In vitro fungicide sensitivity of Alternaria species pathogenic to crucifers and identification of Alternaria brassicicola field isolates highly resistant to both dicarboximides and phenylpyrroles

Blackspot, caused by Alternaria spp. is one of the most important diseases in crucifers and its management mainly relies on fungicide applications. However, growers report increasing prevalence of this disease and decline in its control that might be related to the development of resistance in Alternaria species through over-use of fungicides. To test this hypothesis, field isolates of Alternaria species pathogenic to crucifers were evaluated in vitro for their sensitivity to different fungicides. Toxicity responses were expressed as effective concentration, which inhibits mycelial growth by 50% (EC50). On this basis, three sterol biosynthesis inhibiting fungicides, two of the triazole group (flutriafol and difenoconazole) and one of the imidazole group (prochloraz), performed equally well in reducing the mycelial growth of A. brassicae, A. brassicicola, and A. japonica. Of the two selected dicarboximides, iprodione proved to be more effective than procymidone. Most of the tested isolates were also very sensitive to fludioxonil (phenylpyrroles family). However, in vitro tests for mycelial growth, conidial germination and germ-tube elongation revealed the existence of A. brassicicola isolates highly resistant (EC50 > 100 Mg/l) to both dicarboximides (e.g. iprodione and procymidone) and phenylpyrroles (e.g. fludioxonil). These resistant isolates did not exhibit lower sporulation capacities or reduced aggressiveness towards host-plants compared to sensitive A. brassicicola isolates. To our knowledge, this is the first report of fungal field isolates highly cross-resistant to dicarboximides and phenylpyrroles. (C) 2003 Elsevier Ltd. All rights reserved.