Rainy weather with temperatures in the 80s favors the development of bacterial brown spot. The brown spot bacteria are splash dispersed in the field. Typical leaf symptoms are small brown spots with a small yellow border. When disease is severe, the spots coalesce and large portions of the leaf may be destroyed. Stem lesions occasionally may develop.
Growers ask if a copper fungicide can be used to control bacterial brown spot.
Based on research in Wisconsin, the answer is a guarded yes. These research trials indicate that if a copper fungicide is applied at the first trifoliate leaf stage, and repeated weekly so that brown spot bacterial populations never build up, the disease can be controlled. Of course this is not economic under our conditions. Applying a copper fungicide when the disease is first observed will suppress the disease, but is not as effective as weekly applications beginning at the first trifoliate. If bacterial brown spot is already widespread in the field and actively attacking plants, the bacterial populations are high and it may be too late for effective use of a copper fungicide.