, where the probability of splash impact is greatest.     

Quantifying and Modelling the Mobilisation of Inoculum from Diseased Leaves and Infected Defoliated Tissues in Epidemics of Angular Leaf Spot of Bean

Daily multiplication factor (number of daughter lesions per mother lesion per day) values were experimentally measured in four replications of a monocyclic experiment on angular leaf spot (ALS) of bean, where sources of inoculum were artificially established within a bean canopy, on the ground (defoliated infected leaves), or both. Daily multiplication factor of lesions in the canopy (DMFRc) was higher than that of infectious, defoliated tissues (DMFRd) in all replications. Both DMFRc and DMFRd were strongly reduced under dry compared to rainy conditions. Under rainy conditions for spore dispersal DMFRd was about two to three times smaller than DMFRc. Defoliated leaves may nevertheless represent a significant source of infection, depending on the amount of infectious tissues. Mother lesions within the canopy generated more daughter lesions in the medium (or lower) layers of the canopy than at its upper level (DMFRc higher at the medium and lower layers of a canopy), whereas DMFRd values seemed to decrease with height in the canopy. A mechanistic simulation model that combines host growth and disease-induced defoliation was designed to simulate the respective contributions of the two components of the dual inoculum source of a diseased canopy (infected foliage and defoliated infectious tissues), and varying infectious periods in both sources. Simulations suggest that higher DMFRc values have a large polycyclic effect on epidemics whereas that of DMFRd is small, and that large effects of the infectious period of lesions in the canopy are found when DMFRc is high. Simulations using experimentally measured DMFRc and DMFRd values indicated much stronger epidemics in rainy compared to dry conditions for spore dispersal, but disease persistence in the latter. The implications of considering a dual source of inoculum in the course of a polycyclic process are discussed with respect to epidemic thresholds.     

古尔班通古特沙漠羽毛针禾（Stipagrostis pennata）种群种子雨特征

通过布设种子收集器收集种子,对收集的种子以时空差异进行分组分析,探讨羽毛针禾[WTBX]（Stipagrostis pennata）[WTBZ]种子散布规律以及古尔班通古特沙漠种群的种子雨特征。结果表明：① 羽毛针禾种子雨的积累密度平均达到3 766.30粒·株-1,其中饱满种子占19.56%;② 种子散布的高峰集中在6月25到7月15日,其落种量占整个种子雨的61.96%,其后种子雨密度随时间逐渐减小;③ 种子雨密度与生境之间均不存在显著性差异;④ 种子雨总量与其年龄之间存在显著性差异;⑤ 种子雨的前扩散过程中种子集中降落在背风方向4 m范围和株丛周围1 m内。古尔班通古特沙漠南缘种子雨虽质量不高,但密度大,对种群的自然更新具有一定的意义。     

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.     

Effects of Single-Drop Impactions and Natural and Simulated Rains on the Dispersal of Botryosphaeria dothidea Conidia

ABSTRACT Laboratory and field experiments were conducted to study the dispersal of Botryosphaeria dothidea conidia using single-drop impactions and natural and simulated precipitations. For laboratory studies, 200 single drops were released from a height of 1 m on infected pistachio nuts. On pieces of photographic film, 50% of the droplets were collected within 20 mm (average droplet travel distance) of the target area, and the droplets ranged from 0.041 to 3.19 mm in diameter, with an average of 0.3 mm. Each droplet carried an average of 23 B. dothidea conidia. In 3 years of field experiments, rainwater was collected in funnels connected to bottles positioned at different heights inside the tree canopy and at different distances away from the edge of tree canopy in three commercial pistachio orchards in San Joaquin, Yolo, and Glenn counties in California. Numbers of conidia in rainwater varied among and within sampling seasons by sampling dates and orchards. Up to 67,000 conidia/ml were obtained in rainwater samples collected from an orchard in Yolo County. Rainwater from orchards in Yolo and Glenn counties contained a consistently higher number of conidia than rainwater collected from the orchard in San Joaquin County. Variation in numbers of conidia also existed among heights where bottles were located. There were significantly more conidia in rainwater collected inside than outside tree canopies. Inside tree canopies, bottles located at 100 and 150 cm above ground collected more B. dothidea conidia than those placed at 50 and 200 cm. Conidia were collected as far as 1 m from the tree canopy edge. Based on data from the Glenn County orchard, a linear relationship between number of conidia (Y) and rainfall amount (X) in millimeters was determined as Y = 240X - 3,867, with r(2) = 0.91, which meant that a minimum of 16.1 mm of rain was needed to disperse conidia of B. dothidea. The power law model best described the dispersal gradients of B. dothidea propagules in the 1999-2000 and 2001-02 sampling seasons, with r(2) values of >/=0.73, whereas the exponential law model fit best for the 2000-01 data, with r(2) values of >/=0.81. In a rain simulation experiment, the intensity of the rain generated by a nozzle at 138 kPa of pressure inside the tree canopy was approximately five times higher than rain recorded outside the tree canopy. Rain removed up to 65% of conidia from infected fruit. These results confirmed that B. dothidea is a splash-dispersed pathogen with relatively short distances of spore dispersal within pistachio orchards. Only pycnidia are present in pistachio orchards; therefore, the results also indicate that inoculum of B. dothidea should be entirely splashed dispersed.     

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

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

Calibration and evaluation of an electronic sensor for rainfall kinetic energy

ABSTRACT A novel sensor for measuring the kinetic energy of impacting raindrops, developed based on a soil-mass erosion sensor, was tested in the laboratory, with a rain simulator, and in the field. Drop impactions on the sensor-consisting of a piezoelectric crystal and associated electronics-produce an electrical charge that equals a fixed amount of energy. Calibration of the sensor was done in the laboratory using water drops of known diameter impacting with known velocity, and thus, with known kinetic energy. The relationship between pulse-count output of the sensor minus the background pulse counts when no drops were impacting (O; per min) and kinetic energy flux density (i.e., power [P; mJ cm(-2) min(-1)]) was found to be described by the formula P; = (0.204 + 0.065 . O)(0.67). The measurement threshold was 0.34 mJ cm(-2) min(-1). Using the sensor, generated rains with intensities of 23 to 48 mm/h were found to have powers of 0.4 to 2.2 mJ cm(-2) min(-1). In 2 years of field testing, 85 individual rain episodes were monitored, with mean intensities ranging from 0.1 to 42 mm/h. These rains had mean powers ranging from 0 to 5 mJ cm(-2) min(-1), and the highest power for a 5-min sampling period was 10 mJ cm(-2) min(-1). Both power and intensity varied greatly over time within rain episodes, and there was considerable variation in power at any given rain intensity, emphasizing the importance of measuring rather than simply predicting power. Although there was no known true power measurements for the generated or natural rains, estimates were realistic based on theoretical calculations, assuming that the gamma distribution represents raindrop sizes. The sensor is important in assessing the risk of rain splash dispersal of plant pathogens.     

Effects of wind, relative humidity, leaf movement and colony age on dispersal of conidia of Uncinula necator, causal agent of grape powdery mildew

A wind tunnel was designed to study the effect of wind, relative humidity, leaf movement and colony age on dispersal of conidia of Uncinula necator . Wind speed as low as 2.3 m s⁻¹ instantaneously triggered dispersal of conidia from fixed leaf discs of 18-day-old infections. Conidia were observed on sporulating leaf discs even after exposure to 17 m s⁻¹ wind. The fraction of conidia dispersed at a given wind speed increased with colony age from 12 to 24 days. Conidia of 27-day-old colonies were less easily dispersed. No gradient of maturation of conidia along the conidial chain was observed, suggesting that even newly formed conidia were able to germinate after dispersal. Germination of dispersed conidia decreased slightly with greater colony age. Both wind and simulated rain drops caused dispersal of conidia from infected leaves. Leaf movement at wind speed of 3.5–4 m s⁻¹ increased dispersal, and the first impact of three simulated raindrops caused release of 53% of the total conidia dispersed. Relative humidity had no effect on dispersal of conidia at different wind speeds.     

Influence of environmental factors on field concentrations of<Emphasis Type="Italic">Alternaria solani</Emphasis> conidia above a South African potato crop

Trends in weather variables and concentrations of airborne conidia ofAlternaria solani were monitored in a potato field in South Africa during three potato-growing seasons in 2001 and 2002. Distinct seasonal variation was noted, with a drop in spore numbers during winter. Peaks in spore concentration coincided with periods favorable for spore formation and dispersal; most notable was the effect of interrupted wetting periods. Diurnal periodicity of spore dispersal was also observed, with the peak of spore concentrations between 9h00 and 18h00. Few spores were sampled at night, when wind velocity and temperature are lowest and relative humidity is highest. Increased numbers of spores were sampled during days of harvesting or when other ground-operated farm equipment was used. The results obtained in this study will be useful in establishing decision support systems to control early blight on potatoes in southern Africa. http://www.phytoparasitica.org posting July 10, 2003.     

莱阳地区梨锈病防治适期研究

在山东莱阳地区,梨锈病菌冬孢子角自梨花现蕾期开始陆续成熟,成熟的冬孢子角遇1.5 mm/d以上的降雨,萌发产生担孢子。经2～3次降雨后,冬孢全部萌发,以后不再产生担孢子。梨树初花后35 d内对锈病最敏感,即为防治期。若该期内降雨量大于1.5 mm/次、叶面结露时间长于7 h/次,担孢子即可侵染。其侵染量与梨园周围越冬菌原的数量、降雨持续时间和品种抗病性等因子有关。田间试验表明:担孢子侵染后至病斑显症前(约7 d)是内吸性杀菌剂防治的有效时期。病斑显症后只有少数杀菌剂能抑制病斑扩展,20.67%氟硅唑乳油、10%苯醚甲环唑水分散粒剂和15%三唑酮可湿性粉剂3种药剂在病原菌侵入前喷施能有效地抑制担孢子侵染,持效期约7 d;担孢子侵染后喷药能有效抑制病斑显症,因此,要特别关注梨树初花时第1次降雨后梨锈病的防治。     

Sprinkler irrigation and plant disease under semi-arid climatic conditions1

The effect of overhead irrigation on plant disease can conveniently be discussed under the following headings. Spore dispersal: irrigation effects splash dispersal and provides the necessary moisture. Under semi-arid conditions, spore dispersal takes place during the morning hours. Irrigation early in the day will aid spore dispersal. Spore resistance: irrigation will modify microclimatic conditions and reduce the stress of exposed spores to radiation, heat and dryness. Moisture period required for infection: leaf surface temperatures will determine the moisture period required for infection. Irrigation timing in relation to moisture provided by rainfall or dew may be critical. The macroclimate: the less favourable the macroclimate, the greater is the importance of microclimatic modifications, and its effect on epidemiology. Microclimatic modifications: the lower the relative humidity during the day and the shorter the period of leaf wetness (dew), the greater will be the impact of sprinkling. Foliar density interacts in determining the microclimate. The later the spore dispersal during the morning, the smaller will the effect of sprinkling be. The greater the resistance of the spores to stress conditions during the day, the smaller the effect of sprinkling. The shorter the period required for host penetration, the more pronounced will be the effect of overhead irrigation.     

Temporal spore dispersal patterns of grapevine trunk pathogens in South Africa

Trunk disease pathogens of grapevines, viz. Phaeomoniella chlamydospora, Eutypa lata and several species in Botryosphaeriaceae, Phaeoacremonium and Phomopsis are known to infect fresh pruning wounds by means of air-borne inoculum released after rainfall or prolonged periods of high relative humidity. Recent surveys have demonstrated that most or all of these pathogens are present in climatically diverse grape growing regions of South Africa. However, the factors controlling spore dispersal of these pathogens in vineyards were largely unknown. To address this question, spore trapping was done in a Chenin Blanc vineyard in the Stellenbosch area, South Africa, for 14 weeks during the grapevine pruning period from June to mid-September of 2004 and 2005. Hourly recordings of weather data were done by a weather station in the row adjacent to the spore trap. Spores of E. lata and Phomopsis and species in Botryosphaeriaceae were trapped throughout the trapping periods of 2004 and 2005, with higher levels of trapped spores recorded in 2005. The spores of all three pathogens were trapped during or after periods of rainfall and/or high relative humidity. In neither of the 2 years were spores of Pa. chlamydospora or Phaeoacremonium spp. trapped. Results indicated that spore event incidence, as well as the amount of spores released during a spore event of above-mentioned pathogens, were governed by rainfall, relative humidity, temperature and wind speed prior to and during the spore events.     

Spatial and temporal increase of eastern filbert blight in European hazelnut orchards in the pacific northwest

ABSTRACT Since its first detection in southwest Washington state 30 years ago, eastern filbert blight, caused by Anisogramma anomala, has spread slowly southward ( approximately 2 km/year) into the Willamette Valley of Oregon, an important hazelnut production region. Experiments were conducted to measure gradients of disease spread, rates of disease increase as affected by distance from an inoculum source and variation in host plant resistance, and dispersal of ascospores of A. anomala from diseased orchards. In each of 3 years, 1-year-old hazelnut trees placed from 0 to 150 m north of diseased orchards were infected uniformly and slopes of disease gradients were not significantly different from zero. In 1 year when trees also were placed south of an orchard, the disease gradient was significant (P < 0.05), with disease incidence high at the edge of the orchard and few trees infected at 10 m south of the orchard. Disease gradients were shallower and the magnitude of the area under the disease progress curve (AUDPC) greater in 0.1-ha mini-orchards of highly susceptible cv. Ennis than in mini-orchards of moderately susceptible cvs. Barcelona or Casina. Lower AUDPC values were observed in mini-orchards of Barcelona interplanted with a moderately resistant pollenizer Hall's Giant compared with the highly susceptible pollenizer Daviana. Fungicides applied biweekly starting at bud break reduced AUDPC values in Ennis mini-orchards to values observed in Barcelona and Casina mini-orchards. Data from aspirated spore samplers placed on towers adjacent to severely diseased hazelnut orchards indicated that spores of A. anomala dispersed horizontally and vertically away from the canopy during periods of extended branch wetness and, thus, show potential to be transported long distances in wind currents. Weather patterns in the Pacific Northwest may account for the relatively slow, southward spread of eastern filbert blight within Oregon's Willamette Valley. Of 196 precipitation events greater than 10 h in duration recorded from 1974 to 1995, conditions most favorable for ascospores discharge, periods with wind from the north were rare, representing <6% of total hours.     

Production of conidia by Peronosclerospora sorghi on sorghum crops in Zimbabwe

Factors affecting the production of conidia of Peronosclerospora sorghi , causing sorghum downy mildew (SDM), were investigated during 1993 and 1994 in Zimbabwe. In the field conidia were detected on nights when the minimum temperature was in the range 10–19°C. On 73% of nights when conidia were detected rain had fallen within the previous 72 h and on 64% of nights wind speed was < 2.0 m s⁻¹. The time period over which conidia were detected was 2–9 h. Using incubated leaf material, conidia were produced in the temperature range 10–26°C. Local lesions and systemically infected leaf material produced 2.4–5.7 × 10³ conidia per cm². Under controlled conditions conidia were released from conidiophores for 2.5 h after maturation and were shown to be well adapted to wind dispersal, having a settling velocity of 1.5 × 10⁻⁴ m s⁻¹. Conditions that are suitable for conidia production occur in Zimbabwe and other semi-arid regions of southern Africa during the cropping season.