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Early Season Update for Potato Leafhopper in Alfalfa

Crop and Soil Environmental News, June 1997

Rod R. Youngman
Extension Specialist

Potato leafhopper populations are now present in alfalfa fields throughout much of western Virginia. Our scouting activity reports for the past ten days for alfalfa fields in the piedmont counties of Albemarle, Culpeper, Louisa, Madison, and Orange all have below threshold levels of potato leafhopper. Similarly, only low levels of potato leafhopper have been detected in the Shenandoah Valley (i.e., Augusta, Rockbridge, and Rockingham counties). This past week in Montgomery Co., however, above threshold levels of potato leafhopper were reported in four fields. Potato leafhopper populations are expected to increase rapidly now that the warm weather has finally arrived. The second through fourth cuttings of alfalfa are particularly vulnerable to potato leafhopper feeding. Potato leafhopper immatures (nymphs) and adults damage alfalfa by injecting saliva into the plant during feeding. This saliva causes a toxic reaction in the plant known as "hopperburn" which is readily apparent from the "V"-shaped yellowing of the leaflet tips. Although severe potato leafhopper damage can "burn" back an entire cutting, usually a 30% loss in yield and forage quality can be expected by the time yellowing is observed in the plant stand. To avoid the appearance of hopperburn and subsequent yield penalty to your alfalfa, it is essential to begin scouting your fields from now until the middle of August at intervals of every eight to ten days. Like most crops, healthy growing alfalfa with a foot or more of canopy height is able to withstand higher potato leafhopper levels than drought-stressed or new, regrowth alfalfa following a cutting. However, it does not pay to simply spray every alfalfa field following a cutting because of the sporadic nature of the pest. Information obtained from over 15 years of the Virginia Cooperative Extension Potato Leafhopper Scouting Program has shown that it does pay to scout alfalfa fields for this pest. Potato leafhopper scouting is easily accomplished by relating sweep net counts of nymphs and adults to plant stand height. You can accomplish this yourself by following the easy-to-use guidelines in the 1997 Pest Management Guide for Field Crops (Virginia Cooperative Extension publication 456-016), or by enrolling your alfalfa fields into the 1997 Potato Leafhopper Scouting Program for a nominal fee. Please check with your local county extension agent for more details.

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