CHARACTERIZATION OF IPRODIONE-RESISTANT ISOLATES OF ALTERNARIA-BRASSICICOLA

The resistance of isolates of Alternaria brassicicola to iprodione exhibited a polymodal distribution. Four distinct groups were detected demonstrating ED(50)s of 0 to 7, 10 to 40, 70 to 80, and 300 to 516 mu g a.i. per ml. Most resistant isolates produced smaller colonies on unamended potato-dextrose agar. Fewer conidia were produced per colony for some resistant isolates than for their corresponding wild types, while other resistant isolates produced more conidia per unit of colony area. The resistant isolates produced either smaller lesions or sporulated less, or both, than the sensitive isolates on untreated broccoli leaf disks. Only resistant isolates were able to produce lesions on leaf disks sprayed with 50 mu g a.i. of iprodione per ml. Conidia of resistant isolates germinated in distilled water containing iprodione at a concentration as high as 250 mu g a.i. per ml, while the germination of sensitive isolates was greatly inhibited in distilled water containing only 5 mu g a.i. of iprodione per ml. Iprodione resistant isolates were cross resistant to vinclozolin and dichloran. Resistant isolates exhibited a greater osmotic sensitivity than sensitive isolates; however, osmotic sensitivity was independent of the degree of resistance.