Biological characteristics of dicarboximides resistant isolates of Stemphylium vesicarium from Italian pear orchards

Dicarboximide-resistant isolates of Stemphylium vesicarium have been detected in Italian pear orchards since the early 1990s. Two different resistant phenotypes were observed during a 9 year monitoring study. R1 phenotypes showed Resistance Factor (RF)> 100 towards procymidone and between 3 and 100 to the other dicarboximides (iprodione, vinclozolin and chlozolinate) whereas R2 phenotypes showed RF>100 towards all fungicides of this group. Various morphological and physiological characteristics of the different resistant isolates, such as mycelial colour, mycelial growth rates, sporulation, germination rates and pathogenicity on detached leaves, were compared to those of sensitive isolates. No significant differences could be noted among the phenotypes. Competitive abilities of sensitive and R1 resistant isolates were then evaluated in vitro on unamended medium through mixed inoculations of R/S conidia in 75:25, 50:50 and 25:75 ratios. R1 resistant isolates did not seem to be less competitive in vitro than sensitive ones in the absence of dicarboximide fungicides.