While out in Livingston Parish with Kenny Sharpe earlier in the week, we visited a commercial grower having a problem with mustard greens. My first thought was downy mildew, but it turned out to be white leaf spot, caused by the fungus Pseudocercosporella capsellae. The conidia of this fungus, which are dispersed by wind, were quite abundant on the lesions. Cabrio (pyraclostrobin) is the only fungicide that specifically mentions this disease on the label.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA