Abstract
Iprodione-resistant strains of Botrytis cinerea were detected on tomatoes and cucumbers in greenhouses of Chiba prefecture first in spring 1980 and then more frequently in 1981 within one or two years of practical use of the fungicide. The rate of the resistant strains in all the isolates increased with the frequency of the fungicidal spraying. Percentage of diseased fruits caused by the resistant strains was 50% after five applications of the fungicide. On iprodione-containing potato sucrose agar (PSA) media, the resistant strains raised abundant aerial hyphae and developed mycelia on the media containing 6.25, 12.5, 200, 400, 800μg/ml iprodione respectively, though the mycelial growth was severely suppressed as compared with iprodione-free PSA media. However, the mycelial growth was less developed on the media with 25-100μg/ml iprodione. The resistant strains were as pathogenic as the sensitive strains against excised leaves of kidney beans. Strains resistant to iprodione showed cross-resistance to procymidone, vinclozolin and diclozoline, and they had ability of conidial germination and mycelial growth on the PSA media containing 6.25-400μg/ml of the above fungicides. Three out of four above fungicides except diclozoline untested did not show satisfactory effect against the resistant strains in excised leaf test using kidney bean leaves. Moreover, in greenhouses where the resistant strains of Botrytis cinerea had already been abundant, both iprodione and procymidone showed a reduced effect against gray mold of tomato.
