Triazole sensitivity among South African Puccinia graminis f. sp. tritici isolates

Abstract

Fungicide applications are important in the successful production of winter cereals in South Africa (SA). While fungicide treatments are critical for crop protection, genetic selection and inheritance of fungicide resistance will lead to a decline in disease control. In this study we determined the CYP51 genotypes and in vitro triazole sensitivity of South African Puccinia graminis f. sp. tritici isolates. Sequencing results identified two allelic variants of the CYP51 gene among the isolates. While alleles 1 and 2 differed from each other, both were identical across the isolates, even regarding the mutations and deletions present within the introns. Urediniospore germination percentages were measured for each isolate in replicated experiments against the triazole-based fungicides, propiconazole and tebuconazole, to determine the effective inhibition of half-maximal effective concentration (EC₅₀) values. The EC₅₀ for tebuconazole ranged from 0.004 (isolate 22) to 0.018  mg L^-1 (isolate 28) with a mean of 0.013  mg L^-1 and for propiconazole from 0.0002  mg L^-1 (isolate 44) to 0.0937  mg L^-1 (isolate 41) with a mean EC₅₀ of 0.057  mg L^-1. The highest tested concentrations of propiconazole (166.7  mg L^-1) and tebuconazole (20.6  mg L^-1), effectively inhibited urediniospore germination. The fungicide results did not reveal evidence of decreased sensitivity over time.