Bacterial Spot of Tomato Affected By Polyhalite and Other Potassium Fertilizer Sources.

Improving plant nutrition is one of important practices for disease management. Experiments were conducted in the greenhouse during two growing seasons to evaluate the effect of potassium (K) fertilizer source on bacterial spot disease in tomato (Lycopersicon esculentum Mill.). Treatments included four K sources [muriate of potash (MOP), sulfate of potash (SOP), sulfate of potash- magnesia (SOP-M) and polyhalite (POLY)] plus a control (without K fertilizer)]. All treatments received 224 kg N/ha as urea and 224 kg P₂O₅/ha as triple superphosphate.  Potassium fertilizers were applied at the rate of 200 kg K₂O/ha. Tomato plants were inoculated with Xanthomonas perforans T3 strain at 5.0×10⁷ CFU/ml by foliar sprays. Results showed that application of K fertilizers (except for SOP applied in first experiment) significantly (p<0.05) reduced disease severity of bacterial spot compared to the control during both growing seasons. Tomato plants from the treatment with POLY showed significantly lower disease severity than those received SOP.  Application of K fertilizers significantly increased K concentrations in tomato leaves. Correlation analyses showed that the concentrations of K in leaf tissues were negatively correlated with bacterial spot disease severity (r=-0.373 in first experiment and r= -0.452, p < 0.05 in second experiment), but Ca and Mg concentrations did not exhibit a correlation with disease severity. This study indicated the importance of K nutrition in minimizing damage to tomato plants caused by bacterial spot.  Further research is needed to investigate the mechanisms of K source affecting leaf K concentrations and K in leaves reducing bacterial spot in tomato.