Isolation of resistance source for combined diseases viz., purple blotch, basal rot and white rot in onion (Allium cepa L.) by saprophytic and gametophytic screening

Abstract

With plant pathogens and the insects that spread them are exploding worldwide, finding new ways to fight diseases has become an absolute necessity. The first line of defense against plant diseases in natural resistance, which can often be transferred between and among the species by cross breeding for which resistance sources must exist. Evaluation of resistance for long has been done by sporophytic screening which is time consuming and laborious. Hence, it would be desirable to use strategies that allow an early selection of resistant genotypes, male gametophytic screening is one of them. Apparently, pathogenesis-related mechanisms involved in disease resistance are expressed in both vegetative (sporophytic) and generative (gametophytic) tissues. The primary objective of our study was to develop and evaluate screening methodologies for identifying genotypes within heterogeneous populations that have resistance (Multiple resistances) to purple blotch, basal rot and white rot diseases of the onion caused by Alternaria porri, Fusarium oxysporum f.sp.cepae and Sclerothium cepivorum respectively. Our selection procedures were developed initially with the sporophytic generation where sixteen genotypes were evaluated under field condition, polyhouse condition where the seedlings were subjected to different concentrations of the pathogens (4x104 Conidia ml-1, and 1x104 spores ml-1 and 1-2 Sclerotia per seedling for purple blotch, basal rot and white rot respectively) and then tested the bulbs under storage conditions for the storage diseases namely basal rot and white rot at 45x104 spores ml-1 and 6-7 sclerotia per bulb respectively. And in the gametophytic generation the pollens of eight male fertile genotypes were subjected to the crude culture filtrates of the above said diseases in accordance with the proposal that a possible overlap between generations might result from selection of genes expressed in both stages. Considering the economics losses caused by these three important diseases of Onion, the importance of the work carried out is of significance to Indian agriculture, as some promising results were obtained in accordance with our objective.