ISSUE 3   May 29, 2008

WHEAT STREAK MOSAIC CONFIRMED IN WINTER WHEAT

A sample of winter wheat from south central ND was confirmed on May 23 to have wheat streak mosaic virus (WSMV), with a positive ELISA test done by the NDSU Diagnostic Lab. The leaves tested had the yellow mottling and streaking pattern characteristic of WSMV (see figures).

Wheat streak mosaic virus            Wheat streak mosaic virus close up

However, pictures of this field indicated that symptomatic leaves were a small percentage of the field and, overall, the plants across the field looked green and healthy. Because of the overall appearance of the field, and limited moisture for a new crop, I advised that this field should not be torn up and replanted. This kind of decision is always difficult and has to be made on a field by field basis, based on extent and severity of the disease and replanting options.

 

DROUGHT AND SMALL GRAIN DISEASES

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.

Planting dates: Replanting a spring grain into a field with previous infection at this time would be risky. Wheat curl mites continue to develop and increase populations during warmer months. A late planted spring wheat crop could be infected at emergence and be severely damaged.

Additional information about wheat streak mosaic disease may be found in NDSU Extension Circular PP-646 "Wheat Streak Mosaic" http://www.ag.ndsu.edu/pubs/plantsci/smgrains/pp646.pdf

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. 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 canít 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. Three preventative strategies generally used to combat common root rot are: 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.

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 Ext. Publication A-574".

Seed treatment: Several currently registered seed treatments have suppression activity against the common root rot fungus in wheat and barley. 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, 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

Marcia McMullen
Extension Plant Pathologist
marcia.mcmullen@ndsu.edu

 

ROW CROP DISEASES IN DROUGHT YEARS

Drought impacts many diseases of row crops. In some cases, drought conditions will impact disease by making the environment more (or less) favorable for infection, disease development, and/or disease spread. In other cases, drought may not impact the pathogen at all, but may exacerbate the damage caused by disease in drought stressed plants. Although the effects of drought on disease are variable, certain diseases of some crops tend to be consistently more or less severe in drought conditions (Table 1).

Table 1. General impacts of drought on different diseases.

*Disease Damage

Disease

Crop

Greater

No Effect

Less

Oomycete root rots (Phytophthora, Pythium, Aphanomyces)

Many
 

   

X
 

Fungal root rots causing wilt (Verticillium, Fusarium spp.)

Many
 

X
 

 

X
 

Downy Mildew

Sunflower

   

X

White Mold (Sclerotinia)

Many broadleaf crops

   

X

Sclerotinia Wilt (root infection only)

Sunflower

 

X

 

Charcoal Rot
 

Soybeans, Sunflower, Edible Beans, Corn, others

X
 

   

Leaf Spot Pathogens

All

   

X

Powdery Mildew

Most crops

X

 

X

Rusts

Most crops

X

 

X

Late Blight

Potato, Tomato

   

X

Rhizoctonia Stem Canker

Potato

 

X

 

Early Dying (Verticillium)

Potato

X

   

Black Scurf (tuber)

Potato

   

X

Ascochyta Blight

Pulse crops (pea, lentil, chickpea)

   

X

*These are general effects only; exceptions occur in all catergories. In categories with more than one box checked, drought may cause different effects under different conditions. See text for detail.

Diseases less severe in droughts

Root Diseases. Many different species cause root rots in North Dakota. Oomycete organisms (commonly called water molds, or near-fungi) such as Phytophthora, Pythium, Aphanomyces, and Plasmopara need wet soils to cause disease. Some common diseases caused by oomycete pathogens that are usually less severe in drought conditions include downy mildew of sunflower, Phytophthora root rot of soybeans, Aphanomyces root rot of sugarbeet, and damping off of many plants caused by Pythium species.

Leaf Diseases. Many foliar pathogens are able to infect plants only when leaves are wet. In drought situations, there may be a lack of free moisture on the leaves, which reduces the pathogens ability to infect plants. Fungal or bacterial pathogens causing leafspots and some rusts tend to fall into this category.

Additionally, many foliar pathogens produce spores that are only dispersed by rain splash. Pathogens that need rain to spread are unlikely to cause epidemics in drought years. Many pathogens causing leaf spots fall into this category, but rusts do not.

Stem Diseases. Some stem diseases may be less likely to occur in drought years. White mold (caused by Sclerotinia sclerotiorum) infects many broadleaf crops grown in North Dakota including canola, edible beans, soybeans and sunflower. The soil must be wet for the white mold pathogen to germinate and produce spores, and an extended wet period is needed for those spores to infect plants. When soils are dry and limited wet periods occur in the canopy, white mold is unlikely. One notable white mold exception is sunflower wilt, which is caused when the white mold pathogen infects sunflower plants through the roots. This method of infection is less likely to be influenced by drought.

Diseases more severe in droughts

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.

Dew on leaves often provides enough moisture for rust pathogens to infect plants. While many leaf spot pathogens cause dead spots on leaves, rust pathogens produce an open wound in living plant tissue. This open wound allows moisture to readily escape from the plant, causing a drought stressed plant to lose water more quickly. Additionally, rusts do not need rain to disperse spores, so if adequate dew is available for infection, rust epidemics may occur. In some drought situations, rusts are a major concern.

Powdery mildew is occasionally found on most crops in North Dakota. High humidity provides a favorable environment for the infection and development of powdery mildew. Powdery mildew is likely to occur in drought years when the humidity is high.

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 pathogenís 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.

Sam Markell
Extension Plant Pathologist
samuel.markell@ndsu.edu

Mohamed Khan
Extension Sugarbeet Specialist
mohamed.khan@ndsu.edu


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