INFLUENCE OF NITROGEN, PHOSPHORUS, AND POTASSIUM ON THE SEVERITY OF STRAWBERRY ANTHRACNOSE CROWN ROT

The influence of nitrogen, phosphorus, and potassium on the severity of anthracnose crown rot (causal fungus, Colletotrichum fragariae) was evaluated in three greenhouse studies. Strawberry plants were grown in a greenhouse, fertilized three times weekly with a modified Hoagland’s Nutrient Solution containing the nutrient treatment solution, and inoculated with a suspension of C. fragariae conidia eight weeks after treatment applications began. Disease severity was rated 30 days after inoculation on a scale of 0 (no symptoms) to 6 (plant dead). In the first study 16 treatments were evaluated: eight nitrogen levels (0, 5, 10, 20, 40, 80, 160 and 320 ppm) with either low phosphorus (8.7 ppm) and potassium (16.6 ppm) or high phosphorus (35.8 ppm) and potassium (66.4 ppm). Disease severity scores increased as nitrogen level increased, but the level of phosphorus and potassium did not have a significant effect on disease severity scores. In two other studies, seven nitrogen sources were each evaluated at three rates. Plants receiving 160 ppm N had higher disease severity scores than plants receiving 0 or 40 ppm N. Among plants receiving 160 ppm N, those treated with Ca(NO₃)₂ were the most disease tolerant. Plants treated with nitrate nitrogen sources, including NH₄NO₃, had less severe anthracnose symptoms than plants receiving nitrogen from other ammonium sources. These results suggest that when nitrogen fertilizer is applied to strawberry fields as calcium nitrate, anthracnose crown rot severity should be less severe than when nitrogen is applied in ammonium forms.