2.5.20
FACTORS AFFECTING RATE OF FOLIAR DISEASE INCREASE IN WINTER WHEAT

AM DJURLE1 and JE YUEN2

1Unit of Plant Pathology 1 and 2Unit of Applied Plant Protection, Swedish University of Agricultural Sciences, SE 750 07 Uppsala, Sweden

Background and objectives
The rate of disease increase in a crop is determined by several factors, biotic as well as abiotic. Assuming a logistic model [1], disease increase is a function of the amount of disease and the apparent infection rate r. The objectives of this study were to examine if r) for fungal diseases in wheat is affected by different weather and field characteristics.

Materials and methods
Data on leaf spot diseases as a group (Septoria tritici, Stagonospora nodorum and Drechslera tritici-repentis), brown rust (Puccinia recondita) and powdery mildew (Erysiphe graminis) in winter wheat has been collected from regular field visits to a large number of fields in the wheat-growing areas in Sweden during the growing season. The data covers the period 1988-95. Disease was generally recorded as incidence, sometimes accompanied by severity records. Field-specific data, such as soil type, previous crops, cultivar, nitrogen fertilization, geographic location, crop residues, crop density and sowing date were added to the disease data. Weather data was taken from the nearest suitable synoptic weather station. New weather variables were calculated from those observed in order to relate disease increase to different weather types such as 'wet', 'intermediate' and 'dry', 'hot', 'chilly', etc., and combinations of these. The analyses were done with multiple linear regression and logistic regression, both in SAS, using weather and field data as the independent variables.

Results and conclusions
The results show that the increase in disease incidence for the fields investigated can be described very well as logistic growth. Other factors that affect the rate of development of the respective epidemics of leaf spot diseases, brown rust and powdery mildew are characterized and comparisons are made between the two methods used.

References
l. Vanderplank JE, 1963. Plant Diseases: Epidemics and Control. Academic Press, New York.