Development and Field Evaluation of a Model to Estimate the Maturity of Pseudothecia of Pleospora allii on Pear

Spatiotemporal Dynamics of Stemphylium Leaf Blight and Potential Inoculum Sources in New York Onion Fields

Stemphylium leaf blight (SLB) caused by Stemphylium vesicarium is the dominant foliar disease affecting large-scale onion production in New York. The disease is managed by fungicides, but control failures are prevalent and are attributed to fungicide resistance. Little is known of the relative role of inoculum sources in initiation and spread of SLB epidemics. Plate testing of 28 commercially available organic onion seedlots from 2016 and 2017 did not detect S. vesicarium. This finding suggests that although S. vesicarium has been reported as seed-transmitted, this is unlikely to be a significant inoculum source in commercially available organic seed lots and even less so in fungicide-treated seed used to establish conventional fields. The spatial and spatiotemporal dynamics of SLB epidemics in six onion fields were evaluated along linear transects in 2017 and 2018. Average SLB incidence increased from 0 to 100% throughout the cropping seasons with an average final lesion length of 28.3 cm. Disease progress was typical of a polycyclic epidemic and the logistic model provided the best fit to 83.3% of the datasets. Spatial patterns were better described by the beta-binomial than binomial distribution in half of the datasets (50%) and random patterns were more frequently observed by the index of dispersion (59%). Geostatistical analyses also found a low frequency of datasets with aggregation (60%). Spatiotemporal analysis of epidemics detected that the aggregation was influenced by disease incidence. However, diseased units were not frequently associated with the previous time period according to the spatiotemporal association function of spatial analyses by distance indices. Variable spatial patterns suggested mixed inoculum sources dependent upon location, and likely an external inoculum source at the sampling scale used in this study. A small-plot replicated trial was also conducted in each of 2 years to quantify the effect of S. vesicarium-infested onion residue on SLB epidemics in a field isolated from other onion fields. SLB incidence was significantly reduced in plots without residue compared with those in which residue remained on the soil surface. Burial of infested residue also significantly reduced epidemic progress in 1 year. The effect of infested onion residue on SLB epidemics in the subsequent onion crop suggests rotation or residue management may have a substantial effect on epidemics. However, the presence of an inoculum source external to fields in onion production regions, as indicated by a lack of spatial aggregation, may reduce the efficacy of in-field management techniques.

Stemphylium Leaf Blight: A Re-Emerging Threat to Onion Production in Eastern North America

Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, is a foliar disease of onion worldwide, and has recently become an important disease in the northeastern United States and Ontario, Canada. The symptoms begin as small, tan to brown lesions on the leaves that can progress to defoliate plants. Crop loss occurs through reduced photosynthetic area, resulting in smaller, lower-quality bulbs. Leaf necrosis caused by SLB also can compromise bulb storage, as green leaves are required for the uptake of sprout inhibitors applied prior to harvest. The pathogen can overwinter on infested onion residue and infected volunteer plants. Asymptomatic weedy hosts near onion fields may also be a source of inoculum. Production of ascospores of the teleomorph (Pleospora allii) peaks in early spring in northeastern North America, often before the crop is planted, and declines rapidly as daily mean air temperatures rise. Conidia are usually present throughout the growing season. Application of fungicides is a standard practice for management of the complex of fungi that can cause foliar diseases of onion in this region. Recent assessments have shown that populations of S. vesicarium in New York and Ontario are resistant to at least three single-site mode-of-action fungicides. Three disease prediction systems have been developed and evaluated that may enable growers to reduce the frequency and/or number of fungicide applications, but the loss of efficacious fungicides due to resistance development within S. vesicarium populations threatens sustainability. The lack of commercially acceptable onion cultivars with sufficient resistance to reduce the number of fungicides for SLB also limits the ability to manage SLB effectively. Integrated disease management strategies for SLB are essential to maintain profitable, sustainable onion production across eastern North America.

Modeling Inoculum Availability of Plurivorosphaerella nawae in Persimmon Leaf Litter with Bayesian Beta Regression

Circular leaf spot (CLS), caused by Plurivorosphaerella nawae, is a serious disease affecting persimmon (Diospyros kaki) that is characterized by necrotic lesions on leaves, defoliation, and fruit drop. Under Mediterranean conditions, P. nawae forms pseudothecia in the leaf litter in winter, and ascospores are released in spring, infecting susceptible leaves. Persimmon growers are advised to apply fungicides for CLS control during the period of inoculum availability, which was previously defined based on ascospore counts under the microscope. A model of inoculum availability of P. nawae was developed and evaluated as an alternative to ascospore counts. Leaf litter samples were collected weekly in L'Alcúdia (Spain) from 2010 to 2015. Leaves were soaked and placed in a wind tunnel, and the released ascospores of P. nawae were counted. Hierarchical Bayesian beta regression methods were used to model the dynamics of ascospore production in the leaf litter. The selected model included accumulated degree-days (ADDs) and ADDs taking into account the vapor pressure deficit (ADDvpd) as fixed effects and year as random effect. This model had a mean absolute error of 0.042 and a root mean square error of 0.062. The beta regression model was evaluated in four orchards from 2010 to 2015. Higher accuracy was obtained at the beginning and the end of the ascospore production period, which are the events of interest to schedule fungicide sprays for CLS control in Spain. This same modeling framework can be extended to other fungal plant pathogens whose inoculum dynamics are expressed as proportion data.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Epidemiological Features and Trends of Brown Spot of Pear Disease Based on the Diversity of Pathogen Populations and Climate Change Effects

Brown spot of pear, caused by the fungus Stemphylium vesicarium, is an emerging disease of economic importance in several pear-growing areas in Europe. In recent years, new control strategies combining sanitation practices and fungicide applications according to developed forecasting models have been introduced to manage the disease. However, the pathogenic and saprophytic behavior of this pathogen makes it difficult to manage the disease. In addition, climate change can also result in variations in the severity and geographical distribution of the disease. In this study, ecological and epidemiological aspects of brown spot of pear disease related to inoculum characterization and climate change impact were elucidated. The pathogenic variation in S. vesicarium populations from pear orchards and its relationship to inoculum sources (air samples, leaf debris, and infected host and nonhost tissues) was determined using multivariate analysis. In total, six variables related to infection and disease development on cultivar Conference pear detached leaves of 110 S. vesicarium isolates were analyzed. A high proportion of isolates (42%) were nonpathogenic to pear; 85% of these nonpathogenic isolates were recovered from air samples. Most isolates recovered from lesions (93%) and pseudothecia (83%) were pathogenic to pear. A group of pathogenic isolates rapidly infected cultivar Conference pear leaves resulted in disease increase that followed a monomolecular model, whereas some S. vesicarium isolates required a period of time after inoculation to initiate infection and resulted in disease increase that followed a logistic model. The latter group was mainly composed of isolates recovered from pseudothecia on leaf debris, whereas the former group was mainly composed of isolates recovered from lesions on pear fruit and leaves. The relationship between the source of inoculum and pathogenic/aggressiveness profile was confirmed by principal component analysis. The effect of climate change on disease risk was analyzed in two pear-growing areas of Spain under two scenarios (A2 and B1) and for three periods (2005 to 2009, 2041 to 2060, and 2081 to 2100). Simulations showed that the level of risk predicted by BSPcast model increased to high or very high under the two scenarios and was differentially distributed in the two regions. This study is an example of how epidemiological models can be used to predict not only the onset of infections but also how climate change could affect brown spot of pear. [Formula: see text] Copyright © 2018 The Author(s). This is an open-access article distributed under the CC BY-NC-ND 4.0 International license .

An update on control of brown spot of pear

Brown spot of pear is a fungal disease producing high economical losses in several pear-growing areas in Europe. Fungicide applications during the growing period either at fixed schedule or delivered according to the BSPcast forecasting system are not enough to control the disease under favorable conditions. New strategies have been introduced to control the inoculum production using sanitation methods. These methods are based on combinations of leaf litter removal during winter and biological control agent applications during late winter, spring and summer. These practices reduce both the inoculum pressure and disease levels. Therefore, the resulting optimized disease management consists of a combination of sanitation methods applied during the whole year with chemical fungicides scheduled according to the BSPcast forecasting model during the vegetative period. It is expected that the control of brown spot could be further refined upon availability of rapid methods for inoculum potential analysis. However, this analysis is difficult due to the variability in pathogenicity within the pathogen population.

A Model to Estimate the Amount of Primary Inoculum of Elsinoë ampelina

In Eastern Canada, anthracnose, caused by the fungus Elsinoë ampelina, is a serious disease on susceptible grape cultivars. In the absence of management tools, anthracnose management relies almost exclusively on fungicide applications programmed at fixed intervals. Therefore, a better understanding of the factors affecting primary inoculum release and abundance would help in the timing of the first fungicide applications. The temporal dynamics of airborne primary inoculum released from cane cankers were investigated from 2007 to 2010. One to three times per week, starting in the first week of April, six 12-cmlong cane pieces were randomly selected from diseased canes that had overwintered on a vineyard floor. The concentration of E. ampelina conidia was expressed as the number of conidia per square millimeter of canker. In total, 27, 32, 33, and 118 samplings were conducted in 2007, 2008, 2009, and 2010, respectively, with the 118 samplings conducted on three sites at 49, 35, and 34 samplings for site 1, 2, and 3, respectively. Each year, the number of conidia per square millimeter of canker was expressed as the proportion of seasonal inoculum (PSI) at the same site and analyzed as a function of degree-days (DD; base temperature = 0°C) accumulated since 1 April (cumulative degreedays [CDD]). The nonlinear sigmoid model in the form PSI = 1.003/(1 + e^{-((CDD - 566.133)/139.204)}) provided adequate fit to the observed data (P < 0.0001, R² = 0.97). When the model was validated against independent data, the model adequately predicted PSI; however, reliability was improved by adding a "dry days" threshold of 6 days during which accumulation of DD is stopped. This study shows that primary inoculum of grape anthracnose is available early in the season before bud break; meaning that emerging leaves could be infected provided that weather conditions are favorable. The results also show that there is an overlap in the availability of primary and secondary inoculum, mainly during the period of rapid leaf growth, a situation that may explain the explosive nature of the disease. The results suggest that, on susceptible cultivars and when there is a history of anthracnose in the vineyard, a fungicide spray program should be initiated early in the season, as soon as leaves are present.

Infection Potential of Pleospora allii and Evaluation of Methods for Reduction of the Overwintering Inoculum of Brown Spot of Pear

The capacity for germination and pathogenicity to pear leaves of ascospores of Pleospora allii, the teleomorph of Stemphylium vesicarium, causal agent of brown spot of pear, were studied in vitro. Most ascospores germinated within 1 h at temperatures between 15 and 20°C, and the optimum temperature for germination was 18.9°C. Infections developed on wounded and non-wounded detached pear leaves, but were more frequent on wounded leaves. The minimum infective dose was one ascospore per wound. Biological, chemical, and mechanical methods for decreasing overwintering inoculum of P. allii were evaluated. Ascospores were discharged from March to May, depending on the orchard and year. Leaf shredding or removal were the most effective methods of reducing overwintering inoculum. Biological control methods based on application of Thichodermasp. formulations were partially effective. Chemical methods based on copper and urea treatments were ineffective.

Significance of Preventing Primary Infections by Didymella rabiei and Development of a Model to Estimate the Maturity of Pseudothecia

The significance of preventing primary infections resulting from the teleomorph stage of Didymella rabiei was tested in field experiments in 1998 and 2000. Control efficacy was greater and yield and its components were higher in plots where the fungicide difenoconazole had been sprayed in time to protect the plants from infections resulting from airborne ascospores than in plots where sprays were not applied on time. Forty empirical models reflecting the influence of temperature and interrupted wetness on initial maturation of D. rabiei pseudothecia were developed and verified by using data recorded in chickpea fields in 1998. Seven of the models then were validated with data recorded in 1999 and 2000. The following model provided the best predictions: starting at the beginning of the rainy season (October to December), the predictor of the model was assigned one severity value unit when there was a rain event (1 day or more) with ≥10 mm of rain and an average daily temperature (during the rainy days) of ≤15°C. According to the model, pseudothecia mature after accumulation of six severity values and ascospores will be discharged during the following rain.