ISSUE  13   August 12, 2010

MANAGE WHEAT STREAK MOSAIC NOW

Wheat streak mosaic virus (WSMV) was severe in a number of wheat fields in North Dakota this year. The virus was confirmed in 71 wheat samples sent to the NDSU Plant Diagnostic Lab, with a majority of these samples from counties in the north central region of the state. Keys to managing this disease include understanding its life cycle and directing steps to break the cycle.

Virus and Mite Carrier: The virus causes yellowing and mosaic streaking of the leaves (Figure 1), stunting of the plant, and potentially large yield losses. The virus is transmitted from plant to plant by the tiny wheat curl mite. This mite, only 1/100 of an inch long, needs a green “bridge” to live, reproduce, and survive. The mite can feed, transmit the virus, lay eggs, and complete its cycle within 7-10 days under warm temperatures of 70 degrees Fahrenheit or greater. Starting with one mite, a population of 3 million mites could potentially be produced within 60 days. These virus transmitting mites move from plant to plant by wind. Although mites can be spread a distance of greater than one mile, severely infected fields generally cause problems in fields within one-half mile of the source field. Insecticides or miticides will not control wheat curl mites, and few adapted varieties have some levels of resistance to the mite or virus.


Figure 1.
Leaf streak and mosaic symptoms
of wheat streak mosaic in young winter wheat.

Mite Hosts: The most favored hosts for mites are all the wheat crops, winter, spring or durum. Grassy weeds such as Downy brome and smooth crabgrass are good hosts, and corn, cheat and Japanese brome are fair hosts. Barley, rye and foxtails are poor hosts for the mite, but have been known to be infected with the virus. Mites will feed on corn kernels and can move from green corn into adjacent winter wheat crops.

Wheat Streak Management: Wheat streak mosaic disease is managed by breaking the life cycle of the mite through two key cultural practices: 1) controlling volunteer wheat and grassy weeds at least two weeks prior to planting; and 2) use of appropriate planting dates.

Prior to planting winter wheat this fall, the field to be planted and adjacent fields should be free of any wheat volunteers or grassy weeds. This is generally accomplished with burn-down herbicides applied at least two weeks prior to planting, to assure complete weed control prior to emergence of the new wheat crop. If no host is available in the field, the mite will die.

The second step, date of planting, also is very important. Planting winter wheat too early in the fall generally results in emergence of the new winter wheat crop when mites are still very active, because of warm temperatures. Generally, NDSU recommends planting winter wheat in northern tier counties within the first two weeks of September, and in more southern counties, the last two weeks of September. No volunteer winter wheat should ever be left standing over winter as a potential crop. Figure 2 is evidence of the potential devastating effects of leaving volunteer winter wheat containing mites and virus to infect wheat crops the following spring.


Figure 2.
Volunteer winter wheat harboring
wheat curl mites and wheat streak mosaic virus.

Reasons for More WSMV in 2010: Wheat streak mosaic virus was severe in 2010 because the late harvest made it difficult for producers to kill volunteers and weed hosts with herbicides a solid two weeks prior to planting winter wheat in 2009. Early planted winter wheat emerged in September when warm weather caused higher mite activity. Winter wheat planted at the end of September or later, emerged in October, which was considerably colder than September and led to decreased mite activity and less spread of the virus. The abundant snow cover during the winter had two adverse consequences. It allowed the wheat curl mite and infected spring wheat to survive better, resulting in further spread of the disease. Producers in high risk areas did get some relief in May as cooler temperatures allowed the wheat to grow faster than mite populations.

Sources: Marcia McMullen, NDSU Extension Plant Pathologist; Dan Waldstein, NDSU Extension Area Specialist, NC Research Extension Center, Minot

 

NDSU IPM SMALL GRAIN DISEASE SURVEY, AUG. 2-6

Field scouts are finishing up with the small grain portion of the IPM survey. They surveyed 30 wheat fields in the first week of August, primarily in the north central, northwest, and southwest districts.

Wheat leaf rust was found in 20% of fields surveyed, and stripe rust in 10%. Wheat streak mosaic and loose smut were observed in 17% of surveyed fields. Bacterial leaf streak was observed in 20% of surveyed fields. Scab levels had increased slightly from previous weeks, with 20% of the surveyed wheat fields in that week showing some scab incidence, and an average severity in those fields of 4.6%. However, across all surveyed fields, field severity of scab remained under 2%. Early harvest reports of wheat and barley indicate relatively good yields and quality, with minimal damage due to diseases.

 

STRAW, GRAZING, HAY RESTRICTIONS FOR WHEAT AND BARLEY FUNGICIDES

Hail damage in some areas on small grain fields treated with fungicides has resulted in a few questions about using these crops for hay, forage or straw. The following table contains information about harvest, grazing, hay and straw restrictions for the most commonly used fungicides on wheat and barley.

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

 

BACTERIAL PUSTULE FOUND IN ND SOYBEANS

Bacterial pustule, caused by Xanthomonas axonopodis pv. glycines, is a rare disease to find in North Dakota, largely because it is favored by temperatures between in the high 80’s and low 90’s and wet canopies. However, it has shown up in the SE part of the state. The disease is NOT economically important in our area; however, the pathogen produces a discrete pustule (hence the name bacterial pustule). Because this pathogen produces a pustule, it resembles a rust disease. To someone who has not seen soybean rust before, they may assume it to be soybean rust. Indeed, this is probably the most commonly confused disease with soybean rust.

Soybean rust has not migrated beyond the coastal states, and should not be a problem in our area this year. So if you are seeing something resembling a pustule in your soybeans, think bacteria. Information on soybean rust and lookalike pathogens can be found at www.sbrusa.net.


Fig. 3. Bacterial pustule on the top of a soybean leaf.


Fig. 4. Bacterial pustule on the bottom of a soybean leaf.

 

DRY EDIBLE BEAN RUST IN THE REGION

Common rust on dry edible beans has shown up in much of the bean growing region in the last two weeks. We have confirmation and samples from Cass, Ramsey, Steele, and Traill Counties in ND, and in NW Minnesota. It can be assumed that bean rust is occurring in other areas of the growing region as well.

As edible beans mature, the necessity and/or effectiveness of a fungicide application is reduced greatly. A general rule of thumb would be that when pinto’s stripe or beans are three to four weeks from harvest, fungicide applications are not recommended.

I would not expect much yield loss from rust this year. However, the rust that develops this year will be very important for disease in the future. We believe that most of the rust occurring in the region is the new race (greenhouse tests will be done this fall to confirm), and the new race can infect virtually all beans we grow. The occurrence of rust this year means that the new race of the pathogen has spread throughout the region, and consequently, the amount of inoculum next year will be much higher than in the beginning of 2009 or 2010. In favorable environments, we may expect rust to occur earlier than in 2010, and may be more widespread. Growers will have to be vigilant about scouting next season.

 

WHITE MOLD OCCURRING – DON’T SPRAY

White mold is showing up in dry edible beans, soybeans, and sunflowers throughout the region. There is nothing that one can do to mitigate damage. Research data indicates that fungicides are not effective and are not recommended at late growth stages for dry beans and soybeans. Fungicides are not recommended for sunflowers. Although we won’t know for some time, I believe that growers who aggressively managed this pathogen at appropriate growth stages, particularly in dry beans, will be rewarded for their efforts. However, even those who sprayed may still get some mold; but I expect it will be far less than if the field was not sprayed.

 

SUNFLOWER DOWNY MILDEW SURVEY

Dr. Tom Gulya (USDA Sunflower Research Unit) has been conducting a sunflower-downy mildew race survey this summer. To date, he has 130 samples from ND, SD, MN and NEB tested in greenhouse tests, and 16 of them are a race that can overcome the PI6 gene. This gene is the most commonly used downy mildew resistance gene in commercial sunflower hybrids. This new race was found for the first time last year, but its geographic distribution not totally known. Based on this year’s samples, the new ‘hot’ race appears to be most prevalent in ND (Figure 5), where 16 of 98 samples (38%) were the new race, compared to 6% occurrence in MN and NE samples, and none from SD.


Figure 5. Locations of the new race of the
downy mildew pathogen (J. Nowatski and T. Gulya, NDSU).

Dr. Gulya and his technician will continue to process samples throughout the summer, and they appreciate all the help they have received from extension personnel, and people in the seed industry who have collected and sent samples in for this study.

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


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