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 available, yet their mechanism of resistance is poorly understood. Our objective was 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 and one susceptible cultivar during the spring of 1996. Estimates of alfalfa dry matter, stem length, number of nodes, and hopperburn were taken on each harvest date, and adult potato leafhopper density in each cultivar was determined regularly over the 3-yr study. Potato leafhopper density was significantly greater in the susceptible cultivar compared with one or more tolerant cultivars in only two of the 15 growth intervals sampled. Although there was only one harvest when dry matter differed significantly between alfalfa types, there was a trend for tolerant alfalfa to produce more dry matter than the susceptible alfalfa when the leafhopper pressure was high. Tolerant cultivars had more nodes, longer internodes, longer stems, and less hopperburn 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 translate to greater yields when the leafhopper density is high.