Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 2009
Publication Date: August 31, 2009
Citation: Smith, B.J. 2009. Influence of Nitrogen, Phosphorus, and Potassium on the Severity of Strawberry Anthracnose Crown Rot. Acta Horticulturae. (ISHS) 842:235-238. Interpretive Summary: Anthracnose diseases, caused by species of Colletotrichum, are major diseases of strawberry in the U.S. and often cause devastating losses to the growers. In the southeastern U.S. anthracnose crown rot and fruit rot can be severe in strawberry production fields where the Colletotrichum spp. may spread rapidly during wet harvest seasons, sometimes causing a total crop loss. Fungicides are often used to control anthracnose diseases, but several are no longer effective due to the development of fungicide tolerant stains of the pathogens. Grower observations have indicated that growing strawberry plants in soils with low nitrogen levels can reduce the severity of anthracnose; however, production may be severely reduced. This paper presents the results of greenhouse studies investigating the influence of nitrogen, phosphorus, and potassium on the severity of anthracnose crown rot in which 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, evaluating seven nitrogen sources, plants receiving 160 ppm nitrogen had higher disease severity scores than plants receiving 0 or 40 ppm nitrogen. Among plants receiving 160 ppm nitrogen, those treated with Ca(NO3)2 were the most tolerant. Plants treated with nitrate nitrogen sources, including NH4NO3, had less severe anthracnose symptoms than plants receiving nitrogen from other ammonium sources. When nitrogen was applied as calcium nitrate, anthracnose crown rot severity was less severe than when nitrogen is applied in ammonium forms. This information will be used by strawberry growers, extension agents and research scientists has they develop integrated disease management strategies for strawberry crops.
Technical Abstract: 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 maintained under standard greenhouse conditions with one plant per 10 cm pot fertilized three times weekly with a modified Hoagland's Nutrient Solution containing the nutrient treatment solution. Plants were inoculated with a suspension of C. fragariae conidia eight weeks after treatment applications began. Thirty days later disease severity (DS) was rated on a scale of 0 (no symptoms) to 6 (plant dead). In the first study the effect of nitrogen, phosphorus, and potassium levels was evaluated using 16 treatments: 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. Plants received susceptible ratings (DS > 4) when foliar nitrogen was above 2%. Predicted DS values at N level = 0 ppm to N level = 320 ppm ranged from 2.3 to 5.9. The level of phosphorus and potassium did not have a significant effect on disease severity scores. In two studies, seven nitrogen sources [(NH4)2SO4, NH4Cl, (NH4)2HPO4, NH4NO3, NaNO4, KNO3, Ca(NO3)2] were evaluated at three rates. Plants receiving 160 ppm nitrogen had higher disease severity scores (4.7) than plants receiving 0 or 40 ppm nitrogen (DS = 2.4, 2.7). Among plants receiving 160 ppm nitrogen, those treated with Ca(NO3)2 were the most tolerant (DS=2.9). Plants treated with nitrate nitrogen sources, including NH4NO3, had less severe anthracnose symptoms than plants receiving nitrogen from other ammonium sources. The source and concentration of nitrogen in fertilizers applied to strawberry fields may have a major effect on severity of anthracnose crown rot. When nitrogen is applied as calcium nitrate, anthracnose crown rot severity should be less severe than when nitrogen is applied in ammonium forms.