Genetic variation and survival of Erysiphe necator in tropical India

Abstract

The heterothallic, biotrophic fungus Erysiphe necator causes powdery mildew on grapes which impacts the yield and quality of fruits and their processed products. Two genetic groups, A and B, have been described which overwinter either as flag shoots or chasmothecia. The genetic structure of the Indian isolates and their oversummering mode in tropical regions is not known. In this study, population genetic analysis of 26 and 134 E. necator isolates from temperate and tropical regions of India, respectively, was performed. PCR analysis showed presence of both genetic groups A and B in temperate regions and only genetic group B in tropical regions. Flag shoots were not observed in tropical region. Weather data analysis and temperature sensitivity of E. necator conidia indicate the possibility of the pathogen oversummering as conidia on bark. Sequence analysis of ITS (Internal Transcribed Spacer), IGS (Intergenic Spacer), TUB2 (Beta-Tubulin) and EF1-α (Translation Elongation Factor 1-α) genes covering 1762 nucleotides from 22 isolates showed the presence of four haplotypes, A-IN1, B-IN1, B-IN2 and B-IN3 in temperate regions and only haplotype B-IN3 in tropical regions. Haplotypes A-IN1 and B-IN1 were similar to haplotypes of group A and B, respectively, reported from USA, Europe and Australia. The other two haplotypes, B-IN2 and B-IN3 grouped with haplotypes from USA and are new reports from India. The diversity and haplotype structure suggest that powdery mildew was introduced in north India and then moved to tropical regions corroborating historical reports of the introduction of grapevines in India. Occurrence of a genetic group with lower fungicide sensitivity and greater aggressiveness might explain the difficulties to manage this disease in Maharashtra, India.