Insensitivity of Bremia lactucae to Metalaxyl on Lettuce in Florida. R. N. Raid, University of Florida, Everglades Research and Education Center, Belle Glade 33430. L. E. Datnoff, T. Schettini, and R. W. Michelmore. University of Florida, Everglades Research and Education Center, Belle Glade 33430, and Vegetable Crops Department, University of California, Davis 95616. Plant Dis. 74:81. Accepted for publication 10 September 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0081A.
A widespread outbreak of downy mildew of lettuce (Lactuca sativa
L.), caused by Bremia lactucae Regel, occurred during February 1989
in the Everglades agricultural area of south Florida, the first since
applications of metalaxyl for control were begun in 1986. Actively
sporulating lesions on metalaxyl-treated lettuce and no significant
difference (P≤0.05) between disease indices of a metalaxyl/ chlorothalonil
treatment and the untreated check in a fungicide trial
suggested the presence of a metalaxyl-insensitive strain. Isolates of
B. lactucae were collected from metalaxyl-treated and untreated lettuce
fields. Metalaxyl sensitivity of each Florida isolate was determined
and compared with an isolate of known sensitivity using the seedling
drench method (1). All Florida isolates were metalaxyl-insensitive;
they sporulated even at the highest test concentration of 100 ppm.
This is the first report of metalaxyl-insensitive B. lactucae in Florida.