Identification of Alternaria species associated with purple blotch disease of onions in Southern Punjab of Pakistan and evaluation of plant-based fungicides for disease control

Abstract

Purple blotch of onion, a significant disease affecting major onion cultivating regions globally, is often linked with Alternaria species. In this study, symptomatic onion leaves from 11 different fields in Southern Punjab, Pakistan were sampled, resulting in the acquisition of 25 Alternaria isolates. Six isolates were chosen for DNA sequence analysis, which involved the amplification of three DNA regions: internal transcribed spacer (ITS), glyceraldehyde 3‐phosphate dehydrogenase (GAPDH), and the major allergen gene Alt a1. Our findings indicated the presence of two distinct Alternaria species, with A. alternata demonstrating the highest disease severity on the susceptible cultivar, ‘Phulkari’. In vitro bioassay results revealed that the methanolic leaf extract of Datura stramonium exhibited the most significant antifungal activity (68.52%), outperforming Mentha piperita (43.13%) and Calotropis 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 methanolic extract was 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 compounds, 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 extracts have potential as an alternative disease control agent against purple blotch of onions.