Plants in the small grains disease management research site located near Foxhome on the Hasbargen Farm in west central Minnesota have leaf symptoms that aren’t normally noted in the RRV. Scattered plants have yellow and green leaf striping symptoms (Figure 1). Serology tests conducted in the laboratory by Lorilie Atkinson of the NWROC Extension Plant Pathology Program confirm that symptomatic plants are infected with the virus that causes wheat streak mosaic. The disease is a well-known and persistent problem in arid wheat growing areas - but an infrequent problem here.

How does an arid environment promote the disease?
The virus is spread, or vectored, by wheat curl mites (Figure 2). Wheat curl mites, Asceria tosichella Keifer, are very small mites; <1/100” long, they are not visible to the unaided eye and require a dissecting scope or 10X – 20X hand lens to see. They are whitish yellow, have an elongated tear-drop shape, and have 4 legs near the head that provide only limited movement and a sucker-like structure at the end of their abdomen. They use this structure to attach to leaves and to stand ‘upright’ on a leaf to catch winds for dispersal. Their complete life cycle (egg, 2 larval stages, and adult) takes only 7-10 days resulting in rapid population growth. They feed only on green plants and if none are available at hatching, wheat curl mites will die. It is unusual to have large populations of wheat curl mites develop in Minnesota.

Although they don’t have wings, wheat curl mites can be dispersed by wind. Because they require an adequate food source within 24 hrs, dispersal distance is generally limited (a few miles). Once established in a field, movement within field can be wind borne but is more likely to be plant to plant. Wheat curl mites tend to colonize under leaf sheaths or in furled, new leaves. Their feeding prevents these new leaves from unfurling and gives them a curled appearance (hence the name). As the plant matures, mites will move upward on the plant to feed on the newest growth.

Wheat curl mites are very efficient virus vectors and although they can damage plants through their feeding, their greatest impact comes from spreading viral diseases plant to plant. The potential for crop loss is greater if mites infect plants early in the growing season. Infection later greatly reduces the risk for yield losses.

What can be done to stop the disease from getting more
severe this year?
Nothing. Fungicides don’t have control activity against viral diseases and insecticides don’t control mites. Mites will continue to spread the disease if the weather remains dry. This is a disease that periodic rains help to control. Rain would be a welcome addition in the RRV for more reasons than one.

Charla Hollingsworth
Plant Pathologist - NWROC, Crookston

Ian MacRae
Entomologist - NWROC, Crookston

and Doug Holen
Regional Educator - Crops,
Regional Center. Fergus Falls