Effect of environmental factors on Stenocarpella macrospora (Earle) Sutton and disease-yield loss relationship as affected by site inoculation and inoculum concentration in corn (Zea mays L.)

The laboratory results showed that mycelial growth and pycnidial formation of the fungus were variously affected by the different factors which include the culture medium, light, temperature and pH. Luxuriant mycelial growth was observed in all media exposed under different light conditions with oatmeal agar as the best. Submerged pycnidia were formed singly in carrot agar, corn meal agar and V-8 juice agar but were formed as surface aggregates on oatmeal agar. Malt extract agar and potato dextrose agar did not produce pycnidia under various light conditions. The optimum temperature for mycelial growth, pycnicial formation, conidial germination and discharge of conidia from pycnidia was from 20 to 30 deg C. At higher temperature the conidia need more moisture to trigger germination. The fungus had a wide pH range favorable for growth from pH 3.5 to pH 5.85. Numerous pycnidia were formed in media with added lactic acid (pH 5.65) higher than those adjusted with the addition of NaOH and HCL. The results of the field experiments in two cropping seasons (A and B) indicated that the fungus that causes leaf blight as well as stalk and ear rot of corn caused crop losses as much as 100% regardless of the amount of inoculum (1 x 10E 4, 2 x 10E 4, 3 x 10 E 4 and 4 x 10E 4 conidia/ml) when inoculation was made in ear and in combination with other parts of the plant. The leaf blight severity at 56 days after inoculation (DAI) or 91 days after emergence (DAE), stalk rot at 14DAI (69 DAE), stalk rot at 28 DAI (83 DAE) and ear rot were the strong predictors of percentage crop loss during Season A while the stalk rot at 28 DAI (83 DAE) and eat rot for Season B. The yield (tons/ha), ear diameter and weight of 1000 seeds were significantly affected by the site of inoculation and amount of inoculum in both seasons. Leaf blight increased with time in both seasons. The increase was higher with an increase in the amount of inoculum. During Season A, an apparent increase was observed from 28 to 56 days after inoculation (DAI) irrespective of the site of inoculation and amount of inoculum. During Season B, the increase was gradual from 14 to 42 DAI then abruptly increased at 56 DAI. Leaf inoculated plants were not observed to exhibit stalk rot infection. However, they exhibited a certain degree of ear rot infection, 12.6 and 18.33% with the highest inoculum level (4 x 10E 4) and a corresponding percentage crop loss of 48.31 and 25.68 during the cropping seasons A and B, respectively. Stalk rot occured only in stalk inoculated plants in both seasons