Identification of Alternaria spp. associated with purple blotch disease in onions and evaluation of plant-based fungicides for disease control

Abstract

Purple blotch of onion, a significant disease impacting major onion cultivating regions globally, is often linked with Alternaria spp. In this study, symptomatic onion leaves from 11 different fields were sampled, resulting in the acquisition of 24 Alternaria isolates. Six were chosen for sequence analysis, which involved the amplification of three regions: the internal transcribed spacer (ITS), glyceraldehyde 3-phosphate dehydrogenase (Gpd), and the major allergenic gene Alt a1. Our findings indicated the presence of three distinct Alternaria spp., with A. tenuissima demonstrating the highest disease incidence on the susceptible cultivar, Phulkari. In vitro bioassay results revealed that the methanolic leaf extract of D. stramonium exhibited the most significant antifungal activity (68.52%), outperforming M. piperita (43.13%) and C. procera (40.16%). Greenhouse experiments showed that aqueous extracts of D. stramonium reduced disease severity by 43.75% and 46.23% when used as a protective measure, and by 41.95% and 38.43% when used as a curative measure in two consecutive years. The D. stramonium extract was further fractionated using organic solvents, revealing varying degrees of antifungal activities: n-butanol (99.31%), chloroform (68.76%), n-hexane (50.56%), and ethyl acetate (62.60%). GC-MS analysis of the n-butanol fraction identified nine compounds, primarily alkaloids. From identified conpounds, scopolamine (15.23%), atropine (14.42%), apoatropine (9.55%), 2-methoxy-4-vinylphenol (2.33%) are already reported for their biological activity. Our findings suggest that D. stramonium has potential as an alternative disease control agent against purple blotch in onions.