UK plant diagnosticians have found spring black stem and Lepto leaf spot in alfalfa samples examined in the past week.

Spring black stem produces tiny dark spots on leaflets, causing them to turn yellow and fall off prematurely. It also can produce brown to black blotches on stems. Lepto leaf spot produces tan dead spots with a thin brown border. One or both may be active in alfalfa fields at this time.

Wet weather favors both diseases, and both diseases can cause a loss of yield and forage quality. The only meaningful management recommendation is to cut the alfalfa on a regular basis, anywhere from early bud to first flower. This not only maximizes yield over the course of the season; it also reduces damage from the leaf spotting diseases. By capturing diseased leaves before they fall to the ground, this:

• Reduces yield loss from the diseases
• Reduces buildup of infectious material on the soil surface.

APHIDS, SPITTLEBUGS, AND LEAFHOPPERS IN ALFALFA

SpittlebugPea aphids and spittlebugs are common sap feeders that thrive during cool spring weather. The light green pea aphids can be found in clusters on plant stems and terminal buds. Use of an insecticide to control them might be justified if alfalfa foliage is yellow and wilting and there is an average of 50 to 75 aphids per stem. If many aphids are present, you may see bloated, straw-colored aphid mummies that have been parasitized by a tiny wasp. The wasps and hotter weather will reduce aphid numbers naturally.

Spittlebugs overwinter as eggs inserted between the sheath and stem of grasses. They are most common in mixed grass and alfalfa stands. The nymphs, or immature stages, excrete a frothy liquid that covers them completely, providing protection from drying out and may provide some protection against natural enemies that prefer to look for food that isn’t covered with spit.

An average of one or more spittlebugs per stem is needed to justify concern. Even then, the spittle probably provides protection against and insecticide spray, reducing the effectiveness of any attempted control measure.

The potato leafhoppers is a key pest of alfalfa. They should be present in most fields now and pose the greatest potential for damage in fields that have the longest time until the next harvest. A 35-day harvest schedule generally keeps leafhoppers from building to large numbers. Cutting drives the winged adults out of the field. The wingless nymphs are unable to leave and most starve or die from some other cause before regrowth gets started. Alfalfa fields must be checked carefully with a sweep net to detect damaging numbers before symptoms appear.

See Insect Recommendations for more alfalfa pest recommendations.

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