Tillage effect on seasonal nitrogen availability in corn supplied with legume green manures

Colonization of sorghum by Macrophomina phaseolina in field plots was determined at nitrogen fertilization rates of 0, 56, 112, and 168 kg ha^-1 in 1988 and 0, 84, 168, and 256 kg ha^-1 in 1989. Above ground plant tissue and roots were sampled monthly to determine total nitrogen and percent colonization of root segments by natural inoculum. Root infection was not affected by nitrogen treatment, but was affected by growth stage and environment. High root infection occurred before reproductive development (growth stage 3) in 1988 and was associated with hot, dry weather early in the growing season. In 1989, when the weather was cool and wet, root infection began after reproductive development (growth stage 4). The effect of nitrogen treatments on lesion length was determined in sorghum stalks artificially inoculated with M. phaseolina. Lesion lengths were significantly affected by both nitrogen treatments and growth stage. Lesions were significantly longer with all nitrogen treatments at growth stage 9 than with the no-nitrogen treatment, and lesions tended to increase with increased levels of nitrogen fertilization. Significant increases in lesion length occurred between growth stages 5, 7, and 9 in 1988 and between 7 and 9 in 1989. This study demonstrates that nitrogen fertilization affects colonization of sorghum stalks but not root infection by M. phaseolina.