Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Isolates of Cercospora oryzae were readily obtained by incubating pieces of surface sterilized diseased leaf or leaf sheath samples in a moist chamber at 28 C. Sporulation on culture media was significantly higher following incubation at 28 C on cornmeal agar than on water agar, V-8 juice agar, and potato-dextrose agar. Narrow brown leaf spot lesions developed on leaves when inoculated plants were placed inside a humidified chamber for 1 to 16 days. It was possible to differentiate resistant cultivars from susceptible cultivars when 3-wk-old plants were inoculated. Single spore isolates of C. oryzae were isolated from the leaf and leaf sheath samples of rice collected from seven major rice-growing parishes in Louisiana. Eight differential cultivars were inoculated with conidial suspension from each isolate. Forty-three races in six physiologic race groups were identified among 76 isolates from Louisiana, Arkansas, and Texas. Twenty-two rice lines and cultivars were evaluated for their apparent infection rates (r) and for disease severity values in 1983. The four cultivars Melrose, Nova 76, Nato, and Leah, with low r-values in 1983 were evaluated against a total of five races of C. oryzae in 1984 and 1985 field tests. These cultivars had significantly lower r-values and greatly reduced terminal disease severity when compared to susceptible cultivars. Two cultivars, Nova 76 and Melrose, with rate-reducing resistance had significantly lower relative receptivity, number of lesions per leaf, spore production per lesion, and lesion size when compared with the susceptible cultivar Labelle under greenhouse conditions. The number of lesions per leaf and lesion length were significantly reduced on these resistant cultivars when tested against twenty races of C. oryzae. Comparisons were made of the prepenetration activities of C. oryzae on rice plants of the susceptible cultivar Labelle and the resistant cultivars Melrose and Nova 76. Germ tubes showed positive stomatal tropism and appressoria were only formed on or adjacent to stomata. The number of appressoria formed per conidium was significantly greater on Labelle than on Melrose and Nova 76 at 7 or 9 days after inoculation. There was a highly significant correlation (r = 0.999) between both relative receptivity and lesion size with the number of appressoria formed. Conidiophores only emerged through stomata.