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Physiological studies of Alternaria citri and A. tenuis.

SUMMARY Alternaria citri and A. tenuis grew over a temperature range of 15 to 35 C, the optimum being at 25 C. Both fungi were able to grow at a pH range of 2.7 to 8.0, the optimum being at 5.4. Growth was good on galactose, lactose and mannose. There was an increase in growth with an increase in glucose concentration from 0.5 to 30 g/liter. Both fungi were able to metabolize nitrate, ammonium and organic nitrogen and growth varied with the nitrogen source. No correlation could be drawn between mycelial growth and the rate of assimilation of 9 amino acids. With an increase in C/N ratio, dry weight increased. The disease reaction in plants is the resultant of host-parasite metabolic activity and brings about qualitative and quantitative changes in the host tissue. Knowledge of physiology of the pathogen can be successfully employed in combating plant diseases (Cochrane, 1958). Thus, a comparative knowledge of nutritional patterns and factors influencing sporulation are prerequisite to any study leading to the understanding of host-parasite relationships and species specificity. The present studies deal with the physiology of Alternaria citri Ellis & Pierce, causing black rot of Citrus medica fruits and A. tenuis Nees ex Corda, causing leaf spot of Abelmoschus esculentus. MATERIALS AND METHODS The fungi were isolated from their respective hosts and single spore cultures obtained. Nutritional studies were carried out with a basal medium containing glucose -5 g, KNO3 -3.5 g, KH2PO4 - 1.75 g, and MgSO4 * 7H20 - 0.75 g in 1 liter of distilled water. The carbon and nitrogen sources were in amounts which supplied 2 g carbon and 0.485 g nitrogen per liter. The amount of peptone and starch were similar to that of KNO3 and glucose respectively of the basal medium. Cultures were made by taking 50 ml aliquots of different media in