June 2, 2008

Some areas got much needed rain over the Memorial Day weekend, while other areas are still in great need of rain. Lack of rain and dry soils may favor some diseases. Not much can be done at this time to alleviate these diseases, but growers should be aware of the potential. Small grain diseases that are more severe under dry soil conditions in ND include wheat streak mosaic virus (see above) and certain root rots. These diseases add stress to the small grain plant which is already under stress from lack of moisture and too much heat.

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 in long). Their populations generally increase 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 to adjacent healthy plants. The virus disease severity 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 volunteer wheat plants and grassy weeds with herbicides or tillage at least two weeks prior to planting a new grass 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: At this time of year, control should be directed at any infected fields prior to replanting. Roundup or other glyphosate containing compounds may be used to destroy infected hosts. This should be done two weeks prior to planting a new, susceptible grass crop into that field or adjacent field, because the herbicides 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.

Common root rot: Common root rot is caused by a soilborne 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. When the season provides adequate moisture, wheat and barley plants can often compensate for infected roots, and yield losses may not be noticed in a cool growing season. However, a diseased root system can’t absorb enough water for the plant under stress conditions.

Common root rot management: At this time of year, little can be done to reduce the risk of common root rot.

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, and may be found on wheat, barley, and oats. 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: As with common root rot, crop rotation and seed treatments help reduce the risk of Fusarium root rot. More 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". www.ag.ndsu.edu/pubs/plantsci/smgrains/pp785w.htm

Root Diseases. Most root rot pathogens need some soil moisture to cause infection. However, once disease has been established in a drought stressed plant, the impact of the root rots may be significant. Some fungal species (Fusarium and Verticillium) that cause wilts reduce the plants ability to transport water. The pathogen plugs the xylem tissue that transports water through the plant. When root rots caused by these pathogens occur in drought stressed plants, the damage may be much greater than might occur in plants in a non-drought situation. These wilts are most severe when there is enough moisture to cause infection in the early part of the growing season, but the rest of the season experiences a drought.

Leaf Diseases. Droughts occur when there is a lack of rainfall, but that does not necessarily translate into a lack of humidity. Dew is likely to form if the air is humid and nights are cool. Dew on leaves creates a favorable environment for some pathogens, and in drought stressed plants, damage from some leaf diseases may be extreme.

Stem Diseases. Many stem diseases that begin as infections on leaves or florets (Phomopsis and Phoma of sunflower, white mold) are inhibited by drought conditions. However, stem diseases that first infect roots are largely unaffected. Charcoal rot is a stem disease of corn, soybean, sunflower, edible beans and other crops

grown in North Dakota. The disease begins when microsclerotia (the pathogen’s survival structure) germinate in the soil and infect the plants roots. Once infection progresses into the stem, the disease partially degrades the pith. This limits the plants ability to transport water. Charcoal rot is favored by high temperatures and water stressed plants, so the disease is more likely to be a problem in drought years.

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