Effects of Nitrogen Fertilization on Bacterial Soft Rot in Two Broccoli Cultivars, One Resistant and One Susceptible to the Disease. C. H. Canaday, Associate Professor, Department of Entomology and Plant Pathology, University of Tennessee Institute of Agriculture, Knoxville 37901-1071. J. E. Wyatt, Associate Professor, Department of Plant and Soil Science, University of Tennessee Institute of Agriculture, Knoxville 37901-1071. Plant Dis. 76:989-991. Accepted for publication 22 May 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0989.

Sidedress applications of ammonium nitrate increased the incidence and severity of bacterial soft rot (caused by Pseudomonas marginalis) in a susceptible broccoli cultivar, Premium Crop, but had no effect on a resistant one, Shogun. Plots received N in amounts ranging from 19 to 152 kg/ha and were sprinkler-irrigated two to five times a day during head maturation to create conditions conducive to the disease. The length of time necessary for heads to reach harvest maturity decreased with increased N. Thus, in simulated once-over harvests of Shogun, a large range in head size (6–22 cm in diameter) resulted in apparent increases in both disease incidence and severity as the N rate increased, since these disease indices were positively correlated with head size. Among heads of similar size (10–16 cm in diameter) in all treatments, there were no significant effects of N on disease incidence or severity. In subsequent experiments involving multiple harvests of heads of similar size, disease incidence and severity in Shogun were not significantly affected by the N rate, but in Premium Crop they were significantly increased as the total N applied increased from 38 to 152 kg/ha. The marketable yield of Premium Crop decreased and that of Shogun increased with increased N.