Marcia McMullen

Plant Pathologist

NDSU Extension Service

Fargo, North Dakota

Wheat streak mosaic virus was confirmed in 5 counties of: North Dakota this year, including 6 counties that border Canada. Severe infect ions were observed early in the growing season in winter wheats arid later in the season in spring wheats arid durums. Infected plants were characterized by severe stunting, yellow streaking of the leaves, and poor head formation. Losses were as high as 100% in many winter wheat fields and some late planted durums also had severe losses due to this disease.

The following information provides some answers to quest ions frequently asked in a 1988 about wheat streak mosaic:

HOW does wheat streak mosaic virus spread? The wheat curl mite transmits the virus while feeding on the wheat plant. adult mites, which are only 1/100 inch long, lay eggs which hatch within 7-10 days. Young mites will feed on green wheat and lay more eggs, but if the tissue is maturing or stressed, the mites move up the plant to be caught by wind and are carried to other green host plants. If no green host t issue is nearby, the mites don' t survive arid disease spread stops.

Why was wheat streak so severe this year? Rainfall late last summer encouraged growth of wheat volunteers - volunteers which serve as green bridges for mites to survive between spring and winter wheat crops. Some winter wheat fields were planted too early in 1987; this allowed early mite movement from volunteers into winter wheat and early development of the mite population. also, warm, dry weather last fall into November was ideal for massive curl mite reproduction and dispersal. These mites survived the winter arid then the 1988 spring and summer weather was warm, dry arid windy - all favoring mite movement and plant susceptibility.

What conditions favor mite movement and how far do they move? Ira heat and drought stressed wheat, mites respond by moving to outer surfaces of plant arid upward, where they posit ion themselves for easy movement in a wind. If plant stays lush arid green, mites will stay put to feed and thus the disease stays more localized. also, mite dispersal is positively related to wind speeds greater than 5 mph arid temperatures of 67 F or greater conditions very common in 1988.

Which crops/grasses/weeds are affected by wheat streak mosaic? Wheat crops are the favorite food host of the mite and also the best host for virus reproduction. Barley also is fairly susceptible, and severe infect ions have been seen in North Dakota. Wheat streak was confirmed in some late planted oats this year, a first report of wheat streak in oats in North Dakota.

Winter rye from areas adjacent to infected winter wheat has been tested at NDSU arid found negative for wheat streak. Other states have reported rye to be a possible host. Pros millet, sorghum and sorghum sudan are not reported to be affected by wheat streak.

Corn is a very important host for the mite and virus. Mites move into nearby green corn as wheat matures, then mite moves out of corn into newly emerged winter wheat in the fall. Mite feeding causes kernel red streak in corn. P survey of corn fields in eastern North Dakota in August revealed high mite populations in green corn.

Broad leaved crops such as mustard, flax arid sunflowers are immune to wheat streak mosaic.

Where does the mite overwinter and survive? The mite overwinters or winter wheat arid a few perennial grasses such as Canadian wild rye and Canadian bluegrass. Reports from other states say that the mite arid disease do riot affect the native wheatgrasses, blue gramma, side oats gram, green needle grass or needle arid thread. In the summer, the mites arid disease can survive on grasses such as brome arid wild barley, both of which tested positive for wheat streak at NDSU in 1988.

Although some native arid introduced grasses that are in road side ditches arid CRP acres may be potential reservoirs of the disease generally they are not a source of a major epidemic of wheat streak. In a great majority of the fields that were uniformly arid severely infected in 1988, the source of the disease was nearby infected winter wheat or winter wheat volunteers.

Is wheat streak mosaic a stubble-born disease? No, wheat streak does not survive in the stubble, No, leaving stubble for protection against soil erosion does not increase the risk of the disease. The mite needs live plant tissue for overwintering arid does not survive in wheat or corn stubble. However, the mite does survive in winter wheat volunteers arid occasional by a few weedy perennial grasses that may be found in that stubble. These need to be control led with knock-down herbicides.

What varieties are resistant to wheat streak? No varieties of wheat are resistant to wheat streak but some have more tolerance than others. Tolerant winter wheat varieties are ones that are riot very winter hardy including Siouxland and Thunderbird. Tolerant spring wheats include Butte, Butte 86, Oslo, Pr2369, Cutless arid Challenger. The more tolerant durunis are Ward arid Cando. Tolerance means yield losses with severe infections ranging from 30-47% versus 70-95% for non-tolerant varieties.

How do we control wheat streak mosaic? No chemicals are now available which can be sprayed on a crop to control either the mite or the virus. CONTROL IS BY CULTURAL PRACTICES. DATE OF PLANTING IS VERY IMPORTANT. Winter wheat should be planted late; no sooner than mid September and preferably later in southern counties. However, spring wheat arid durum should be planted early to escape severe infect ion.

ALL WHEAT VOLUNTEERS AND WEEDY GRASSES SHOULD BE DESTROYED WITH A KNOCK-DOWN HERBICIDE OR TILLAGE TWO WEEKS PRIOR TO PLANTING WINTER WHEAT IN THE FALL. This breaks the green "bridge " that mites survive on. avoid planting winter wheat next to green corn in an area where wheat streak has been identified.

Will wheat streak be severe next year? Severity ':'f wheat streak in 1989 will depend on how well the management practices are followed in the fall of 1988, plus be dependent on the weather this fall arid next spring. Very dry weather this August and into September already has prevented volunteer wheat growth in many areas of North Dakota. This will help reduce the risk of high mite populations surviving in these areas. Also, there has been less winter wheat acreage planted, arid when planted, has generally been planted according to recommended dates, instead of too early. This will help reduce the risk. Survival of remaining mite populations arid spread of the disease next spring is still dependent on the weather of the upcoming 6-8 months.

FOOD FOR THOUGHT

In 1896, William Jennings Bryan said it best, "Burn down your cities and leave our farms, and your cities will spring up again as if by magic, BUT destroy our farms and the grass will grow in the streets of every city in the country".

As long as our farmers can supply our food at a price that allows us to spend the greatest part of our income for our cars and TVs and recreational items, our high standard of living can continue. BUT, when our farmers can no longer produce our low-cost food, or when they have no economic incentive or ability to produce this food----truly, again,---"the grass will grow in the streets of every city in the country".