The Effect of Insect Defoliation on the Presence and Severity of Fusarium Crown-Rot in Alfalfa

Alfalfa Medicago sativa L. (‘Fla. 77’) was inoculated with three different isolates of Fusarium and defoliated to varying levels with yellowstriped armyworms, Spodoptera ornithogalli (Guenée), to determine the effect of insect defoliation on the development of crown-rot under greenhouse conditions. There were no significant interactions between short-term insect defoliation and Fusarium crown-rot on forage quality, yield, or root carbohydrate reserves. Although insect defoliation alone did reduce plant height, yield, and maturity (18, 33, and 30% respectively) at the first harvest, no significant effects were observed at two subsequent harvests. Of the three isolates tested, Fusarium oxysporum Schlecht was the most virulent. Short-term defoliation did not increase the severity of Fusarium crown-rot in alfalfa.