ISSUE 6 June 10, 2004
SMALL GRAIN DISEASE SURVEY
Wheat leaf rust was observed on June 8 by NDSU IPM survey scout Tammy Link, in two spring wheat fields in Cass county. The number of plants per field showing symptoms was 4% and there were only one or two pustules of rust on these plants.
The NDSU IPM survey scouts looked at a little over 100 wheat fields and 10 barley fields up through 6/4. In their survey of wheat, they found about a little more than 1//4 of the wheat fields showed some level of tan spot, with leaf severities all less than 15%; incidences of plants infected in a field were slightly higher in the southwest and northwest counties. Some spot blotch fungal infections were also seen in 8% of the counties surveyed, primarily in the southwest.
The NDSU scouts also looked at 8 barley fields last week and net blotch was observed in 2 fields in the central region of the state and spot blotch was observed in 2 fields in the northwest counties.
WHEAT DISEASE FORECASTING SITES INDICATE DISEASE RISK
The NDSU small grain disease forecasting site ( http://www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm ) indicated that favorable weather for infection by the wheat tan spot fungus occurred in most areas of the state this past week. Some areas also had favorable weather for leaf rust.
Scab (FHB) and the forecasting sites:
The NDSU disease forecasting site indicated many areas of eastern North Dakota had a high risk for infection of Fusarium head blight (scab = FHB) as of June 8. However, it is most likely that only winter wheat fields in these areas may be flowering at this time.
This scab forecast is dependent on rainfall duration, relative humidity, and temperature, and will change on a daily basis. The map is interpolating weather data from the individual NDAWN weather stations to create a map of risk across a large area, and some areas within a risk zone may have had drier or wetter weather and be at lesser or greater risk. Producers need to consider how closely their weather matched the nearest NDAWN station.
The Penn State- US Wheat Barley Scab Initiative web site for FHB forecasting (http://www.wheatscab.psu.edu/riskTool.html) indicated that on June 9, several of the same areas in North Dakota also had intermediate to high risk of FHB, including portions of Barnes, Steele, Griggs, Grand Forks, and Nelson counties.
Again, it must be remembered that these are web sites that use models that base the risk on the previous 7 days weather for the particular day you check – neither model uses future events – and both models change risk on a daily basis.
WHEAT STREAK MOSAIC VIRUS
The NDSU diagnostic lab received a sample of spring wheat from Sargent county that had severe wheat streak mosaic virus. The typical symptoms of stunted plants, yellow streaking and green/yellow mosaic discoloration on the leaves were evident. The leaves also were curled along the margin and within those curled leaf edges were many wheat curl mites, the vector of the virus. The wheat curl mites are only 1/100 of an inch in length, so are almost impossible to see with the naked eye or a field hand lens.
This virus disease must be controlled with prevention, planting wheat into clean fields with no winter wheat volunteers or grassy weed hosts present, or no infected winter wheat fields or volunteers nearby. At this time, there is nothing that can be done for the infected field. If the disease is occurring along field edges or in small patches, the producer may decide to plow under the infected parts to prevent spread to adjacent plants and fields. A burn-down with a glyphosate-containing product will only result in the wheat curl mite to more rapidly seek healthy tissue and be carried by wind to adjacent healthy plants.
Extension Plant Pathologist
BLACKLEG ON CANOLA
Blackleg, a fungal disease caused by Leptosphaeria maculans, has already been observed on canola this season in research plots at Minot. Early symptoms of blackleg may appear as lesions on the leaves that are gray in color with pycnidia (small black dots) in the center (Fig. 1). Symptoms on young plants may also occur as cankers on the lower stem (Fig. 2). The best practices for blackleg management are crop rotation and planting resistant varieties. During the season, the only available management option is to apply Quadris fungicide at the 2 to 4 leaf stage. Quadris has not been evaluated for blackleg control in any NDSU trials, so data are not available to provide information on efficacy and economic return. A new very aggressive strain or pathogenicity group of blackleg has recently been found in parts of Manitoba and North Dakota. If severe outbreaks of blackleg are observed, please contact NDSU Extension. Blackleg samples are being collected to determine pathogenicity groups in which they belong
Fig. 1. Blackleg lesion on canola
leaf with pycnicia (black dots) in
the center of lesion. (Photo by G. Ash)
Fig. 2. Lower stem canker caused
by blackleg. (Photo by G. Ash)
SEEDLING DISEASES ON BEANS CAUSING STAND REDUCTIONS
As dry beans are starting to emerge, reports of fields with poor stands are coming in. One of the possible culprits of poor dry bean stands could be due to seedling diseases. Seedling diseases can be caused by a variety of pathogens which include Fusarium, Rhizoctonia, and Pythium. The cool, wet conditions observed after some bean fields were planted have been conducive for seedling diseases. Roots of plants able to emerge may still be infected, which could lead to poor yields. Root rot symptoms can include reddish-brown distinct lesions on the main root (Fig. 3) or non-distinct brown lesions affecting the root tip and lateral roots (Fig. 4).
Fig. 3. Distinct sunken lesions
on the main root.
Fig. 4. Root rot affecting root
tip and lateral roots.
Extension Plant Pathologist