Physiologic Specialization and Genetic Differentiation of Puccinia triticina Causing Leaf Rust of Wheat in the Indian Subcontinent during 2016-2019.

Wheat is the second most cultivated cereal in the world and is equally important in India. Leaf (brown) rust, caused by Puccinia triticina, was most prevalent among the three rusts in all the wheat-growing areas of India, Bhutan, and Nepal during 2016 to 2019. Leaf rust samples from wheat crops in these countries were pathotyped using the wheat differential genotypes and binomial Indian system of nomenclature. To facilitate international communication, each pathotype identified was also tested on the North American differentials. A total of 33 pathotypes were identified from 1,086 samples, including 3 new pathotypes, 61R47 (162-5 = KHTDM) and 93R49 (49 = NHKTN) from India and 93R57 (20-1 = NHKTL) from Nepal. Two pathotypes, 121R60-1 (77-9/52 = MHTKL) and 121R63-1 (77-5 = THTTM), accounted for 79.46% of the population. Virulence on Lr19 was identified in 0.27% of the samples and from Nepal only. The proportion of pathotype 121R60-1 (77-9 = MHTKL) increased during these years to 57.55%. Virulence was not observed to Lr9, Lr24, Lr25, Lr28, Lr32, Lr39, Lr45, and Lr47 in the population of the Indian subcontinent. Eighteen polymorphic simple sequence repeat (SSR) primer pairs tested on the isolates amplified 48 alleles with an average of 2.66 alleles per primer pair. Based on SSR genotyping, these pathotypes could be grouped into two clades with further two subclades each. Many of the Lr genes present in Indian wheat germplasm (Lr1, Lr3a, Lr10, Lr11, Lr14a, Lr15, Lr16, Lr17, Lr20, Lr23, and Lr26) were ineffective to a majority of the pathotypes. Most of these varieties possessed a high degree of leaf rust resistance. The field resistance of wheat varieties could be attributed to the interaction of genes, unknown resistance, or adult plant resistance.