ISSUE 14  August 27, 2009

NOW IS THE TIME TO MANAGE WHEAT STREAK MOSAIC VIRUS (WSMV)

NDSU field scouts and the NDSU Diagnostic Lab saw more wheat streak mosaic virus this year than has been seen for many years. This disease was observed statewide, in winter wheat, spring wheat and durum. Levels of severity ranged from slight to severe. The July 8th edition of the NDSU Crop and Pest Report outlined many possible reasons for this high incidence, including: 1) a wet fall in 2008 with lots of green grasses, volunteers, and corn present late into the season, allowing a continuous green bridge for survival of the mite vector and virus; 2) heavy snow cover in many areas allowing good survival of the mite and virus in host plants; 3) and a cool, wet spring which hampered herbicide "burn-down" of weeds and volunteers and delayed spring planting.

To reduce the risk of carrying this disease over into winter wheat this fall and subsequent spring crops next year, management steps must be taken now to break the green bridge that allows the mite and virus to survive. The green bridge consists of host plants, such as wheat, corn, volunteer wheat, and grassy weed hosts. Those management steps are three-fold:

  1. Plant winter wheat during the recommended planting windows: ie. Sept. 1-15 for the northern half of ND, and September 15-30 for the southern half of the state. Earlier planting will more likely result in winter wheat emerging when mites are still very active and when grassy weeds and volunteers in the field or adjacent fields have not been controlled or destroyed adequately.
  2. Control volunteer wheat and grassy weeds in a field two weeks prior to planting the next susceptible crop. A two-week window of not having a host present assures that the mite has gone through its life cycle and not found a subsequent host to feed on and transmit the virus.
  3. If you plan to plant winter wheat following spring wheat this fall, especially in the northern tier of counties where spring wheat harvest will be late, plan on using a glyphosate burn-down on the crop prior to harvest, to help control late maturing tillers and green grassy weeds.

Late harvest of wheat and corn this year is again going to perpetuate the risk of the mite and virus surviving continuously along the green bridge until freeze-up. Weeds and volunteers may be difficult to control in some areas prior to planting winter wheat and or prior to winter freeze-up. Corn also will be green late into the season. With all these risks, it is essential that growers do the best job possible in controlling volunteers and following the winter wheat planting date recommendations. Next spring, the earlier planted spring wheat will avoid the highest risk of subsequent WSMV infection.

A 2008 Extension publication from Nebraska, titled "Managing Wheat Streak Mosaic", publication EC1871, has an excellent description of mite movement from a source field. The following is a quote from that Nebraska Extension publication:

"The risk of mite dispersal and virus spread from a source field tends to follow an oval-shaped pattern extending from the northwest to the southeast according to prevailing winds. The size of this oval pattern depends on the size and mite density of the mite source. If mite populations in the source fields are low, spread to neighboring fields will only occur for a short distance and an edge effect of the spread will be evident. If mite populations in the source field are high and the mite source field is large, mites and virus will be spread across entire fields....The extent of this distance is not known, but anecdotal evidence indicates that in some instances, it may be up to one to two miles".

This above information may indicate to producers where they should plant their winter wheat this fall, if they have had a WSMV confirmation this year. Of course, the highest risk for WSMV is planting winter wheat into a field with remaining green wheat volunteers or some grassy weed hosts, such as downy brome, barnyard grass, or green foxtail. Barley and oats, although not commonly thought of as good hosts, also may harbor the disease, and their volunteers also should be controlled.

More information about wheat streak mosaic virus may be found in the NDSU Extension publication PP- 646 (revised) "Wheat Streak Mosaic Virus", found on-line at:

http://www.ag.ndsu.edu/pubs/plantsci/smgrains/pp646.pdf

Or in the Nebraska Extension Publication, "Managing Wheat Streak Mosaic", found on-line at:

www.ianrpubs.unl.edu/epublic/live/ec1871/build/ec1871.pdf  

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

Joel Ransom
NDSU Extension Agronomist
joel.ransom@ndsu.edu

 

SUMMARY OF WHEAT AND BARLEY DISEASE SURVEYS - 2009

NDSU IPM field scouts surveyed 1086 wheat fields and 281 barley fields across the state, as of Aug. 14. A complete summary of disease and insect occurrences will be given in the fall publication "ND Crop Production Guide", but the key findings on disease occurrence were:

Wheat:

Tan spot fungal leaf spot was detected early, and was the most frequent disease detected and often was severe, especially where wheat was the previous crop. Approximately 85% of all fields surveyed showed tan spot symtpoms. Leaf rust was almost non-existent this year, observed in only 1.4% of the commercial fields surveyed, and most often in winter wheat. Fusarium head blight (scab) also was fairly infrequent in 2009, found in a little over 13% of the post-flowering fields, with an average field severity in those fields of 2.1%. Loose smut was found in 19.7% of post-flowering fields, with an average of 4.9% of the tillers in those fields with smut.

Bacterial leaf stripe was observed in 45 fields or 4.1% of all fields, but was often severe where observed. Two virus diseases were fairly common, Barley yellow dwarf (BYDV) symptoms were observed in 88 fields and wheat streak mosaic virus (WSMV) was observed in 102 fields. Most, but not all, BYDV symptoms were observed post-boot stage, but 24 of the 102 fields observed with WSMV symptoms were observed in fields prior to boot stage.

Barley:

Net blotch and or spot blotch were again the most frequent disease symptoms observed on barley, with about 71% of fields showing some symptoms, although, generally, severity was low. Of the 54 post-heading fields surveyed, only 3 had symptoms of Fusarium head blight and 10 had symptoms of loose smut, with average of 6% of tillers showing smut symptoms in those fields. Barley yellow dwarf virus was observed in 19 barley fields.

 

CEREAL DISEASE BULLETIN, FINAL 2009 ISSUE

The USDA/ARS Cereal Disease Labís Cereal Rust Bulletin of August 18, 2009 summarizes cereal rust occurrences across the US for 2009. The authors report that wheat stem rust was at low levels in plots and in fields of susceptible winter and spring wheats across the US. Their surveyors found trace levels of wheat stem rust in plots of susceptible wheats in eastern and central North Dakota. They did not find stem rust in plots at Minot. In the NDSU IPM survey, we did not find stem rust in any commercial fields. The stem rust races found in the US by the Cereal Disease Lab were "common races found in the US the past several years."

For wheat leaf rust, the level of leaf rust varied across the US. Pathologists at the Cereal Disease Lab in St. Paul found leaf rust at trace levels in spring wheat fields throughout ND during the third week of July. They returned to ND in August, and found "increased amounts of leaf rust in plots of the cultivars Knudson and Briggs." "No leaf rust was observed on cultivars with Lr21(gene), eg., Faller, Glenn, Steele ND, RB07, and others." Thus, their findings on stem and leaf rust of wheat were close to what we found in our NDSU IPM disease survey.

This Bulletin has further information on detections of other rusts across the US in 2009, as well. It can be found at the Cereal Disease Labís web site:

http://www.ars.usda.gov/mwa/cdl.

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

 

DRY EDIBLE BEAN RUST CONFIRMED IN NORTH DAKOTA

Last week samples of common rust on dry beans were sent to the NDSU diagnostic lab by John Jones (ADM Seedwest). Samples were from a field in Traill County, planted to a variety thought to have the Ur-3 resistance gene. The identification of rust on a variety with this gene indicates that the new rust race (20-3) discovered in the fall of 2008 is likely causing disease. As such, all bean varieties should be assumed to be susceptible.

Fortunately, rust has been found relatively late this season. However, for some late maturing beans, rust has potential to reduce yield. It is important to protect your beans until approximately three weeks from harvest. This roughly corresponds to R7, or striping in pintos. At this time, rust will not impact the crop.

The first step for managing rust is to scout your fields. Even though the samples we received were from Traill County that does not mean rust is limited to that area. Conditions in Eastern ND have been favorable for rust. Scout the middle of the canopy and on the undersides of the leaves, as this is most likely place where rust develops first on the beans. Pay particular attention to field edges bordering either last yearís bean fields or shelter belts.

If you donít find rust, good, scout again later. I would not recommend applying a preventative application this late in the season, unless you know your field is adjacent to a field where rust is developing. Although preventative applications were a good practice in the past, current fungicides are much more effective at managing disease once it is occurring. Plus, it is so late in the season that it is likely that most fields will escape infection altogether. However, if you find rust before R7 (before striping or 3 weeks or more prior to harvest) multiple fungicides are available for use.

Strobilurin fungicides (Headline and Quadris) will work well, but are best when applied as protectants or when rust severity is low. As disease severity increases, these products do not manage rust as well as some of the triazoles. The lower rate of strobilurin should provide adequate protection, but the higher rate may provide more or longer protection.

Tebuconazole (Folicur and generics) have been shown to be a very good option for managing rust when disease is present at low or moderate disease severities. A 4 fl oz rate has proven to be very effective. Proline has been shown to manage rust of other crops (wheat, sunflower), and will likely do so on dry edible beans, but little data exist on beans. The low rate (4.3 fl oz) should be adequate. Other products like Maneb and Sulfur are protectants only.

The PHI for each product varies but all products mentioned are 21 days or less. Protection of the crop should not take place within three weeks of harvest, so the PHIís should not be an issue. Read and follow directions of the label if you apply a fungicide.

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


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