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An Analysis of Plant Breeding Procedures for Obtaining Curly Top Resistance in Tomato. P. E. Moser, Graduate Assistant, Department of Botany, Utah State University, Logan, Utah 84321, Present address of senior author: Department of Plant Pathology, MacDonald College of McGill University, MacDonald College P.O., Quebec, Canada; O. S. Cannon, Professor, Department of Botany, Utah State University, Logan, Utah 84321. Phytopathology 62:564-566. Accepted for publication 27 December 1971. DOI: 10.1094/Phyto-62-564.

Methods of breeding tomato varieties for curly top resistance were evaluated. Some genetic factor or factors had eliminated or modified the expression of genes for curly top resistance in the modified backcross to Lycopersicon esculentum Mill. by the hybrid L. esculentum × L. peruvianum var. dentatum Dun. Resistance of the USDA breeding lines, CVF4 and C5, was also less than that of L. peruvianum var. dentatum, which was included in their ancestry. Failure of breeding methods to retain the high level of curly top resistance of L. peruvianum var. dentatum was attributed to the consistent loss of resistance in the modified backcross of the original interspecific hybrids, and sharing of genes for resistance by later intercrossed lines. In additional curly top tests, F2 progeny (from seed set in the field on F1 plants which were either selfpollinated or received pollen from hybrid offspring of L. peruvianum var. dentatum) expressed considerable resistance. It will be necessary, however, to overcome the factor or factors causing the loss of resistance in order to achieve greater success in future breeding programs. This will require considerable research with original interspecific crosses and less with existing, partially resistant breeding lines.