ISSUE 8 June 22, 2006
WHEAT STREAK MOSAIC VIRUS UPDATE
Reports of wheat streak mosaic virus (WSMV) infection in spring wheat are fairly common in a few locations across the state now. In most cases, the source of the virus infection appears to be some volunteer winter wheat that was not destroyed far enough in advance (at least two weeks) of planting spring wheat, either in the same field or in the immediate vicinity. Incomplete burn down of volunteers can result in unexpected outbreaks of wheat streak, because the "green bridge" for the mite and virus has not been broken.
Warm, dry weather makes WSMV much worse, because the wheat curl mite that transmits the virus multiples more rapidly. Under these conditions, the mite doesn’t like stressed plants and seeks new, greener tissue. The virus makes damage worse if the plant is already stressed from heat and drought
Chemicals are not available to control the mite or the virus. Cool, wet weather would favor the crop and not the disease or mite - but continued, hot dry weather will make for additional crop damage.
In most cases, it is too late to destroy the crop and plant something else. If growers chose to destroy a field, they should first consult with their insurers to see about levels of compensation. They may be asked to leave a check strip. Yield losses will depend on the extent of infection across a field and also on how early the crop was infected. Damage is greater when the plant is infected young, but little yield loss occurs if infections occurs at flag leaf stage or later.
Destruction of infected volunteers will be a must this fall, prior to any planting of winter wheat in these areas.
Prior to planting is the time to control WSMV.
WHEAT DISEASE OBSERVATIONS, June 12-20
NDSU IPM’s summer field scouts primarily found tan spot in wheat fields across the state during the week of June 12-16. Leaf rust was observed in Richland county on June 19, at severity levels between 1 and 5%. Dry conditions in many areas has prevented severe development of fungal leaf spots or leaf rust, although some high risks of infection are still indicated on the NDSU small grain disease forecasting web site for those areas that have had rain.
On June 13, the NDSU Plant Pathology Department hosted a group of wheat pathologists from around the US, to talk about research results and research needs. On a field tour to spring wheat and winter wheat plots in Cass Co., a small amount of stripe rust was found by Dr. Tim Murray, wheat pathologist from Washington State Univ. The group also found a fair amount of Septoria leaf blotch in the winter wheat plots in Prosper. Dr. Shaukat Ali, NDSU Plant Pathology Dept., confirmed that the species of Septoria was Septoria tritici, a species we usually find on wheat in the northeast part of the state.
Blake Vandervorst, of Ducks Unlimited, reported severe powdery mildew on winter wheat at plots near Ellendale. On June 20th, he reported that fungicide treatments (early season followed by flowering application) reduced the powdery mildew significantly. Powdery mildew is rarely a problem in wheat in our region, but I have received several reports of its occurrence across the eastern half of ND and into Minnesota this year. Most fungicides that are registered for wheat have good activity against powdery mildew, but early detection is the key for successful control.
NDSU DISEASE FORECASTING SITE
On June 20, the NDSU forecasting site (http://www.ag.ndsu.nodak.edu/cropdisease/) had some areas indicated to be of moderate risk for Fusarium head blight on very susceptible or susceptible spring wheat varieties. This area extended from Dickey and LaMoure counties in the south, northward to the Canadian border. The risk was associated with recent rains and high relative humidity across those counties. Other areas of the state were at low risk for Fusarium head blight (scab).
Dr. Shaukat Ali of the NDSU Plant Pathology Dept. has begun his Fusarium graminearum spore counts across several locations. For example, the June 14-16 air samples from Carrington indicated high spore counts, but the subsequent collections on June 17-19 resulted in low spore counts. The environment at Carrington on June 14-16th also was favorable for FHB infection, because of long dew periods. These same days also had weather favorable for leaf disease infections.
The spotty rain showers and subsequent variable risks of Fusarium head blight or leaf disease for individual locations makes it harder to predict disease severity across a region this year. Good field scouting and attention to disease risk as indicated by the NDSU disease forecasting web site will be critical for good fungicide decisions in 2006!
Extension Plant Pathologist
KEEP AWARE OF SOYBEAN CYST NEMATODE
Although the soybean cyst nematode (SCN) has only been confirmed in Richland County, soybean growers in adjacent and other counties also need to be on the lookout for this pest. Fields infested with high populations of SCN in Richland County are already exhibiting signs and symptoms of infection. The white to cream-colored SCN females are already visible on soybean roots in infested fields (see figure) and yellowing and stunting is already occurring. With the high temperatures and low moisture occurring this year in parts of Richland County, susceptible varieties planted in infested fields may suffer some big yield losses. Crop rotation with a non-host (such as corn or wheat) and planting resistant varieties are the two major methods of managing SCN. If you suspect SCN in your field, send soil samples from the affected area and the margins of the affected area to a laboratory (such as the NDSU Plant Diagnostic Lab) that can test for SCN.
Tiny, oval shaped SCN females on a soybean root
(courtesy Craig Grau, Univ. of Wisconsin)
SCLEROTINIA RISK MAP FOR CANOLA
The Sclerotinia Risk Map for canola has begun this season. Maps will be posted twice per week at the Northern Canola Growers Association website and through the NDAWN website at:
http://www.northerncanola.com/ and http://www.ag.ndsu.nodak.edu/aginfo/sclerotinia/sclerotinia.htm
The maps base their risk on the prediction of the emergence of apothecia in the soil, which are the "fruiting" structures of the Sclerotinia sclerotiorum fungus that produce the airborne spores that cause infections. Canola is only at risk if it is flowering, as the spores must first infect a senescing flower petal to cause disease. Fungicides registered for control of Sclerotinia stem rot of canola include Topsin M, Ronilan, and Endura. These fungicides should be applied when the canola plants are between the 30 to 50% flowering stages.
Carl A. Bradley
Extension Plant Pathologist