EFFECTS OF NITROGEN-FERTILIZATION ON BACTERIAL SOFT ROT IN 2 BROCCOLI CULTIVARS, ONE RESISTANT AND ONE SUSCEPTIBLE TO THE DISEASE

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.