Models for predicting pseudothecium maturity and ascospore release of Phyllosticta spp. in South African citrus orchards

Ascosporic infection plays a major role in the epidemiology of citrus black spot (CBS) in South Africa, a disease caused by Phyllosticta citricarpa. Phyllosticta pseudothecium maturation and ascospore release models have been integrated in infection models to predict the availability of the primary inoculum source. However, these models have not been validated on a broader data set and this study aimed to validate and improve these epidemiological models. New pseudothecium maturation and ascospore release models for P. citricarpa were developed, based on weather and ascospore trap data from 13 locations and up to five seasons. From the 29 data sets analysed, 3775 3-hourly periods with ascospore events were recorded on 1798 days; 90% of these events occurred between 16.0 °C and 32.1 °C (daily Tmin and Tmax of 15.4 °C and 33.5 °C, respectively) and 75% occurred above a relative humidity (RH) of 55.9% (daily RH > 47.9%). Rain was recorded during 13.8% of these ascospore events and 20.0% of ascospore days. Using logistic regression, a Gompertz model that best predicted pseudothecium maturation, or the probability of onset of ascospore release, was developed and was markedly more accurate than the previously described models. The model consisted of DDtemp [cumulative degree-days from midwinter (1 July) calculated as (minimum + maximum daily temperature) / 2 – 10 °C] and DDwet (DDtemp accumulated only on days with >0.1 mm rain or vapour pressure deficit <5 hPa) as variables in the formula: probability of first ascospore event = exp(-exp(-(-3.131 + 0.007 × DDtemp - 0.007 × DDwet))). A Gompertz model [PAT = exp(-2.452 × exp(-0.004 × DDwet2))] was also developed for ascospore release; DDwet2 = DDtemp accumulated, from first seasonal ascospore trap day, only on days with >0.1 mm rain or vapour pressure deficit <5 hPa. Similar to the DDwet2 model described in a previous study, this model adequately predicted the general trend in ascospore release but poorly predicted periods of daily, 3-day and 7-day ascospore peaks.

Potential and actual ascospore release of Erysiphe necator chasmothecia in Austria

Ascospores of grape powdery mildew (Erysiphe necator Schw.) play a crucial role in the disease onset in spring in many vine-growing areas. We investigated the physiological maturation of chasmothecia and the time of the first potential ascospore release in three grape-growing areas in Austria by providing standardized conditions for ascospore release in the laboratory and excluding the environmental influence for the release itself. In the overwintering season 2017/2018, the potential ascospore release started in March 2018 in all three investigated wine-growing areas, while in 2018/19, the potential ascospore release was already possible in autumn 2018. Autumn 2018 was characterized by higher temperatures than autumn 2017. We related accumulated degree days (base 8 °C) after chasmothecia formation with the time of first potential chasmothecia dehiscence and found that more than 480 degree days are necessary to reach physiological maturity of chasmothecia. Temperature significantly influenced the dynamics of the potential of ascospore release. More than 50% of the total potential of ascospore release occurred before bud break in both years. Furthermore, weather factors affecting the actual ascospore release in the field were studied. Precipitation and leaf wetness showed a significant positive correlation with ascospore release in the vineyard. In contrast to the potential release, only a small percentage of actual release in the field occurred before bud break, while 84 and 95% of total trapped ascospores were found between bud break and flowering in 2018 and 2019, respectively. Our results reveal that the potential release and actual release have to be combined to predict ascospore release in spring.

Phyllachora maydis Ascospore Release and Germination from Overwintered Corn Residue

Tar spot of corn, caused by Phyllachora maydis, has been reported in several upper Midwest states in the United States. This has led to expanded efforts to more thoroughly understand the biology of P. maydis and the epidemiology of tar spot. This study determined the potential for P. maydis ascospore release and germination from overwintered P. maydis-infected corn residues from various locations in the upper Midwest. Corn residue samples collected in the spring of 2019 from 12 fields in four states were examined. Ascospore release and germination were observed in all residue samples collected. The mean total number of ascospores released per milliliter of water ranged from 3.6 × 103 to 4.8 × 106 after 4-h incubation and 3.7 × 103 to 4.4 × 106 after 24-h incubation. The mean percent spore germination ranged from 0.7 to 24.1% after 4-h incubation and 2.4 to 24.9% after 24-h incubation. There was a significant inverse relationship between total numbers of ascospores released and the percent germination of the ascospores. Samples from Illinois consistently yielded the greatest total ascospore release but were also consistently among the samples with the lowest percent ascospore germination. Samples from Wisconsin and Indiana were among the lowest for total ascospore numbers but were among the highest for total ascospore germination. These findings provide evidence that P. maydis can overwinter in multiple areas of the upper Midwest. Future research should focus on reducing infested corn residue to reduce initial infection by P. maydis.

Assessment of Erysiphe necator Ascospore Release Models for Use in the Mediterranean Climate of Western Oregon

Predictive models have been developed in several major grape-growing regions to correlate environmental conditions to Erysiphe necator ascospore release; however, these models may not be broadly applicable in regions with different climatic conditions. To assess ascospore release in near-coastal regions of western Oregon, chasmothecia (syn. cleistothecia) were collected prior to leaf drop and placed onto natural and artificial grape trunk segments and overwintered outside. Ascospore release was monitored for three overwintering seasons using custom impaction spore traps from leaf drop (Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie [BBCH] 97) until the onset of the disease epidemic in the following growing season. Airborne inoculum was concurrently monitored in a naturally infested research vineyard. Weather and ascospore release data were used to assess previously developed models and correlate environmental conditions to ascospore release. Ascospore release was predicted by all models prior to bud break (BBCH 08), and was observed from the first rain event following the start of inoculum monitoring until monitoring ceased. Previously developed models overpredicted ascospore release in the Willamette Valley and predicted exhaustion of inoculum prior to bud break. The magnitude of ascospore release could not be correlated to environmental conditions; thus, a binary ascospore release model was developed where release is a function of the collective occurrence of the following factors within a 24-h period: >6 h of cumulative leaf wetness during temperatures >4°C, precipitation >2.5 mm, and relative humidity >80%. The Oregon model was validated using field-collected ascospore datasets, and predicted ascospore release with 66% accuracy (P = 0.02). Extant methods for estimating ascospore release may not be sufficiently accurate to use as predictive models in wet, temperate climatic regions.

Predicting Ascospore Release of Monilinia vaccinii-corymbosi of Blueberry with Machine Learning

Mummy berry, caused by Monilinia vaccinii-corymbosi, causes economic losses of highbush blueberry in the U.S. Pacific Northwest (PNW). Apothecia develop from mummified berries overwintering on soil surfaces and produce ascospores that infect tissue emerging from floral and vegetative buds. Disease control currently relies on fungicides applied on a calendar basis rather than inoculum availability. To establish a prediction model for ascospore release, apothecial development was tracked in three fields, one in western Oregon and two in northwestern Washington in 2015 and 2016. Air and soil temperature, precipitation, soil moisture, leaf wetness, relative humidity and solar radiation were monitored using in-field weather stations and Washington State University’s AgWeatherNet stations. Four modeling approaches were compared: logistic regression, multivariate adaptive regression splines, artificial neural networks, and random forest. A supervised learning approach was used to train the models on two data sets: training (70%) and testing (30%). The importance of environmental factors was calculated for each model separately. Soil temperature, soil moisture, and solar radiation were identified as the most important factors influencing ascospore release. Random forest models, with 78% accuracy, showed the best performance compared with the other models. Results of this research helps PNW blueberry growers to optimize fungicide use and reduce production costs.

Neonectria ditissima spore release and availability in New Zealand apple orchards

Conidia and ascospore release of Neonectria ditissima, the causal agent of European canker, was investigated using rainwater traps and Vaseline®-coated glass slides in the Tasman region, New Zealand. Trapping of spores was carried out from May 2013 to June 2015 in three separate apple orchard blocks planted with ‘Scifresh’/Jazzâ„¢, ‘Royal Gala’ and ‘Braeburn’, respectively. Conidia and ascospores were both produced at any time of the year when rainfall occurred. The numbers of both conidia and ascospores trapped peaked in April—May, but were produced throughout the year. There was a trend for lower spore numbers with increasing number of non-rainy days prior to rainy event 2mm, while more spores were trapped under frequent rainfall conditions. There was a significant correlation between mean conidia trapped and total monthly rainfall in all three orchards, but ascospore numbers were significantly correlated with rainfall on glass slides only in the ‘Braeburn’ orchard.