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Seasonal Patterns of Spore Dispersal of Phaeosphaeria spp. and Stagonospora spp.

The spatial and temporal patterns of discharge and dissemination of airborne spores of Phaeosphaeria spp. and Stagonospora spp. were studied. Both ascospores and pycnidiospores of the pathogens were deposited at various densities on microscope slides used as spore samplers. The maximum deposition of the spores was observed during the period of August to October. A multiple regression analysis was used to determine which weather factors significantly explained the variation measured in the numbers of ascospores that settled on microscope slides. Rainfall, air temperature, and relative air humidity were influential in the release of Phaeosphaeria spp. ascospores into the air. The amount of airborne ascospores was a function of the variables and remained largely under their control. The liberation of ascospores was favored by air temperature above 0°C, rainfall greater than 1 mm, and high relative humidity. The range of atmospheric conditions stimulating air dispersal of ascospores was wider than that for pycnidiospores. Pycnidiospores were sampled only during rainy days. Their release was affected adversely by air temperature below 5°C. Multiple regression models based on weather data were developed and verified for their predictive ability and accuracy by jackknife and cross-validation procedures, as well as by comparisons of observed and predicted mean numbers of deposited ascospores per microscope slide after a substitution of each period data set with a set of data of the other respective time interval. The numbers of airborne ascospores predicted by the regression models were in a good agreement with the observed values. The jackknife and cross-validation techniques allowed use of the limited data sets for both the parameter estimation and validation processes in a development of simulation models. The airborne inoculum appeared to be omnipresent over cereal areas year round, except during periods with freezing temperatures and a snow cover. Such an omnipresence of inoculum of the pathogens poses a danger to crops and could be of importance in the epidemiology of Stagonospora (= Septoria) blotches under the climatic conditions of central Poland.