Influence of rate of soil fertilization on Alternaria leaf blight (Alternaria dauci) in carrots

The possibility of suppressing Alternaria dauci (Kuhn) Groves & Skolko, the causal agent of Alternaria leaf blight in carrot, by excess application of fertilizer was examined in greenhouse and field experiments. Reducing the rate of fertilization by one half from the optimal rate (100 ppm N, 19 ppm P and 74 ppm K) resulted in a 23-30% increase in the severity of Alternaria leaf blight. However, doubling the rate of fertilization resulted in only a 10-15% decrease in disease severity. Inoculating with different concentra tions of A. dauci spores (10(3) or 10(4) spores/ml) did not alter the respo nse of the plants to the fertilization rate, although significantly higher disease severity was observed in plants inoculated with the higher spore co ncentration. These results were corroborated in the field, where neither di sease severity nor harvested yield was significantly affected by tripling t he amount of soil fertilization. Application of foliar fungicides, on the o ther hand, had substantial effects on both disease and yield. Therefore, it was concluded that carrot crops should be fertilized and maintained for op timum yield, and that A. dauci should be managed by properly timed applicat ions of fungicides during the growing season.