Response of Sorghum Genotypes to Charcoal Rot and Fusarium Infection Under Three Nitrogen Fertilization Regimes.

Stalk rots are complex diseases affected both by genetic and environmental factors. Genotypes that maintain functional green leaf area during maturity (stay-green) tend to possess greater resistance to these diseases. The onset of senescence on the other hand has been partly implicated to nitrogen nutrition. The objective of this study was to assess the reaction of sorghum genotypes with variable stay-green properties to infection by Macrophomina phaseolina and Fusarium thapsinum under various levels of nitrogen fertilization. Twelve sorghum genotypes (six hybrids and six inbreds) were grown under three levels of nitrogen fertilization 0, 45, and 90 Kg ha^-1 during the 2010 and 2011 season at two locations in Kansas. The experiment was a randomized complete block design with four replications. At 14 d after flowering, three plants in each plot were inoculated on the basal stalk using M. phaseolina-infested toothpicks and three were inoculated with liquid inoculum of F. thapsinum (5 x 10⁴ conidia ml^-1) using a modified syringe. At maturity, infected plants were harvested and rated for disease severity by measuring lesion length and determining the number of nodes crossed by lesions. Also, total nitrogen was determined from tissue samples collected at flowering and maturity. Stay-green inbreds had lower infection compared to high yielding hybrids, but mean lesion length was generally greater in F. thapsinum inoculated plants. The impact of nitrogen fertilization on disease development varied between genotypes and locations. There were positive and negative relationships between tissue nitrogen content and stalk rot development.