Small Grain Diseases: 
Management of Those More Common and Severe in Dry Years

Reviewed and Revised April 2008

Marcia McMullen - Plant Pathologist

Small grain diseases that are more severe under dry soil conditions in North Dakota include wheat streak mosaic virus and common root rot.  These two diseases add stress to the small grain plant which is already under stress from lack of moisture and too much heat.  Wheat streak mosaic is a virus disease primarily attacking wheat crops, and is transmitted by wheat curl mites.  Common root rot is caused by a fungus ordinarily found in North Dakota soils, a fungus that may attack roots and crowns of wheat and barley.  Another fungal root disease, Fusarium crown rot, is not as frequently observed as common root rot, but under dry conditions, it also may cause damage on wheat, barley and oats. 

Wheat streak mosaic: Wheat streak mosaic virus is carried from plant to plant by wheat curl mites.  Wheat curl mites are very tiny (1/100 inch in length).  Their population generally increases rapidly under extended warm dry weather, and with high mite numbers, the chance of infection of susceptible crops becomes higher.  The mite transmits the virus during feeding and the virus infection causes yellow streaking of leaves, stunting of the plant, and reduced yields.  The mite needs a green bridge for survival; it frequently overwinters on winter wheat or perennial grassy weeds, and then moves from these plants to adjacent spring grains in the spring or early summer.

Mites move more frequently during heat and drought stress, seeking green, healthy plant tissue. They move to the outer edge of leaves where they are positioned for easy movement in the wind.  The wind moves them to adjacent plants or fields, where mite transmission of the virus reoccurs.  The virus disease severity also is greater under drought stress because the plant has fewer nutritional and water resources to compensate for virus infection. 

Wheat streak mosaic management: Wheat streak mosaic is managed by two cultural practices: elimination of wheat volunteer plants and grassy weeds with herbicides (or tillage) at least two weeks prior to planting a new crop; and use of appropriate planting dates.  Each practice helps to break the green bridge needed for mite survival.

Elimination of volunteers and grassy weeds: Roundup or other glyphosate-containing compounds may be used to destroy these hosts of the mites and virus.  This should be done two weeks prior to planting a new crop into that field or adjacent field.  If infected volunteers, grassy weeds, or even infected fields are left standing, they may be a serious source of infection.   Roundup or other glyphosate products act slowly to kill plants, and mites on these plants will move out of the infected volunteers for up to 10 days after herbicide treatment. 

Planting dates: Winter wheat should be planted in mid-September, or later in southern North Dakota counties.  This reduces risk of exposure to high mite populations dispersing from infected plants during August and early September.  Green corn also may be an important reservoir for the mite and virus, and planting winter wheat next to corn that is still green increases risk of movement of the mite from the corn into the winter wheat.  On the other hand, spring wheats should be planted early to escape infection prior to summer mite buildup. Severe damage is most likely if the crop is infected at a young growth stage. 

Additional information about wheat streak mosaic disease may be found in NDSU Extension Circular PP-646 “Wheat Streak Mosaic”.

Common root rot: Common root rot is caused by a soil-borne fungus, Bipolaris sorokiniana. The common root rot fungus is widespread in North Dakota soils, causing root rot of wheat and barley.  Common root rot is characterized by a dark-brown discoloration of the roots, sub-crown internode, and often the crown.  The disease interferes with water uptake and subsequently affects grain fill.  Moist soil and cool temperatures often allow wheat and barley plants to compensate for infected roots, and yield losses may not be noticed in a cool growing season.  However, early infection of roots can result in severe yield loss if the plant is subsequently exposed to high temperatures and dry soils.  A diseased root system can’t absorb enough water for the plant under these stress conditions. 

Common root rot management: Three strategies are generally used to combat common root rot:  crop rotation, variety choice, and seed treatment. 

Crop rotation: Crop rotation is the most effective method of reducing the risk of common root rot.  Non-host crops include broadleaf crops such as soybean, canola, dry beans, and flax.  Oats also are a good rotation crop if small grains must be grown.  Increasing the time between re-cropping wheat or barley also is effective; each additional year break from these two crops reduces the level of the fungus in the soil.

Varieties: Varieties of spring wheat, durum, and barley differ in their susceptibility to common root rot.  The latest information on spring wheat variety susceptibility can be found in the NDSU Extension publication on spring wheat varieties “NDSU Extension Publication A-574".

Seed treatment: Several currently registered seed treatments have suppression activity against the common root rot fungus in wheat and barley.  The most likely conditions where seed treatments would be beneficial are: under continuous wheat or barley production; short rotational cycles between wheat or barley; or in soils or areas where moisture stress is likely.  Further information on seed treatments for wheat and barley may be found in NDSU Extension Circular P-622, “ND Field Crop Fungicide Guide”.

Fusarium root rot: Root rot caused by several fungi in the genus Fusarium may be associated with very dry soils or areas that get low annual precipitation.  Fusarium root rot on wheat often is called dryland foot rot.  Fusarium root rots are characterized by a brown to reddish-brown discoloration of the roots and crowns.  Affected plants may be in patches, appearing as prematurely ripened plants.  As with common root rot, plants infected with Fusarium root rot cannot absorb enough water to carry plants through grain fill.

Fusarium root rot management: Crop rotation and seed treatments help reduce the risk of Fusarium root rot. 

Crop rotation: All non-cereal crops are good rotation choices to reduce the risk of this disease.  As with common root rot, the longer break between cereal crops reduces the risk, as well.

Seed treatment: Several currently registered seed treatment products have activity against root rot caused by Fusarium species.  Further information on seed treatments for this disease may be found in NDSU Extension Circular P-622, “ND Field Crop Fungicide Guide”.

Additional information about common root rot and Fusarium dry rot may be found in NDSU Extension Circular PP-785 “Root and Crown Rots of Small Grains”.