ISSUE 7 June 21, 2007
BLACKLEG OF CANOLA
Two diseases that can be particularly damaging to canola are Blackleg and Sclerotinia. Both diseases are caused by fungi and can do significant damage the stem of the plant. Wet weather favors both diseases, and Blackleg has already been reported this year in the northwest part of the state.
Symptoms of Blackleg can sometimes be observed on leaves as tan to buff colored lesions. The lesions may contain small black structures that resemble pepper flakes (pycnidia). Blackleg lesions on the stem occur near soil line or where a petiole was attached. Lesions are gray with a dark or black border that may contain pycnidia. Lesions may become sunken and girdle the stem, producing the characteristic ‘blackleg’ symptom. Early infection may result in premature death and lodging.
Sometimes Blackleg can cause infection to a stem but produce few symptoms. If plants die for no apparent reason, take a sharp knife and slice open the roots. If blackleg is to blame, roots will be black, gray, or streaked with gray. Healthy roots have a cream color.
Symptoms of Blackleg on a leaf
Symptoms of Blackleg on a stem
Crop rotation, clean seed, and seed treatments can be effective in reducing the amount of inoculums in your field and limiting the spread of the disease into new areas. Selection of a resistant cultivar can aid in control. However, the blackleg fungus has different strains. Prior to 2002 only two strains were known in North Dakota, an aggressive or virulent strain, and a weak strain. In 2003 two additional aggressive strains were identified. Although not common (7% of canola isolates in 2003 surveys were new strains), the new strains may have the ability to cause disease on canola that was considered resistant. If you observe high levels of blackleg on varieties that were considered resistant, it may be one of the new strains.
Research done in Langdon demonstrated that a Quadris application at the high rate (15.4 fl oz/acre) during the 2-4 leaf stage can reduce blackleg. However, it was more economical to plant a resistant variety (as long as resistance holds up).
SCLEROTINIA OF CANOLA
Sclerotinia infections begin during flowering, and the first symptoms are a light-brown, mushy, target-like pattern on cast petals. The infection spreads down infected petioles to the stem. Unlike blackleg, symptoms of sclerotinia on canola stems usually occur 6-18 inches off the ground, lesions are white, and infected stems take on the appearance of a dry bone. Hard black structures (sclerotia) that resemble rat droppings are sometimes found in the stem. Lodging can be a concern.
Sclerotinia infected canola stems
Crop rotation and tillage can help to manage Sclerotinia, and foliar fungicides have been shown to be effective at reducing disease. Quadris (Azoxystrobin), Endura (Boscalid), Ronilan (Vinclozolin), Topsin M and T-Methyl, (Thiophanate methyl) and Proline (Prothiconazole) are registered for control of Sclerotinia in Canola. Multiple years of research at different locations have been done to evaluate the effect of fungicides on control of sclerotinia. Throughout these studies, a fungicide application at the high rate generally resulted in significantly greater yield than that of the untreated control. Only in one location-year were significant yield differences observed between the fungicides used.
When data were summarized from 13 location-years of fungicides applied at the high rate, yield increased about 400 lb/acre over the untreated control but yield differences between fungicides were not statistically significant. Yields averaged 2475lb/acre for Proline, 2355 lbs/acre for Ronilan, 2421 lbs/acre for Endura, 2359 lbs/acre for Topsin M, and 1975 lbs/acre in the untreated control. Yield of Quadris treatments, which were not included in all studies, appeared to be similar to other fungicides.
A Sclerotinia risk map in available and can aid in evaluating sclerotinia risk. The map uses available weather data to assess the Sclerotinia risk to a crop in flowering. The risk map is available at the NDSU extension website at http://www.ag.ndsu.edu/sclerotinia/ and the Northern Canola Growers Association at http://www.northerncanola.com/maps/index.asp
For more information on rate and timing of fungicide applications consult the 2007 North Dakota field crop fungicide guide (PP-622).
Extension Plant Pathologist
NDSU IPM CROP SURVEY
The NDSU field scouts examined 111 wheat fields last week across all parts of the state; 14 of these fields were identified as winter wheat.
Tan spot leaf infection was again the most common disease observed, found in 89% of the fields surveyed, with moderate to high severities observed on flag leaves of winter wheat, but severities of 1-5% on leaves of most spring wheat fields. Detections of Septoria leaf blotch also were picking up, with 6.3% of fields reported with symptoms. Leaf rust was observed in 16.2% of the fields surveyed, but average severity of leaf rust remained low in spring wheat (1-2%). Winter wheat fields showed a higher percentage of tillers with rust symptoms, and a higher level of severity on the flag leaves.
Grain aphids were most abundant in observations in Traill and Steele counties, with a range of 4 - 70% of tillers showing at least one aphid. Aphids were also common in some areas of the north east and south central regions, but almost absent in the west.
The NDSU field scouts observed 25 barley fields during the week of June 11-15. These barley fields had a wide range of growth stages, from the 2 leaf stage to early heading in other fields. The most common disease reported for these barley fields was spot blotch, with a few showing net blotch and bacterial blight, as well. Fungal leaf spot severity ranged from 1% to 25%. Two fields showed very low levels of barley leaf rust. Grain aphids also were observed in half of the scouted fields, with 1 - 20% of tillers showing at least one grain aphid.
WHEAT FORECASTING INFORMATION
On June 20th, the NDSU small grain forecasting site indicated moderate to high risk of Fusarium head blight infection for flowering wheat of susceptible or moderately susceptible cultivars, risk found primarily in the eastern half of ND. Most days in the past week had also been favorable for tan spot and leaf rust infections across the whole state.
However, high temperatures predicted in the latter part of the week of June 18-22 will reduce the risk of fungal infections over that time period. Spotty showers or thunderstorms across the state are also predicted, and in locations were those occur, fungal disease risk will likely remain high. Because the risk changes on a daily basis, individuals making fungicide decisions would benefit by looking at the NDSU forecasting models, based on NDAWN weather station data, found at:
Example: NDSU forecasting site map of Fusarium head blight risk for moderately susceptible wheat cultivars on June 19, 2007.
The other web site that also provides information on Fusarium head blight risk for the region, with weather information coming both from NDAWN stations and predicted weather, can be found at:
UPDATE ON BACTERIAL BLIGHT ON WHEAT
The June 14th edition of the NDSU Crop and Pest Report indicated that bacterial leaf blight (stripe) was observed in some wheat fields, and pictures of symptoms were provided. Four wheat samples brought to NDSU this past week, three winter wheat and one spring wheat, also had symptoms of bacterial blight (brown streaking, brown spotting, and water soaking), and bacterial streaming from dissected tissue was very evident under a compound microscopic.
These symptoms have been confused with fungal leaf diseases, such as rust or Septoria. A reminder that fungicides will not control bacterial infections, and symptoms of bacterial blight may occur in the presence or absence of fungicide treatment.
Ext. Plant Pathologist