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 214.5μgSO₂m⁻³ concentration for 5hday⁻¹ on alternate days for 3 months in open top exposure chambers to evaluate resistance/tolerance against the gas. SO₂ at 71.5 and 143μgm⁻³ concentrations did not incite any measurable injury to mustard cultivars. However, 143μgSO₂m⁻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 143μgSO₂m⁻³, but decreased at 214.5μgSO₂m⁻³. The interaction between the gas and the fungus was found to be dependent of SO₂ concentration. The gas at 71.5 and 143μgSO₂m⁻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 143μgSO₂m⁻³; the gas injury was also relatively greater in this treatment. The gas promoted sporulation of the fungus. Interaction of 214.5μgSO₂m⁻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 143μgm⁻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 143μgSO₂m⁻³ and the fungus was recorded on six cultivars out of ten cultivars tested, whereas the interaction at 214.5μgSO₂m⁻³ was antagonistic on two cultivars (Alankar and Mahyco Bold) and on rest near to additive. At 214.5μgSO₂m⁻³, the sporulation was suppressed but increased at 143μgSO₂m⁻³. The study has shown that lower concentration of SO₂ (143μgSO₂m⁻³) stimulated the Alternaria blight but the higher concentration (214.5μgSO₂m⁻³) suppressed the disease.