DEVELOPMENTAL STAGE RESPONSE TO FUNGICIDES OF PLASMOPARA-HALSTEDII (SUNFLOWER DOWNY MILDEW)

The response of Plasmopara halstedii to anti-oomycete fungicides varied during fungal ontogeny, particularly between host-dependent (HD) and host-independent (HI) stages in the life-cycle. Mycelial growth was found to be the most sensitive to phenylamides. Among HI stages of development, sporangia were the least sensitive to fungicides, the ED50 values being two-fold higher than those for cystospore germination. Zoospores were very sensitive to fosetyl-Al but relatively insensitive to all other fungicides even after prolonged exposure. The initiation of the germ-tube of cystospores was less sensitive than germ-tube elongation. When comparing developmental stage responses with fungicides by means of principal component analysis, the presence of five independent components has been demonstrated. Of these, the first and second components accounted for 72% of the total variation to which HD and HI stages contributed differently.