Glandular-Haired Alfalfa Resistance to Potato Leafhopper (Homoptera: Cicadellidae) and Hopperburn: Development of Resistance Indices

Abstract

Eight proprietary genotypes of glandular-haired alfalfa, Medicago sativa L., supplied by two different companies, were compared for the degree and types of resistance to the potato leafhopper, Ernpoasca fabae (Harris), and hopperburn. A tube cage no-choice bioassay was developed to test leafhopper mortality, feeding, settling preferences, severity of hopperburn symptoms (in this case, defined as both yellowing and stem growth reduction), and trichome density and type on feeding sites. Leafhopper mortality was both strongly and significantly associated with feeding and leaf trichome density; decreased hopperburn symptom severity was weakly, although significantly, associated with increased mortality. To quantify hopperburn in terms of both yellowing and stem growth reduction, we developed a ranking system that reduces overall hopperburn expression to a single number that considers the varying responses to both types of symptoms. Great variability in leafhopper settling, leafhopper mortality, and stem glandular trichome density was detected among alfalfa genotypes, suggesting that genotypic differences may be based on the concentration and/or chemical constituency of the trichome exudates. We postulate that, among variably resistant genotypes of glandular-haired alfalfa, differences among leafhopper responses and hopperburn severity are linked to forced movement from the stems to the leaves as refuge feeding sites. Principal component analysis was performed to reduce the 10 variables down to five biologically significant factors. Scores for these factors were then used to develop resistance indices for potato leafhopper resistance, hopperburn resistance, and an overall glandular-haired alfalfa resistance index.

Repellency to the Potato Leafhopper (Homoptera: Cicadellidae) by Erect Glandular Trichomes on Alfalfa

“…Insects were reared on greenhouse-grown broad beans, Vicia faba L. ÔWind-sorÕ, in a growth chamber (25 Ϯ 2ЊC, 16:8 [L:D] h). Ten-day old, conditioned, adult females, treated as in Shockley et al (2002), were used. All leafhoppers were starved for 1 h before use.…”

“…All leafhoppers were starved for 1 h before use. The same eight genotypes of glandular-haired alfalfa in Shockley et al (2002) were used (Cal/West (Woodland, CA): G98A, G98B, G98C, G98D; Forage Genetics (W. Salem, WI): 1Ð27Ð1, 1Ð27Ð 4, 1Ð27Ð7, and 3Ð73Ð1). The genotypes were selected as representing two spectra of resistance to hopperburn, one from each company.…”

“…All other studies were limited to the genotypes from Cal/West. Alfalfa plants were vegetatively propagated and grown in growth chambers, according to the methods of Shockley et al (2002) and Kabrick and Backus (1990). Plants were cut once and allowed to regrow until they reached 10 Ð12 internodes in height.…”

“…In this study, we continued the work described in Shockley et al (2002) by comparing among proprietary, variably resistant genotypes of glandular-haired alfalfa for leafhopper settling preferences (Þve genotypes) and trichome morphology (eight genotypes). Our objectives were to gain insights into the mechanisms of resistance by testing the roles of putative repellency via volatile chemicals, and chemistry versus morphology of the hairs.…”

“…Recent work has demonstrated that variation in levels of resistance of some alfalfa genotypes cannot be explained by trichome density alone (Hogg and McCaslin 1994, Elden and McCaslin 1997, Lenssen et al 1999, Shockley et al 2002. Glandular trichomes in alfalfa consist of two basic types: erect and procumbent (Ranger and Hower 2000a).…”