Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Milton C. Rush


Nine rice cultivars with different levels of resistance to sheath blight, caused by Rhizoctonia solani, were evaluated in the field and greenhouse for quantity of epicuticular wax (EW) from leaves, infection cushion (IC) formation by the pathogen on leaves, and the relationship of these factors to disease resistance. Comparisons among resistant and susceptible cultivars using scanning electron microscopy suggested that more EW was found on the leaf blade and sheath surfaces of resistant cultivars. Gravimetric analysis of EW from rice leaves supported these observations. EW production at different growth stages increased until the boot stage and then dropped. Correlation coefficients for EW and percent disease, EW and IC number, and IC number and percentage of tissue diseased were highly significant and ranged from r = $-$0.88 to $-$0.93; r = $-$0.82 to $-$0.88 and r = 0.95 to 0.97, respectively. The susceptible cultivar Labelle produced significantly less EW when compared to more resistant cultivars grown under the same cultural conditions. Tetep, the most resistant cultivar evaluated, had average IC numbers of 2.0 and 9.2 per cm$\sp2$ compared to 66.0 and 73.4/cm$\sp2$ for the susceptible cultivar Labelle in the field and greenhouse evaluations, respectively. IC production on Labelle was twice as high or more than on the moderately resistant cultivars Mars and Saturn. Highly significant positive correlation coefficients, ranging from r = 0.56 to 0.92, were shown for the relationship of IC number and disease rating. Significant negative coefficients ranging from r = $-$0.57 to $-$0.94 resulted from the correlation of EW quantity and IC numbers. Significant correlation coefficients ranging from r = $-$0.52 to $-$0.90 resulted from the correlation of disease rating and EW quantity suggesting that surface wax may be functional in resistance. Biomass production measured as plant weight, tiller number, and leaf area was significantly increased with increasing nitrogen. Associated with this increase in biomass production, was a concomitant increase in disease incidence, disease progress, and lesion size resulting in significant decreases in yield. However, relative changes in disease with increasing nitrogen were cultivar dependent with susceptible cultivars showing greater increases in disease.