EFFECT OF BLACK PLASTIC MULCH AND NITROGEN SIDE-DRESSING ON VERTICILLIUM WILT OF EGGPLANT

Eggplants were grown in soils naturally infested with Verticillium dahliae and in soils that had been planted to rye for more than 15 yr. Plants were grown using four cultural treatments with or without black plastic mulch, and either side-dressed by hand with nitrogen (112 kg N/ha) 35-45 days after planting or not side-dressed. The date when visual symptoms first appeared, the percentage of chlorotic and/or wilted leaves, plant size, and yield were monitored over the season. Colonization of vascular tissue was assessed at final harvest by sampling nodes from the main stem of each plant and isolating V. dahliae on a selective medium. Fifty percent disease incidence occurred on mulched plants 13 days sooner than plants grown on bare ground; nitrogen supplements had no effect on 50% disease incidence. Area under the disease progress curve was not significantly affected by any treatment despite the early onset of visual symptoms in mulched plants. However, plants were larger in mulched plots. Systemic colonization of eggplant stems by V. dahliae was variable and not affected by any treatment or combination of treatments. Compared with yields from diseased plants that were not mulched and did not receive supplemental nitrogen, fruit weight was significantly greater when both treatments were used together. In 1990, Verticillium wilt caused a three-fold decrease in yield when compared with asymptomatic plants growing in adjacent plots that had previously been cropped to rye. These asymptomatic plants had greater yield than untreated plants only when they were mulched and not treated with nitrogen. Yield was correlated with the area under the plant growth curve (r = 0.74, P = 0.001) and less correlated with AUDPC (r = 0.55, P = 0.01).