Wetness Duration Research Articles

Abstracts, The Canadian Phytopathological Society annual meeting, London, Ontario, 2001

Abstracts, The Canadian Phytopathological Society annual meeting, London, Ontario, 2001

Septoria tritici Blotch Development as Affected by Temperature, Duration of Leaf Wetness, Inoculum Concentration, and Host.

The effects of incubation temperature, leaf-wetness duration, inoculum concentration, and interaction between leaf-wetness duration and inoculum concentration on the development of Septoria tritici blotch were evaluated at the seedling stage in two bread wheats (Katepwa and 6 Lacos-78) and two durum wheats (AC Melita and Kyle). The study was conducted to assess if bread and durum cultivars widely grown in Manitoba and a resistant cultivar from South America react differently to the disease at temperatures characteristic of Manitoba summers, and to obtain information on conditions that would be used in differentiating resistant and susceptible cultivars under controlled conditions. The experiments were carried out under three temperature regimes. Factors that evaluated included inoculum concentration and duration of leaf wetness. Increasing incubation temperature, duration of leaf wetness, and inoculum concentration resulted in an increase in disease severity. There were significant (P < 0.05) differences for duration of leaf wetness and inoculum concentration within each cultivar. Pycnidia were observed 4 days earlier when incubation temperature increased from 18°C day/15°C night to 22°C day/15°C night or when inoculum concentration increased from 1 × 106 spores/ml to 1 × 107 spores/ml. There were more pycnidia when duration of leaf wetness was 72 h as opposed to 48 h and 60 h. The cultivar that was presumed to be resistant maintained its resistance under environmental conditions that are characteristic of Manitoba summers. We found that the optimal conditions for screening spring wheats for Septoria tritici blotch reaction were incubation temperatures of 18°C day/15°C night, and 22°C day/15°C night. Leaf wetness duration of 48 or 72 h and inoculum concentration of 1 × 107 spores/ml consistently produced a susceptible reaction on Katepwa, AC Melita, and Kyle the three cultivars that were susceptible to Septoria tritici blotch.

Modelling the progress of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in relation to leaf wetness and temperature

A compartmental model was developed to describe the progress with time of light leaf spot (Pyrenopeziza brassicae) on leaves of winter oilseed rape (Brassica napus) during the autumn in the UK. Differential equations described the transition between the four compartments: healthy susceptible leaves, infected symptomless leaves, sporulating symptomless leaves and leaves with necrotic light leaf spot lesions, respectively. The model was fitted to data on the progress of light leaf spot on winter oilseed rape at a single site during the autumn of the 1990–1991 season. Model parameters were used to describe rates of leaf appearance, leaf death, infection by airborne ascospores (primary inoculum) and infection by splash‐dispersed conidiospores (secondary inoculum). Infection was dependent on sufficient leaf wetness duration. The model also included delay terms for the latent period between infection and sporulation and the incubation period between infection and the appearance of necrotic light leaf spot lesions. This modified SEIR model formulation gave a reasonable fit to the experimental data. Sensitivity analysis showed that varying the parameter accounting for the rate of infection by ascospores affected the magnitude of the curves after the start of the epidemic, whilst including a parameter for conidiospore infection improved the fit to the data. Use of ascospore counts from different sites and different years showed variation in spore release patterns sufficient to affect model predictions.

Development of phoma lesions on oilseed rape leaves inoculated with ascospores of A‐group or B‐groupLeptosphaeria maculans(stem canker) at different temperatures and wetness durations

In controlled‐environment experiments, ascospores of both A‐group and B‐groupLeptosphaeria maculanswere able to infect leaves of oilseed rape and produce phoma leaf spot lesions at temperatures from 5 to 20°C and wetness durations from 8 to 72 h after inoculation. Lesions formed on leaves inoculated with B‐group ascospores had few pycnidia and were darker, smaller and less noticable than the larger, pale grey lesions with many pycnidia produced by A‐group ascospores. Lesions formed by A‐group or B‐groupL. maculanson naturally infected winter oilseed rape experimental crops were similar to lesions produced by the two groups on inoculated plants. The greatest numbers of lesions were produced with a leaf wetness duration of 48 h and at temperatures of 15–20°C for both A‐group and B‐group ascospores. As leaf wetness duration and temperature decreased below the optimal values, the number of lesions decreased. The incubation period, estimated as the time from inoculation to the appearance of the first lesions (t1), or the time to the appearance of 50% of the lesions (t50), of B‐group was often shorter than that of A‐groupL. maculans. As temperature decreased below 20°C, the length of the incubation period of both A‐group and B‐groupL. maculansincreased.

Modelling the effects of temperature and wetness duration on development of light leaf spot on oilseed rape leaves inoculated withPyrenopeziza brassicaeconidia

A model was developed to describe the effects of temperature and leaf wetness duration in controlled‐environment experiments on the development of light leaf spot on oilseed rape (cv. Bristol) leaves inoculated withPyrenopeziza brassicaeconidial suspensions. A Gompertz function was used to describe the progress with time in percentage leaf area with sporulation, and included the parameters maximum percentage leaf area with sporulation (c); maximum rate of increase in percentage leaf area with sporulation (r); and latent period (l, the time from inoculation until the leaf area with sporulation reached 37% ofc). The effects of leaf wetness duration oncandrwere also described with Gompertz functions, which included the parameters minimum leaf wetness duration (vcorvr); and maximum ofc(mc) or maximum ofr(mr). The effects of temperature onmc,vcandvrwere described by quadratic functions, and the effect of temperature onmrwas described by a linear function. The combined model described the progress with time in percentage leaf area with sporulation, including the effects of temperature and leaf wetness duration on the parametersc,randl. It generally fitted well to the observed data. Latent periods in previously published experiments were predicted accurately by the model, but percentage leaf area with sporulation was not. Assuming a great number of conidia were dispersed and infection occurred when there was 2 mm h−1rain for 0·5 h, the model estimates for latent period were used to predict the dates when large increases in light leaf spot severity occurred in experiments at Rothamsted on winter oilseed rape (cv. Bristol) under natural conditions in 1998/99 and 1999/2000. The predictions agreed with the observations.

Modelling the effects of temperature and wetness duration on development of light leaf spot on oilseed rape leaves inoculated with Pyrenopeziza brassicae conidia

Modelling the effects of temperature and wetness duration on development of light leaf spot on oilseed rape leaves inoculated with Pyrenopeziza brassicae conidia

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.

Influence of Temperature and Leaf Wetness Duration on Infection of Strawberry Leaves by Mycosphaerella fragariae

ABSTRACT In controlled environment studies, the influence of temperature and wetness duration on infection of strawberry leaves by Mycosphaerella fragariae was quantified by inoculating plants with a conidial suspension and incubating them at various combinations of temperature (5 to 35 degrees C) and leaf wetness duration (0 to 96 h). Infection was expressed as the number of lesions per square centimeter of leaf surface and relative infection was used to develop an infection model. Younger leaves were more susceptible to infection. Regardless of temperature and duration of leaf wetness, only few lesions developed on the oldest (19 to 21 days old) and intermediate leaves (12 to 15 days old), respectively (maximum of 1.7 and 2.3 lesions per cm(2)) as compared to the youngest leaves (5 to 7 days old; maximum of 12.6 lesions per cm(2)). On the youngest leaves, lesions developed at all temperatures except at 35 degrees C, and the number of lesions, for all leaf wetness durations, increased gradually from 5 to 25 degrees C and decreased sharply from 25 to 30 degrees C. For temperatures of 15 and 20 degrees C, the number of lesions increased gradually when leaf wetness duration increased from 12 to 96 h. At 25 degrees C, the number of lesions increased with increasing leaf wetness from 12 to 48 h and then at a higher rate from 48 to 96 h. The optimal temperature for infection was 25 degrees C. For most temperatures, a minimum of 12 h of leaf wetness was necessary for infection (more than 1 lesion per cm(2)). Relative infection was modeled as a function of both temperature and wetness duration using a modified version of the Weibull equation (R (2) = 0.98). The resulting equations provided a precise description of the response of M. fragariae to temperature. The model was sufficiently flexible to account for most characteristics of the response of M. fragariae to wetness duration. The model was used to construct a risk chart that can be used to estimate the potential risk for infection based on observed or forecasted temperature and leaf wetness duration.

Canopy Microenvironments and Applied Bacteria Population Dynamics in Shaded Tall Fescue

Shading of the turfgrass canopy is assumed to support increased microbial activity and therefore more potential disease problems. The goal of this research was to determine how shading affects microenvironmental conditions, and how these conditions affect phylloplane colonization by applied bacterial strains. Shade cloth was suspended over tall fescue (Festuca arundinacea Schreb.) turf in field plots and environmental parameters were measured in shaded and full sun areas. Shading increased leaf wetness duration and relative humidity (RH), but decreased the average and range of canopy air and foliage temperatures. Three species of bacteria (Bacillus megaterium, Stenotrophomonas maltophilia, and Enterobacter cloacae) were applied to all plots. Populations of B. megaterium were the lowest of the three species in both study years. Data showed that measurable differences in microclimate occur as a result of shading and that bacterial colonization was generally favored by shading. The lack of consistency between the two assay methods and among bacterial species regarding the effects of shading suggests that differences in response to microclimate conditions may exist among bacterial species or strains. Results indicate that shading should be considered when screening biological control agents for disease management.

Effects of temperature and wetness duration on conidial infection, latent period and asexual sporulation ofPyrenopeziza brassicaeon leaves of oilseed rape

Experiments in controlled environments were carried out to determine the effects of temperature and leaf wetness duration on infection of oilseed rape leaves by conidia of the light leaf spot pathogen,Pyrenopeziza brassicae. Visible spore pustules developed on leaves of cv. Bristol inoculated withP. brassicaeconidia at temperatures from 4 to 20°C, but not at 24°C; spore pustules developed when the leaf wetness duration after inoculation was longer than or equal to approximately 6 h at 12–20°C, 10 h at 8°C, 16 h at 6°C or 24 h at 4°C. On leaves of cvs. Capricorn or Cobra, light leaf spot symptoms developed at 8 and 16°C when the leaf wetness duration after inoculation was greater than 3 or 24 h, respectively. The latent period (the time period from inoculation to first spore pustules) ofP. brassicaeon cv. Bristol was, on average, approximately 10 days at 16°C when leaf wetness duration was 24 h, and increased to approximately 12 days as temperature increased to 20°C and to 26 days as temperature decreased to 4°C. At 8°C, an increase in leaf wetness duration from 10 to 72 h decreased the latent period from approximately 25 to 16 days; at 6°C, an increase in leaf wetness duration from 16 to 72 h decreased the latent period from approximately 23 to 17 days. The numbers of conidia produced were greatest at 12–16°C, and decreased as temperature decreased to 8°C or increased to 20°C. At temperatures from 8 to 20°C, an increase in leaf wetness duration from 6 to 24 h increased the production of conidia. There were linear relationships between the number of conidia produced on a leaf and the proportion of the leaf area covered by ‘lesions’ (both log10‐transformed) at different temperatures.

Sooty Blotch and Flyspeck of Apple: Etiology, Biology, and Control

Sooty Blotch and Flyspeck of Apple: Etiology, Biology, and Control

Effects of inoculum concentration, wetness duration and plant age on development of early blight (Alternaria solani) and on shedding of leaves in tomato plants

Effects of inoculum concentration, wetness duration and plant age on the development of tomato early blight were evaluated in relation to host susceptibility under controlled environmental conditions. The main effect of early blight was premature defoliation, which was linearly related to the percentage of leaf area showing symptoms. As ln(inoculum concentration, conidia mL−1) increased from 6·2 to 11·5, the percentages of leaf area affected and of defoliation increased linearly. Four h of leaf wetness after inoculation were sufficient to initiate the disease on plants of hybrid Skala RZ but not on those of cv. Rio Rojo, for which at least 6 h leaf wetness were needed. As wetness duration increased up to 24 h, there was an increase in the percentage leaf area showing symptoms and in the percentage of defoliation, but thereafter there was no significant increase in either parameter. Tomato plants were susceptible to Alternaria solani at all growth stages, but susceptibility increased as plants matured. There were no significant differences in susceptibility between tomato cultivars and hybrids.

Environmental Factors Affecting the Severity of Alternaria Brown Spot of Citrus and Their Potential Use in Timing Fungicide Applications.

Alternaria brown spot affects many tangerines and their hybrids, causing lesions on leaves, twigs, and fruit resulting in reduced yield and fruit quality. Field studies were conducted in a severely affected Minneola tangelo grove in central Florida from 1996 to 1998 to determine the environmental factors associated with infection of field trees and potted trap plants. Conidial production peaked following large flushes of new leaves, which were heavily infected. Most infections occurred during the summer rainy season, but trap plants became infected nearly every week of the year. When plants were exposed for 1-week periods, linear regression analysis indicated that disease severity on trap plants was positively related to the amount of rainfall, duration of leaf wetness, and average daily temperatures, and negatively related to the number of conidia trapped. Similar relationships occurred with trap plants exposed for 24-h periods on 141 different dates, except that temperature was not a significant factor. Nevertheless, these factors individually or combined in stepwise multiple regressions explained only a low percentage of the variability in disease severity with both weekly and daily trap plant sampling. When daily environmental data were categorized as: (i) rain versus no rain, (ii) <10 h or >10 h leaf wetness duration, and (iii) average temperature <20°C, 20 to 28°C, and >28°C, relationships to disease severity on trap plants were clearer. Disease severity on days with rain was nearly double that of days without rain, but considerable infection occurred on days with >10 h leaf wetness duration and no rain. Infection was greatest on days with temperatures of 20 to 28°C and slightly less at lower or higher temperatures. A point system, called the ALTER-RATER, was designed whereby each day would be assigned a severity value according to the prevailing environmental conditions. A fungicide application would be made after a predetermined number of points had accumulated. Simulated spray programs based on accumulation of 50, 75, 100, and 150 points from historical weather data at several locations indicated that from 8 to 15, 6 to 8, 5 to 6, or 3 to 4 sprays, respectively, would be needed depending on year and location in Florida. Such a weather-based control system could reduce the number of fungicide applications and improve control of Alternaria brown spot of tangerine.

Optimization of chemical control of Pseudoperonospora cubensis on cucumber in Ukraine

Downy mildew (Pseudoperonospora cubensis) is one of the most harmful diseases of cucumber in Ukraine. At high disease severity, the growing season is considerably shortened and yield is reduced. The most effective method of control is the development of resistant cultivars but, in the absence of absolutely resistant forms, control with fungicides remains very important. The effectiveness of such treatment depends on the dates of application, the number of sprays and the efficacy of the fungicides used. An improved scheme is presented, based on three components: use of cultivars with increased resistance; application of highly effective systemic + contact fungicides; and reliable short-term forecasting of disease appearance. The predictors for downy mildew forecasting are: sum of effective temperatures, weather index, duration of leaf wetness, concentration of sporangia in air and latent period of disease development. These conditions determine‘critical days’for disease development. The first fungicide spray on less susceptible cultivars is recommended on the first critical day, and the next on the day of appearance of first disease symptoms. This scheme allows the fungicide load to be approximately halved, while ensuring higher yield.

Infection of onion leaves by Alternaria porri and Stemphylium vesicarium and disease development in controlled environments

Infection of onion by Alternaria porri and Stemphylium vesicarium was investigated under a range of controlled temperatures (4–25°C) and leaf wetness periods (0–24 h). Conidia of A. porri and S. vesicarium germinated within 2 h when incubated at 4°C. Terminal and intercalary appressoria were produced at similar frequencies at or above 10°C. The maximum number of appressoria was produced after 24 h at 25°C. Penetration of leaves by both pathogens was via the epidermis and stomata, but the frequency of stomatal penetration exceeded that of epidermal penetration. There was a strong correlation (R2 &gt; 90%) between appressorium formation and total penetrations at all temperatures. Infection of onion leaves occurred after 16 h of leaf wetness at 15°C and 8 h of leaf wetness at 10–25°C, and infection increased with increasing leaf wetness duration to 24 h at all temperatures. Interruption of a single or double leaf wetness period by a dry period of 4–24 h had little effect on lesion numbers. Conidia of A. porri and S. vesicarium separately or in mixtures caused similar numbers of lesions. Alternaria porri and S. vesicarium are both potentially important pathogens in winter‐grown Allium crops and purple leaf blotch symptoms were considered to be a complex caused by both pathogens.

Effects of Host, Inoculum Concentration, Wetness Duration, Growth Stage, and Temperature on Anthracnose of Lentil.

The effects of concentration of conidia, duration of the wetness period, plant growth stage, and temperature on the development of anthracnose (Colletotrichum truncatum) on lentil (Lens culinaris) were assessed in growth-chamber and greenhouse studies using cv. Indianhead and line 458-57, which have partial resistance, and susceptible cv. Eston. Each genotype was assessed for incubation period (IP), latent period (LP), number of lesions (LN) per stem, and disease severity (DS). Both IP and LP decreased linearly with increasing conidial concentration, wetness duration, and temperature. Both IP and LP also became progressively shorter as the plants aged between 2 to 6 weeks and increased between 6 to 8 weeks after seeding. Both LN and DS increased linearly with increasing inoculum concentration, wetness duration, and temperature. Both LN and DS also increased with plant age between 2 to 4 weeks and decreased between 4 to 8 weeks. The growth stage and temperature required for optimal disease development ranged, respectively, from 4 to 6 weeks and 20 to 24°C when plants were inoculated at a concentration of 4 × 104 conidia/ml and provided with a wetness period of 24 h. Generally, cv. Indianhead and line 458-57 had significantly longer IP and LP and lower LN and DS than cv. Eston.

Estimating disease risk at the whole plant level with General Circulation Models

General Circulation Models (GCMs) have been developed to assess the impacts of potential global climate change. However, these models do not provide specific weather information at the whole-plant level and thus provide only very gross estimates of conditions that affect plant and disease development. Also, climatic change may increase the frequency of extreme events that influence plant production more than changes in daily or monthly averages. One solution is a simulation approach that can scale weather information from the global down to the plant scale. Over the last 4 years, we have been developing methods to hierarchically define current and forecast weather conditions down to the whole-plant level based on nested high-resolution atmospheric (mesoscale) numerical models. Two hierarchical mesoscale model approaches were tested to downscale weather data in a vineyard. The first, known as the Localized Mesoscale Forecast System (LMFS) uses surface databases to ‘localize’ mesoscale output. The second, known as the Canopy-Mesoscale Forecast System (CMFS), uses a boundary layer model to downscale mesoscale output. To illustrate the utility of this approach we focused on surface wetness duration (SWD), a variable with high spatial and temporal variability. SWD is also a critical variable for prediction of plant disease. Simulations of SWD with on-site input data were compared to those derived from the mesoscale models and to on-site sensors. Forecasts of atmospheric variables by the two systems were compared to on-site observations. Success in this effort leads us to extend this method to GCMs where factors such as temperature, rainfall, relative humidity, and surface wetness can be estimated within plant and crop canopies. We explore the implications of this work on evaluating the assessment of climate change on the risk of plant disease development.

STREP: a brown-spot disease predictor for scheduling fungicide sprays against Stemphylium vesicarium on pear

Brown spot of pear (Pyrus communis) caused by Stemphylium vesicarium is an important disease in fruit-growing areas of Europe. The control of brown spot is based on protectant sprays of fungicide applied, at 7- to 15-day intervals depending on the type of fungicide, from fruit set to preharvest regardless of the risk of infection. A forecasting model has been developed and can be used for timing the fungicide applications and to eliminate unnecessary sprays. The model quantifies the relationship between wetness duration and temperature in disease severity and can be used as a tool for the prediction of disease infection periods. The capacity of risk prediction was validated over 2 years. The fungicide sprays scheduled by the STREP forecaster were evaluated in commercial orchards during 3 years in different climatic areas in Spain and Italy. The results showed consistently that use of the STREP model minimized the number of fungicide sprays compared with a protectant schedule, but without decreasing the quality of commercial disease control. The susceptibility of selected European pear cultivars to infection by S. vesicarium and the influence of fruit age were also determined. The implementation of the model in warning stations will require the combination of predictions by the STREP model, the level of inoculum and the sensitivity of phenological stage and pear cultivar.

Phytophthora Brown Rot of Citrus: Temperature and Moisture Effects on Infection, Sporangium Production, and Dispersal.

Sporadic epidemics of Phytophthora brown rot occur in citrus orchards in Florida and are caused primarily by Phytophthora palmivora and secondarily by P. nicotianae. With P. palmivora, fruit wetness durations of 3 h or more resulted in maximum infection at favorable temperatures. The optimum temperature range for fruit infection and brown rot development was 27 to 30°C. No brown rot developed at 22°C or less. The optimum temperature for sporulation in culture and on the fruit surface was 24°C, with sporangium production decreasing rapidly at higher or lower temperatures. A few sporangia were produced with 18 h of fruit wetness, and numbers increased as duration of wetness increased up to 72 h. Propagules were splash-dispersed by single droplets of water from infested peel disks horizontally about 350 to 450 mm and vertically about 30 to 45 mm with most isolates. Sporangia were not dispersed by air currents alone. Few sporangia of P. nicotianae were produced on fruit surfaces, and they were not readily dispersed by falling water droplets. Brown rot caused by P. palmivora can become epidemic in Florida from July to October during the rainy season because of its high temperature optimum and prolific sporulation on the fruit surface.

Neural Network Classification of Tan Spot and Stagonospora Blotch Infection Periods in a Wheat Field Environment

ABSTRACT Tan spot and Stagonospora blotch of hard red spring wheat served as a model system for evaluating disease forecasts by artificial neural networks. Pathogen infection periods on susceptible wheat plants were measured in the field from 1993 to 1998, and incidence data were merged with 24-h summaries of accumulated growing degree days, temperature, relative humidity, precipitation, and leaf wetness duration. The resulting data set of 202 discrete periods was randomly assigned to 10 modeldevelopment or -validation (n = 50) data sets. Backpropagation neural networks, general regression neural networks, logistic regression, and parametric and nonparametric methods of discriminant analysis were chosen for comparison. Mean validation classification of tan spot incidence was between 71% for logistic regression and 76% for backpropagation models. No significant difference was found between methods of modeling tan spot infection periods. Mean validation prediction accuracy of Stagonospora blotch incidence was 86 and 81% for backpropagation and logistic regression, respectively. Prediction accuracies of other modeling methods were </=78% and were significantly different (P = 0.01) from backpropagation, but not logistic regression, results. The best backpropagation models of tan spot and Stagonospora blotch incidences correctly classified 82 and 84% of validation cases, respectively. High classification accuracy and consistently good performance demonstrate the applicability of neural network technology to plant disease forecasting.