Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Twenty nine isolates of Cochliobolus miyabeanus (Ito & Kuribayashi) Drechsler ex Dastur cultured from leaves and seeds of several rice, Oryza sativa L., varieties obtained from rice growing areas of Louisiana, Arkansas and Texas were tested for pathogenicity on 12 test varieties to determine if physiologically specialized races of this fungus occur in the United States. This study was conducted in the greenhouse at Louisiana State University, Baton Rouge, Louisiana. The results obtained from inoculating 12 test varieties with 29 isolates of C. miyabeanus as expressed in lesion size, lesion number and disease rating indicated no clear evidence for physiologic specialization in C. miyabeanus. The results indicated differences in virulence occurred among the isolates ranging from highly virulent to weakly virulent biotypes. The morphology of conidia of six isolates cultured on rabbit food agar were studied and the conidia of one isolate was significantly larger than those of the other isolates. The average conidia size ranged from 60 x 10 (mu)m to 150 x 24.5 (mu)m and number of septa per conidia ranged from 5 to 12. The results indicated there was a wide range in length, width and number of septa for conidia from isolates of C. miyabeanus produced on artificial media. The inheritance of plant reaction to C. miyabeanus was studied in the F(,1) and F(,2) populations from the hybridization of the resistant cultivar Taichung Native #1 and a susceptible cultivar, Dular. The F(,2) progeny showed a continuous distribution for plant reactions to C. miyabeanus as expressed in lesion size, lesion number and general disease rating under field conditions which indicated a quatitative type of inheritance for this trait. The number of segregating genes for plant reaction to C. miyabeanus in the F(,2) population was estimated using the Castle-Wright formula and the number of parental phenotypes recovered in the F(,2) population. A minimum of three major genes appears to condition the differences in lesion size between these two varieties. A minimum of one gene appears to condition the difference in lesion number. Three to six genes seem to condition the differences in the disease rating which includes factors for both lesion size and lesion number. Broad sense estimates of heritability for plant reaction to C. miyabeanus calculated from the variance of an F(,2) population transplanted to the field were 59% for lesion size, 87% for lesion number and 46% for disease rating. Heritability estimates calculated from an F(,2) population seeded directly into the field under relatively close spacing were 55% for lesion size, 29% for lesion number and 48% for disease rating.