Detection of resistance to demethylation inhibitor fungicides in Erysiphe necator from tropical India by biological and molecular assays.

Abstract

Fungicides of demethylation inhibitor (DMI) group are used worldwide for the management of Erysiphe necator but are associated with medium to high risk of development of resistance in the pathogen. Till date there was no report on the presence of DMI resistance in E. necator isolates from the major grape growing regions in tropical India, though there were instances of DMI fungicides providing less than accepted levels of powdery mildew control. In this study, 54 E. necator isolates were collected during 2015–2017 from vineyards located in different geographical regions of India. The isolates were tested for their sensitivity to the commonly used DMI fungicide, myclobutanil, using leaf disc bioassay. Four isolates were sensitive (MIC <1.0 μg/ml), nine were moderately resistant (MIC 1.0 to < 10 μg/ml) and 41 were resistant (MIC > 10 μg/ml) to the fungicide myclobutanil. The resistance factor (RF) ranged from 1.5 to 295. In PCR amplification of a specific allele, the product specific for A495T mutation was produced only in the 43 isolates with RF > 4. The CYP51 gene sequence analysis confirmed A495T mutation leading to Y136F change associated with high levels of resistance to DMI fungicides. Cross resistance studies among the DMI fungicides showed that 11 out of 13 myclobutanil resistant isolates were also resistant to difenoconazole and tetraconazole. Three myclobutanil sensitive isolates were also sensitive to difenoconazole and tetraconazole. Detection of resistance in E. necator isolates from the major grape growing region of tropical India stresses on the need for developing resistance management strategies.