Sensitivity of Newly Identified Clades in the Sooty Blotch and Flyspeck Complex on Apple to Thiophanate-methyl and Ziram

Abstract

Sensitivity to the fungicides thiophanate-methyl and ziram was characterized for newly identified clades in the sooty blotch and flyspeck (SBFS) disease complex. Isolates obtained from apple orchards in Illinois, Iowa, Missouri, and Wisconsin were previously grouped into clades based on parsimony analysis of the internal transcriber spacer (ITS) and large subunit (LSU) regions of rDNA. Two isolates from each of eight clades were cultured on water agar amended with a range of fungicide concentrations. Radial growth of colonies was measured after incubation for 21 days at 25°C. Fungicide sensitivity (ED 50 ) did not differ significantly (P = 0.05) between isolates within a clade, but several clades differed significantly. There was a greater variability among clades for sensitivity to thiophanate-methyl than for ziram. Significant differences in fungicide sensitivity among, but not within, clades provide evidence that these clades differ physiologically. Understanding differences in fungicide sensitivity among SBFS clades could have practical implications for improved management of SBFS. Accepted for publication 12 November 2003. Published 9 December 2003.