Fitness and cross-resistance of Alternaria alternata field isolates with specific or multiple resistance to single site inhibitors and mancozeb

42 field isolates of Alternaria alternata collected from infected tomato fruit of processing plants located in the region of Pelloponisos, South Greece, were characterized in terms of sensitivity to mancozeb, tebuconazole, fludioxonil, iprodione and cyprodinil. Fungitoxicity tests in vitro revealed that 63 % of the isolates tested were sensitive to all the above fungicides whereas the remainder of the isolates exhibited reduced sensitivity to one or more of these fungicides. The observed resistance phenotypes were confirmed by in vivo experiments on artificially inoculated tomato fruit. EC₅₀ in vitro values ranged from 2.34 to >100 μg/ml for mancozeb, 0.16 to >10 μg/ml for iprodione, 0.43 to 20 μg/ml for tebuconazole, 0.007 to >5 μg/ml for cyprodinil and from 0.002 to >10 μg/ml for fludioxonil. Sensitivity distributions of A. alternata isolates to mancozeb, iprodione, cyprodinil and fludioxonil was bimodal indicating a probable initiation of a discrete shift of the population towards reduced sensitivity to these fungicides. Correlation analysis of respective EC₅₀ values showed positive cross-resistance relationships between fludioxonil and iprodione while positive correlation was observed between mancozeb and cyprodinil, mancozeb and tebuconazole, and cyprodinil and tebuconazole. Interestingly, a number of A. alternata isolates were specifically resistant to the multi-site inhibitor mancozeb, a fact reported for the first time. Isolates resistant to mancozeb were more aggressive while those with reduced sensitivity to both fludioxonil and iprodione produced significantly less conidia than sensitive ones. A considerable portion of the sample exhibited reduced sensitivity and no apparent fitness penalties, a fact that should be considered in future planning and implementation of anti-resistance strategies.