ISSUE 5 May 30, 2002
SCLEROTINIA RISK MAP FOR CANOLA
The first Sclerotinia risk map for canola will be available on June 3. The first map will provide information on planting dates of canola (when 50% of the crop was planted in each county) and also will provide data on the soil moisture conditions. Moisture in the top four inches of soil is critical to the model for Sclerotinia since the upper two inches of soil must be saturated for 10 days for the Sclerotinia spore-producing structures, the apothecia, to form. The apothecia, tiny mushroom bodies resembling diminutive golf tees, release millions of air borne spores that initiate Sclerotinia infections. The spores initiate growth on dead plant tissue, primarily the cast dead petals. The infection areas must be wet for the better part of 40-48 hours.
Subsequent risk maps will address the risk of Sclerotinia infection in various areas of North Dakota and northwestern Minnesota. We will have better weather information this year than last with the addition of North Dakota Agricultural Weather Network (NDAWN) stations at Fingal and Pillsbury in Barnes County, Wishek in McIntosh County, Karlsruhe in McHenry County, Roseglen (Plaza) and Ross (Stanley) in Mountrail County, Berthold in Ward County and Crosby in Divide County, as well as Roseau in Roseau County, Minnesota and access to University of Minnesota weather station data at Karlstad in Kittson County and Williams in Lake of the Woods County, Minnesota. Actual risk will not begin in any given area until the earliest planted canola in the area begins flowering or until petal cast occurs.
The risk map will be posted on the web on the Northern Canola Growers Association web site:
and on the NDSU Extension Service Web site:
Plant Pathologist Emeritus
SOYBEAN SEED WITH WHITE "POWDERY" APPEARANCE
A few instances of soybean seed with a white "powdery" appearance have been reported this spring. There are 3 diseases that may cause this "powdery" appearance:
*Downy mildew, caused by Peronospora manshurica. Infected seed will be encrusted with oospores. Under cool conditions, infected seed can produce systemically infected seedlings. Yield reductions due to downy mildew have been reported to be between 4 and 9 bu/A when greater than 95% of the plants were infected. Downy mildew is typically not a disease that draws major concern, but to avoid a severe outbreak, infected seed should be treated with mefenoxam or metalaxyl, soybean residue should be plowed under, and a non-host crop should be rotated to for at least 1 year.
*Phomopsis seed decay, caused by different species in the genera Phomopsis and Diaporthe. Infected seeds may also be shriveled. Infected seeds will usually not germinate, and a seed treatment should be applied if 15% or more of the seeds are infected. Most broad-spectrum seed treatments will have activity against this disease.
*White mold, caused by Sclerotinia sclerotiorum. Mycelia on infected seed may form sclerotia in the soil, which could eventually form apothecia. Planting infected seed is an important method of introducing the disease to new areas. The seed treatment fungicides thiram, fludioxonil, and captan + PCNB + TBZ have been reported to reduce sclerotia formation from infected seed.
PINK SEED OF PEA DISEASE
A relatively new disease of pea has been reported in Alberta, Saskatchewan, and Montana. The disease, caused by the bacteria Erwinia rhapontici, causes pea seed to be discolored to a pale pinkish brown to bright pink. This same bacteria causes a similar disease to wheat seed. The disease has been reported to cause reduced germination and stunting. Unlike pink coloration due to fungicide treatment, the pink color caused by disease cannot be washed away. This disease has not been reported on pea in North Dakota, and using disease-free seed is the best method to avoid introducing the disease to this area. If you suspect that your pea seed is infected with Erwinia rhapontici, please send samples to the NDSU Plant Diagnostic Lab at: Box 5012, Fargo, ND 58105.
Extension Plant Pathologist
UPDATE ON EARLY SEASON FUNGICIDES FOR TAN SPOT CONTROL
Considerable tan spot lesions were observed on the first three leaves of a 5-6 leaf winter wheat crop in Ransom county on May 28, 2002. This winter wheat crop was planted into wheat stubble, and the potential for substantial tan spot infection exists if recent rain showers hit that field. That field and similar winter wheat or spring wheat fields planted into wheat stubble would be good candidates for early season fungicide application, if rainfall occurs.
I recently received a question about comparison of efficacy of products available and whether even a lower rate than half the full label rate has been tested by NDSU. In 2001, John Lukach at the Langdon Research Extension Center compared Stratego and Tilt for early season leaf disease control on Alsen HRSW, and I compared Stratego, Tilt and Folicur for early season leaf disease control at Fargo on Oxen HRSW. In both locations, we used half the full label rate, 2 fl oz of Tilt or Folicur and 5 fl oz of Stratego. We did get significant reductions in leaf disease compared to the untreated control with these treatments, but generally no significant differences among the fungicides were observed. John did see a slightly lower level of leaf disease on the F-2 leaf with Stratego as compared to Tilt. Other NDSU studies at Dickinson and Minot in previous years had also used half the full label rate of products.
We are doing studies in 2002 with rates lower than half the full label rate, but the only data I have from previous studies with lower than half the full label rate was with 1 fl oz of Tilt early vs 2 fl oz Tilt early on winter wheat in 2001. In these fungicide studies on winter wheat varieties near Lisbon, ND, a 1 fl oz rate of Tilt at the 5 leaf stage, followed by 3 fl oz at early heading improved leaf disease control and yields more so than a treatment of 2 fl oz rate of Tilt applied early followed by 2 fl oz rate of Tilt at heading. These results could be indicating that a heading application of only 2 fl oz may not be adequate to control late development of leaf disease, instead of indicating that an early application of 1 fl oz of Tilt was adequate. We had no straight comparisons of 1 fl oz vs 2 fl oz alone.
Further information on fungicides available for early application to small grains was provided in NDSU Crop and Pest Report # 3.
Extension Plant Pathologist