Effect of intermittent exposures of SO2 on the leaf blight caused by Alternaria brassicicola on Indian mustard

Effects of low levels of SO2 were investigated on the Alternaria blight on Indian mustard cultivars under artificial treatment condition. Ten cultivars of Indian mustard, Brassica juncea L. viz., Alankar, BS-2, Kalamoti, Karishma, Kranti, Mahyco Bold, Pusa Bold, Rohini, Swarna and T-59 were exposed to 5.7 (ambient), 71.5, 143 and 214.5 μg SO2 m−3 concentration for 5 h day−1 on alternate days for 3 months in open top exposure chambers to evaluate resistance/tolerance against the gas. SO2 at 71.5 and 143 μg m−3 concentrations did not incite any measurable injury to mustard cultivars. However, 143 μg SO2 m−3 caused visible injuries to all cultivars screened and led to a significant reduction in plant length, yield, and oil contents (P ≤ 0.05). Effects of intermittent exposures of SO2 on leaf blight caused by A. brassicicola on the mustard cultivars were also studied. The fungus inoculated plants developed dark brown to almost black necrotic circular lesions, and severity of the blight increased on plants exposed to 143 μg SO2 m−3, but decreased at 214.5 μg SO2 m−3. The interaction between the gas and the fungus was found to be dependent of SO2 concentration. The gas at 71.5 and 143 μg SO2 m−3 promoted pathogenesis of A. brassicicola with result the cv. Kalamoti, expressing tolerance to the fungus, becoming susceptible and exhibiting greater blight and plant growth reductions at 143 μg SO2 m−3; the gas injury was also relatively greater in this treatment. The gas promoted sporulation of the fungus. Interaction of 214.5 μg SO2 m−3 concentration and A. brassicicola was found to be antagonistic. The cv. Kalamoti showed resistance to A. brassicicola and did not exhibit significant suppression in the yield. The exposures especially 143 μg m−3 apparently broke the tolerance reaction of the cv. Kalamoti and greater lesions (32%) developed on the leaves of inoculated plants, fungus inoculation, however, did not influence the sensitivity of the cultivars to SO2. Synergistic interaction between 143 μg SO2 m−3 and the fungus was recorded on six cultivars out of ten cultivars tested, whereas the interaction at 214.5 μg SO2 m−3 was antagonistic on two cultivars (Alankar and Mahyco Bold) and on rest near to additive. At 214.5 μg SO2 m−3, the sporulation was suppressed but increased at 143 μg SO2 m−3. The study has shown that lower concentration of SO2 (143 μg SO2 m−3) stimulated the Alternaria blight but the higher concentration (214.5 μg SO2 m−3) suppressed the disease.