ISSUE 11 July 11, 2002
WHEAT ROOT ROT SYMPTOMS
White heads in wheat at the milk to soft dough stage are apparent now in SE North Dakota. Some are due to wheat stem maggot injury, others are due to root rot infections. When root rot causes white heads, the whole plant generally turns whitish to pale grey-green and the plant easily pulls from the soil. Roots and crowns often are discolored brown on these symptomatic plants, and the crown area is brown and "cheesy or punky", instead of healthy crowns which are cream to white colored with the cell structure intact. Incidence of white heads due to root rot or stem maggot in most fields was under 1% and generally was worse on the headlands.
SMALL GRAIN SURVEY UPDATE
NDSU IPM field scouts continue to find leaf rust and tan spot in most wheat fields, and low levels of spot blotch in western barley fields. Wheat leaf rust is most common and severe in central, east central and northeast counties, with a few detections in the north central area now, too. In areas with more leaf rust, heaviest infections have been reported on the spring wheat variety Ingot and Gunner. In Crop and Pest Report # 9, June 27th issue, I had a table listing variety response of spring wheats to leaf rust. I neglected to include Gunner spring wheat in the S-MS column. On July 8, at the Langdon REC off-station site near Tulna in Nelson County, Terry Gregoire and I rated the spring wheat varieties for leaf rust and found the highest severity on Ember. Ingot was not present in that variety plot. The varieties with the lowest levels of leaf rust at that site and time were Alsen, Norpro, Knudson Briggs, and Keystone.
Tan spot is common in most fields at some level, and more severe in areas that received more rainfall the past week. The spore bearing stalks and spores of the tan spot fungus can be seen with a hand lens in fields with more severe infections.
A very low level of barley leaf rust was observed in variety plots at Tulna in Nelson County on July 8. Trace levels of net blotch also were seen on barley varieties in these plots. For the most part, barley fields have had few disease symptoms so far this year, except for reports of spot blotch in the southwest.
The NDSU field scouts have not observed stem rust in wheat, head scab, or Septoria yet in their field surveys. However, on July 9, in winter wheat plots near Lisbon, ND, I observed trace levels of stem rust on Seward winter wheat and 5% severity of stem rust on an out-of-state experimental line. (Jim Miller, USDA, had observed stem rust the previous week in winter wheat plots at Casselton and Fargo). I also observed a very low incidence (less than 1%) of head scab in two adjacent spring wheat fields in northern Richland county; the crop was in the early dough stage.
DISEASE FORECASTING UPDATE
Rains in parts of the state the past week have increased the risk of head scab in some parts of the state, although, as of July 9, no area was in a high risk zone, but many were in moderate risk zones. Much of the grain in the southern part of the state is past the growth stages where fungicides would benefit the crop, but a lot of grain in the northern half may be entering the flowering stage. Please keep advised of the NDSU small grain disease forecasting site for information on favorable infection periods for leaf diseases and scab. Again, the web site is at:
Extension Plant Pathologist
MANAGEMENT OF WHITE MOLD ON DRY BEANS
White mold or Sclerotinia stem rot, caused by the fungus Sclerotinia sclerotiorum, can be a very damaging disease of dry bean under favorable conditions. Keeping disease levels to a minimum can be a difficult task some years. Tools used to manage the disease include variety selection, fungicide application, and good irrigation practices.
Although no variety is resistant to white mold, some varieties may perform better than others in the presence of the disease. These "tolerant" varieties should be planted in areas where white mold can be a potential problem. The NDSU Extension bulletin A_654, North Dakota Dry Bean Performance Testing, lists which varieties have a tolerant or susceptible reaction to white mold.
Topsin M (Cerexagri) and T-methyl (Micro Flo) fungicides, which contain the active ingredient thiophanate-methyl, are labeled for white mold management in dry beans. Timing of the fungicide application is critical in achieving the best results. Fungicides should be applied when 10 to 30% of the plants have at least one open bloom. Fungicides should not be applied within 28 days prior to harvest.
Applications of a fungicide may be made via airplane, ground rig, or through irrigation sprinklers. Fungicides applied by airplane appear to work better with spray volumes ranging from 7 to 10 GPA compared to volumes of 5 GPA or less. When applying by ground, drop nozzles between the rows and a nozzle over the top of the row provide the best coverage. Applying fungicides with a pressure of 100 PSI or greater will also increase coverage. When applying through sprinkler irrigation, the system should be set to deliver between 0.1 and 1.25 inches of water per acre. The treated area should not be irrigated 24 to 48 hours after the application to prevent washing the fungicide off.
Scheduling irrigation properly can help reduce white mold development. Research conducted by the University of Minnesota in Staples, MN showed that scheduling irrigation events when the average soil water tension in the upper 10 inches of the soil reached 65 to 75 centibars could reduce the potential for white mold development.
RHIZOCTONIA ROOT ROT OF SOYBEAN
Rhizoctonia root rot of soybean, caused by the fungus Rhizoctonia solani, has been reported in soybean fields located in Cass, Traill, and Steele counties. Symptoms appear as reddish-brown sunken lesions located on the hypocotyl and roots. Fields that have been in continuous soybean or in short rotations with soybean are those most likely to have root rot. Crop rotation with cereal crops and fungicide seed treatments will help manage the disease.
QUADRIS ON CHICKPEA
A section 18 label was granted to control Ascochyta blight of chickpea with Quadris fungicide in North Dakota. Quadris can be applied at a rate of 6.2 to 9.2 fl. oz./acre. The first application should be applied at the onset of disease. A second application may be made at a minimum of 14 days later.
TOPSIN ON CANOLA
Topsin M fungicide was granted a section 3 label for management of white mold (Sclertonia stem rot) of canola. Topsin may be applied once at 20 to 50% flowering at 1 to 2 lbs/acre. Topsin may also be applied as a split application at 1 lb + 1 lb/acre. The first application should be applied at 20 to 30% flowering, and the second application should be applied at 40 to 50% flowering.
POTATO DISEASE FAX SERVICE INITIATED
A new potato disease fax service was initiated on July 1. The late blight forecast fax provides forecasts maps for late blight and will be faxed each Monday, Wednesday, and Friday until mid-September. The service (Dithane/Gavel Forecast FLASH) is sponsored by Dow AgroSciences and is provided through cooperation with Drs. Gary Secor and Neil Gudmestad of the NDSU Plant Pathology Department and Dr. John Enz, NDSU Climatology Department. The service distributes late blight forecast maps from the new NDAWN late blight forecasting website which became operational a few weeks ago. The maps are generated using weather information from NDAWN stations across the state.
The new fax service is designed to distribute info quickly and when computer access is not available or convenient. Users can also access the web site ( http://ndawn.ndsu.nodak.edu/application/potatoes.html ), to view color maps which display information on late blight severity values, late blight favorable day values, and early blight p-values for the past two days and season-long accumulation.
If you are not now receiving the service and would like to subscribe, please call the Dow AgroSciences Customer Information Center, 1-800-258-3033.
Extension Plant Pathologist
BEAN POD MOTTLE VIRUS IN SOYBEANS
There are several viruses that can infect soybean, one of which is Bean Pod Mottle Virus (BPMV). This virus is transmitted by leaf feeding beetles, the most important of which is the bean leaf beetle (Cerotoma trifurcata) in the north central region. This beetle feeds on soybeans and other legumes infected with the virus, and then moves those virus particles to other plants. BPMV is believed to overwinter in perennial weed species and forages such as alfalfa and clover. In the spring when bean leaf beetles become active, they feed on these overwintering hosts and pass the virus on to soybean crops. I donít believe we seen this virus yet in ND but it has been increasing in frequency in SD, moving steadily into northern counties in recent years. In 1998, only 4 counties reported BPMV in soybeans. In 1999, 11 counties reported the virus with incidence in fields ranging from 2-100%. By 2000, 15 counties reported BPMV infection in soybeans, with the average disease incidence being 9-86%.
BPMV appears on leaves as a green and yellow blotchiness called leaf mottle. Pictured at: http://www.ag.ndsu.nodak.edu/diaglab/bpmv_leaf_mottling.htm .
Younger leaves usually show more severe symptoms than older leaves. These younger leaves may also show a raised, bumpy or blistered look that distorts the appearance of the upper canopy. Pictured at: http://www.ag.ndsu.nodak.edu/diaglab/bpmv_and_bean_leaf_beetle.htm .
These symptoms are most obvious when there is rapid growth under cool conditions. These symptoms may also appear similar to those caused by herbicide drift injury, so it important to scout for the beetle and distribution of symptoms across the field.
In addition to plant distortion, BPMV may decrease pod formation, and reduce seed size, weight, and number. Infected seed may also be mottled (although not all mottled seed is infected with the virus). Pictured at: http://www.ag.ndsu.nodak.edu/diaglab/bpmv_and_seed_mottling.htm .
BPMV infected seed is more susceptible to infection by Phomopsis, a fungal pathogen. A condition called green stem has been associated with BPMV, where stems and petioles remain green after pod formation and maturity, causing problems at harvest. Pictured at: http://www.ag.ndsu.nodak.edu/diaglab/green_stem.htm
More information is available at: http://www.ncsrp.com/planthealth/virus/virussymp.htm . Leaf samples can be sent to the lab for testing. Collect 5-10 leaves showing symptoms. These can be stored overnight in a bag (kept refrigerated or cool) and brought to the lab, or press leaves between sheets of paper and send to:
NDSU Plant Diagnostic Lab
PO Box 5012
Fargo, ND 58105
NDSU Plant Diagnostic Lab