Alternaria Blight (Alternaria dauci):
Alternaria leaf spots first appear at the margin of the leaflets and are dark brown to black and irregular in shape. Lesions produced on the petioles and stems are dark brown and often coalesce and girdle the stems. As the disease progresses entire leaflets may shrivel and die, appearing scorched. Alternaria leaf lesions are generally more prevalent on older foliage and plants than on young foliage. The disease spreads rapidly on the older leaves of a maturing crop after the rows have closed. This is due in part to poor air circulation among the older lower leaves in the canopy and to the moisture-holding capacity of the dense foliage.
Control:
The disease can be kept under check if a well-drained soil is selected and suitable crop rotation is adopted. Since the fungus can survive in the seed, hot water treatment at 50°C for 15 minutes is recommended. Seed treatment with Thiram (3g/kg of seed) before sowing is effective to control the disease. Crop rotation and destruction of infected plant material in the field will minimize the disease infection, Fungicidal applications with Foltaf (0.2%), Copper Oxychloride (0.3%) satisfactorily controls the disease.
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