Fumigation and Fertilizer Nitrogen Source Effects on Potato Yield, Quality, and Early Dying

Research has shown that while fumigation and use of ammonium N can both reduce the severity of verticillium wilt (Verticillium dahliae Kleb.) of potatoes (Solanum tuberosum L.), the use of the two practices together raises concerns over feeding the crop only ammonium N under reduced nitrification conditions. To assess the validity of this concern, we conducted two 3-year field split-plot experiments with both using metam sodium fumigant (none, fall or spring applied) as the main plot. For the first experiment, N source (134 kg N ha(-1) as ammonium sulfate, urea, or ammonium nitrate) was the split, whereas for the second trial in-season N rate (0, 67, 134, or 202 kg N ha(-1) all as ammonium sulfate) was the split. For both trials, in 2 of the 3 years, fumigation significantly increased tuber yield by an average of 9.9 Mg ha(-1) and decreased late-season verticillium severity ratings from 77 to 45%. In some years, fumigation also increased the proportion of U.S. No. 1 tubers and tubers > 170 g. No differences in crop yield or quality were observed between the various N sources applied. This was true even on spring-fumigated areas with the highest rate of ammonium N applied. These experiments confirm that the choice between in-season potato N fertilizer should be based on factors such as potential for benefits or N losses, cost, and convenience of use rather than concern over an interaction between fumigation and ammonical N. While both fumigation and N rate reduced verticillium severity ratings in some years, the lack of interaction suggests these factors are functioning independently.