Conidial dispersal of Gremmeniella abietina: climatic and microclimatic factors

The conidia dispersal started in Suonenjoki, in central Finland, in 1997–99 by the end of May or beginning of June, and continued occasionally at least to the middle of September. The temperature sum, day degrees (d.d., threshold temperature = 5°C) was between 100 and 165 d.d. at the beginning of dispersal. In years 1997–99, 80, 94 and 82% of the dispersal had occurred by the end of July – beginning of August when the temperature sum reached 800 d.d. All the spore data are coming from the spore traps. Cumulative number of conidia increased linearily with logarithm of temperature sum. A binary logistic regression model with temperature sum and rainfall as explanatory variables predicted accurately the date of the first spores in the spring: the prediction error was at most 3 days. The model classified 69% of all the days in the analysis correctly to the spore‐free days and correspondingly 74% to the days of at least one spore caught. A regression model for the number of spores per day explains 21, 5 and 51% of the within‐season variation in 1997–99 (24, 37 and 62% on a logarithmic scale). The explanatory weather variables in the model were d.d., rain and year. The very low explanatory coefficient of determination in 1998 results from one exceptionally high number of conidia. The between‐differences in the total number of spores were large and could not be explained by the measured weather variables. In the regression model, these differences were taken into account by adding a constant for each year in the model. Rain increased conidia dispersal significantly but conidia were found also in consecutive rainless days.