Epidemiological development of Drechslera tritici-repentis in wheat crops

During 1987-1989, seven studies in wheat fields were conducted to explore the population dynamics and epidemiology of Drechslera tritici-repentis. Results of the studies were based on data recorded in observations made every 7 days from growth stage EC 22 up to EC 87. The parameters-frequency of attack and sporulation, the number of lesions and the percentage of necrotic leaf tissue as well as the number of conidia per leaf - were evaluated per leaf insertion. The amount and distribution of rainfall and the temperature were recorded by electronic weather stations. First infections caused by ascospores take place from growth stage EC 21 up to EC 31-33. After 2-3 weeks, as soon as necrotic leaf tissue has developed, conidia may occur, which cause secondary infections. Under favourable conditions, a rapid transfer from older to younger leaves is typical. Additionally in this period and because the conidia are spread by wind, the fungus can be carried over greater distances. Already a small inoculum for these processes is sufficient. Frequent rainfall producing leaf wetness and a high air humidity as well as temperatures above 10 degrees C supply both, the primary infection including ejection of ascospores and the production of conidia. Dry weather conditions of 5 or more days inhibit the disease progression. The dynamics of the causal agent of this disease may be characterized by a typical exponential function. As a consequence, the highest disease incidence occurs in the later part of the crop life cycle, in particular from flowering to grain filling in wheat. The relationships between the development of necrotic leaf tissue and sporulation, frequency and severity (number of lesions) of attack, the number of lesions and conidia production are described by mathematical functions