VIEW ARTICLE

Altered Host Specificity in Race 1 of Xanthomonas malvacearum by Passage Through a Resistant Variety of Gossypium hirsutum. W. C. Schnathorst, Plant Pathologist, Crops Research Division, ARS, USDA, University of California, Davis 95616. Phytopathology 60:258-260. Accepted for publication 5 September 1969. DOI: 10.1094/Phyto-60-258.

A single-cell culture of race 1 of Xanthomonas malvacearum was inoculated to young leaves of Stoneville 20, a resistant variety of Gossypium hirsutum. Isolations were made from small, partially or wholly necrotic lesions 14 and 56 days later. No detectable change in colony traits or in pathogenicity occurred in race 1 in 14 days (even after three successive passes through leaves), but colonies similar to race 2 were apparent in isolations made after 56 days (on the first pass). Purification of buff (race 1 type) and yellow (race 2 type) colonies and inoculation to Stoneville 20 demonstrated that the yellow colonies were race 2. Race 2 comprised 77% of the cells in necrotic lesions 56 days after inoculation. Since no spontaneous change from race 1 to race 2 had been observed in culture over a 10-year period, and since none was apparent in hundreds of isolations from lesions from susceptible varieties, the altered specificity is associated with the B₇ (resistance) gene in Stoneville 20.