VERTICAL DISTRIBUTION OF VERTICILLIUM DAHLIAE AND THE IMPACT OF NITRATE NITROGEN ON COTTON GROWTH, YIELD, QUALITY AND DISEASE DEVELOPMENT
Eight field studies were conducted during the 2011 and 2012 growing seasons to evaluate the effect of increasing nitrogen rates (0-280 kg/ha) on Verticillium wilt incidence, lint yield and fiber quality of the varieties Deltapine 0912B2RF, Deltapine 104B2RF, Fibermax 9160B2F (both years), NG 334B2RF (2011) and Deltapine 1032B2RF (2012). Soil samples were collected from each field at depths of 0 to 16, 16 to 31, 31 to 46 and 46 to 61 cm and analyzed for residual nitrate nitrogen and Verticillium dahliae densities throughout the duration of the study. Drought conditions throughout the 2011 and 2012 growing seasons negatively affected all agronomic factors measured. The application of nitrogen did not result in taller plants or an increased number of nodes compared to the non-treated control. Appreciable foliar symptoms of Verticillium wilt were observed at one of the locations for both years, where the application of nitrogen generally increased disease incidence in 2011. At Halfway, disease pressure for susceptible varieties (DP 0912B2RF and DP 1032B2RF) was 13% disease incidence compared to the partially resistant varieties (DP 104B2RF and FM 9160B2F) at 6%. In 2012, there was no plant response to nitrogen regardless of rate. Lint yields (kg/ha-1) were increased with the application of nitrogen over the non-treated control in 2011; however, differences in yield between the application rates were not observed in 2012. Yields were greatest for the susceptible DP 0912B2RF and similar for the other three varieties. Yields improved in 2012 over 2011 due to the moderation of growing conditions. Yields were generally better for DP 0912B2RF, FM 9160B2F and DP1032 B2RF compared to DP 104B2RF. Subtle differences in fiber quality parameters were observed for the varying nitrogen rates, while varietal effects were observed for most parameters when analyzing the data independently or combined. Microsclerotia densities on average increased from 2010 to 2012. Furthermore, lower propagules of the fungus were generally found in the upper soil strata compared to the lower soil strata; however, the fungus was recovered from the entire profile. Residual nitrate nitrogen increased over the three years with the highest concentrations observed at the 0 to 16 cm depth. Nitrate concentrations were greatest at the upper strata and generally decreased at deeper depths. Although confounded by adverse environmental conditions, these studies suggest that interactions between V. dahliae and nitrogen may exist and that variety selection and the distribution of microsclerotia within the soil profile may affect disease development. Additional studies need to be conducted in more conducive fungal growing conditions to better understand the interactions between yield, fiber quality and growth habit of the plant in this region.