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Reduction of Bacterial Spot Disease Severity on Tomato and Pepper Plants with Foliar Applications of Ammonium Lignosulfonate and Potassium Phosphate

Bacterial spot is a serious and persistent disease problem of tomato and bell pepper in both the United States and Canada. Current disease management practices, based primarily on fixed copper bactericides, do not give consistent, effective protection. Foliar applications of ammonium lignosulfonate (ALS), derived from the wood pulping process, and the fertilizer potassium phosphate (KP) were tested for their ability to control this disease under both greenhouse and field conditions. Acibenzolar-S-methyl was included as a control. Greenhouse-grown tomato transplants treated with acibenzolar-S-methyl, 2 or 4% (vol/vol) ALS, 25 mM KP, or 2% ALS plus 10 mM KP and then inoculated with Xanthomonas campestris pv. vesicatoria had significantly less disease than the unprotected controls. Weekly foliar applications of acibenzolar-S-methyl, ALS, or KP significantly reduced disease severity on the foliage of inoculated field-grown tomato and pepper plants; although less disease appeared on the fruit of these plants, the effect was not always statistically significant except for the acibenzolar-S-methyl treatment. Acibenzolar-S-methyl increased the yield of marketable tomato fruit in 2 of 3 years of the study and that of pepper fruit in 1 of 2 years. There was a marked increase in the yield of marketable fruit on all ALS-treated pepper plants in 2001. None of the treatments significantly increased total tomato or pepper yield. ALS and KP had no observable phytotoxic effect on tomato or pepper foliage. Our results indicate that future integrated disease management programs for bacterial spot may be enhanced by including foliar sprays of these two products.