VIEW ARTICLE

Variability in Host-Parasite Relationships of Heterodera trifolii. N. D. Singh, Department of Botany and Plant Pathology, Iowa State University, Ames 50010, Present address of senior author: Lenora, West Demerara, Guyana; D. C. Norton, Department of Botany and Plant Pathology, Iowa State University, Ames 50010. Phytopathology 60:1834-1837. Accepted for publication 24 July 1970. DOI: 10.1094/Phyto-60-1834.

Significant differences in cyst production existed among Heterodera trifolii isolates from Canada, Illinois, Kentucky, Oregon, and Story and Union counties, Iowa, when propagated on Trifolium repens in a greenhouse. Quantitative differences in cyst production were greatest on Trifolium pratense, T. fragiferum, and one cultivar of Melilotus officinalis. Differences in cyst production were as great among eight isolates from western Iowa as were differences among isolates obtained from the more widely distributed localities. Cysts of the Canadian isolate were significantly longer than those from Illinois and from Story and Union counties, Iowa. The length: breadth cyst ratio of the Union County isolate was significantly less than from Canada, Illinois, Kentucky, Oregon, and Story County. Differences in pathogenicity of three isolates were correlated with histopathological differences and cyst production. Histological studies indicated that cyst production was high only when stelar syncytia were formed. When either hypersensitive cell necrosis or syncytial abortion or both occurred, cyst production was relatively low.