Effects of Nitrogen Application Rate and Plant Density on Severity of Tar Spot of Corn

Abstract

Tar spot of corn, caused by the fungus Phyllachora maydis, is an economically important foliar disease recently reported in the United States and Canada. Due to the recent introduction of Phyllachora maydis, the impacts of cultural management practices on disease development are still unknown. Separate field studies were conducted to determine the effects of nitrogen (N) application rate and plant density on disease development. Field trials were conducted across six site years in Michigan with two corn hybrids of differing disease susceptibility. The relative area under the disease progress curve was used to compare disease development between N application rates and plant densities. Nitrogen application rate had no significant effect on disease at any location. Plant density and disease had a significant ( P < 0.05) inverse relationship at five of six site years, with an average 41% decrease in the relative area under the disease progress curve for every 1,000 plants per hectare increase. The economically optimal planting density ranged from 73 to 77 thousand plants per hectare for US$150 to 300 per metric ton corn prices, demonstrating that relatively low planting densities were more profitable despite greater disease. Therefore, other disease management practices including hybrid selection may be more effective at protecting yield than increasing plant density.