Doctor of Philosophy (PhD)
Plant Pathology and Crop Physiology
Nitrogen nonutilizing mutants were used to assess vegetative compatibility of 58 isolates of Cercospora kikuchii, 55 of which were isolated from soybean plants in Louisiana. Only 16 of 56 self-compatible isolates were assigned to six multi-member vegetative compatibility groups (VCGs), 01-06, with two or three isolates in each VCG. The other 40 isolates were not vegetatively compatible with any isolates other than themselves. All six multi-member VCGs contained isolates from different soybean cultivars, and three included isolates from different locations. Only one VCG included isolates both from soybean leaves and from seeds, while two and three multi-member VCGs included isolates only from leaves or from seeds, respectively. Population structure of C. kikuchii was further examined in 164 isolates, 161 of which were from Louisiana, with random amplified polymorphic DNA (RAPD) and microsatellite-primed PCR (MP-PCR). All isolates in the VCG study were included. Based on analysis of molecular variances, isolates from different host cultivars or different locations in Louisiana were not significantly different, but the Louisiana population was significantly different from isolates collected outside the state. Leaf and seed populations were significantly different. In the clustering analysis, isolates from Louisiana were grouped into four lineages, clades A-D. Clades A-C were further grouped into a large clade (ABC) with moderately strong bootstrap support. Clade B was the most dominant lineage in Louisiana. Only seven isolates from Louisiana were in the lineage, clade D, that included all three isolates from outside sources. Multilocus gametic disequilibrium tests did not reject the null hypothesis of random mating in clade B, but it was rejected in Clades A and D and the total collection. Some isolates within a VCG were closely related, but isolates within a VCG were not clustered together according to VCG in general. Representive isolates in clades A, B, and D were used to screen six commercial soybean cultivars, HBK R5588, AG5701, DP 5806 RR, TV59R85, SS RT 6299N and DP 6880 RR. Cultivars AG5701 and TV59R85 were significantly more resistant than other cultivars, and cultivar DP 6880 RR was most susceptible. Clade D was significantly more virulent than the other two lineages.
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Cai, Guohong, "Cercospora leaf blight of soybean: pathogen vegetative compatibility groups, population structure, and host resistance" (2004). LSU Doctoral Dissertations. 3605.
Raymond W. Schneider