Pathotyping of Bremia lactucae in Florida. L. E. DATNOFF, Associate Professor of Plant Pathology, Departments of Plant Pathology and Horticulture, University of Florida, Institute of Food and Agricultural Sciences, Everglades Research and Education Center, P.O. Box 8003, Belle Glade 33430-8003. R. T. NAGATA, Associate Professor of Horticulture, and R. N. RAID, Associate Professor of Plant Pathology, Departments of Plant Pathology and Horticulture, University of Florida, Institute of Food and Agricultural Sciences, Everglades Research and Education Center, P.O. Box 8003, Belle Glade 33430-8003. Plant Dis. 78:854-857. Accepted for publication 6 June 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0854.

Isolates of Bremia lactucae were obtained from infected lettuce coming from California into Florida for fresh market sale and preparation of fresh mixed salads and from naturally infected lettuce fields in Florida during 1991 and 1992. Using an established lettuce tester set containing the 13 Dm resistant genes, isolates of B. lactucae were grouped into distinct pathotypes based on presence or absence of or infrequent sporulation. Among all the isolates of B. lactucae tested, sporulation was absent on Dml, Dmll, and Dm15. These isolates usually produced infrequent or sparse sporulation on seedlings of lines containing Dm4, Dm10, and Dml6. Based on this information, 72% of the isolates obtained from infected lettuce coming from California into Florida were pathotype IV. Of the isolates from Florida field-grown infected lettuce, 50% were determined to be pathotype IV and 28%, pathotype III. These data suggest that infected lettuce arriving from California may be an important source of primary inoculum for epidemics of downy mildew in Florida.

Keyword(s): Lactuca sativa