Symptoms and growth of potato leafhopper-tolerant alfalfa in response to potato leafhopper feeding

Alfalfa (Medicago saliva L.) cultivars marketed as resistant, or tolerant, of the potato leafhopper (Empoasca fabae [Harris]) are increasingly availab le, yet their mechanism of resistance is poorly understood. Our objective w as to determine if growth characteristics differ between potato leafhopper- tolerant and susceptible alfalfa cultivars under different pest pressures, and how differences may contribute to a greater yield potential in tolerant cultivars. Two field experiments, one near Ames, IA, and one near Charlton , IA, were planted with four potato leafhopper-tolerant alfalfa cultivars a nd one susceptible cultivar during the spring of 1996. Estimates of alfalfa dry matter, stem length, number of nodes, and hopperburn were taken on eac h harvest date, and adult potato leafhopper density in each cultivar was de termined regularly over the 3-yr study. Potato leafhopper density was signi ficantly greater in the susceptible cultivar compared with one or more tole rant cultivars in only two of the 15 growth intervals sampled. Although the re was only one harvest when dry matter differed significantly between alfa lfa types, there was a trend for tolerant alfalfa to produce more dry matte r than the susceptible alfalfa when the leafhopper pressure was high. Toler ant cultivars had more nodes, longer internodes, longer stems, and less hop perburn than the susceptible cultivar when the leafhopper number was high. Alfalfa cultivars tolerant of potato leafhopper feeding were not immune to this pest; however, they had favorable growth characteristics that could tr anslate to greater yields when the leafhopper density is high.