Isolation and characterization of Alternaria brassicicola from Assam (India) and screening for resistance in Sinapis alba

Alternaria blight, caused by the necrotrophic fungus Alternaria brassicicola is a devastating and widespread disease of oilseed Brassicas. Conventional breeding to develop cultivars resistant to the disease has not been successful due to the non-existence of resistance sources within the cultivated species of Brassica. Sinapis alba, a wild cultivar of the Brassicaceae family, reportedly exhibits resistance to Alternaria blight disease. Here, we report the isolation and characterization of A. brassicicola from Brassica rapa, from the state of Assam (India). We also characterized the S. alba - A. brassicicola interaction through morpho-histopathological studies and, screened for host resistance in S. alba against the pathogen. Detached leaf assay and artificial pathogen inoculation resulted in the development of typical blight symptoms on the leaves of the highly susceptible cultivar B. rapa within 24 hours post inoculation (hpi) whereas no symptoms were observed in S. alba. Trypan blue staining was negligible in S. alba but showed pronounced cell death in B. rapa. Light microscopy and scanning electron microscopy studies revealed the presence of an extensive conidial mass and mycelial spread in B. rapa while very less mycelia with only a few spores were detected in S. alba. Furthermore, 10 days post inoculation (dpi), the % Leaf Disease Incidence (%LDI) in B. rapa was found to be 82.50% but this value was only 15% for S. alba. The study established that the wild relative S. alba is a potential source of resistance to Alternaria blight and offers the prospect of gene mining to develop oilseed Brassica varieties resistant to the disease.