ISSUE 2 May 8, 2003
WET WEATHER MAY BRING ON EARLY SEASON TAN SPOT OF WHEAT
Recent rains across the state will favor development of the tan spot fungus in wheat stubble. Wheat crops that have emerged could develop symptoms on even the first leaf within 7 days of rainfall. Symptoms of tan spot early in the season may be similar to later season symptoms, if warm temperatures are common. Typical symptoms are the 1/8" to 1/4" diameter tan spots on the leaves, surrounded by a yellow halo.
Tan spot symptoms
These spots spread and join together if favorable weather continues for infection. Under cool temperatures, the spots may have more of a purple halo. Early season tan spot can affect yield potential and tillering ability. Early season tan spot is more common and severe if:
FUNGICIDE APPLICATIONS TO CONTROL EARLY SEASON TAN SPOT
Fungicide applications at the 4-5 leaf stage have been effective in reducing tan spot and increasing yield in research plots across the state. Fungicides are generally applied at a one-half the full label rate and are tank mixed with herbicides that are commonly applied at the early leaf stages of wheat. Herbicide and fungicide labels should be checked for specific instructions on tank mixing.
In 2002 at Fargo, several fungicides treatments were tested for early season tan spot control. These fungicides were applied once, at one-half their full label rate, to Oxen wheat at the 4-5 leaf stage. The previous crop had been wheat and the residue had been chiseled twice before planting. Early season tan spot did develop in the untreated wheat. Response to each of the fungicides was similar - all reduced leaf spot severity and all improved yields (from 3.3 to 4.9 bu/acre). Differences among fungicides were not significant. The fungicides and rates tested were:
Tilt: 2 fl oz/acre
Stratego: 5 fl oz/acre
Headline: 3 fl oz/acre
Similar responses were seen in 2001 at Fargo, with the highest yield increase of 5.8 bushels/acre. Roger Ashley, at the Dickinson REC found similar response to early season application of Tilt in 2000. The fungicide was applied to 4-5 leaf wheat on wheat or sunflower ground. When the wheat was planted back into wheat ground, 4.3 and 6.9 bu/acre yield responses were observed at Regent and Mandan, respectively. NO beneficial response of early season fungicide application was observed when the wheat was planted into previous year’s sunflower ground at Beach.
Supplemental (2ee) labels are available for early app. for leaf disease control for Tilt, Contend, Propimax, Stratego, Headline, and several mancozeb fungicides.
SPRING WHEAT VARIETY RESPONSE TO LEAF DISEASES
Wheat varieties vary in response to tan spot. The following information indicates the response of some spring wheat varieties to tan spot.
WHEAT STREAK MOSAIC VIRUS (WSMV)
Jack Riesselman, Extension Plant Pathologist at Montana State University, has recently reported an outbreak of WSMV in winter wheat in the Golden Triangle area of Montana. WSMV symptoms include stunting of the plant and a yellow streaking or stippling of the wheat leaf.
This virus disease is vectored by tiny (1/100") wheat curl mites. Both the virus and the mite development are favored by warm, dry conditions. These conditions were predominant last fall when the winter wheat was established and early this spring. Some of the affected fields in Montana may be destroyed and replanted to spring wheat.
Similar infections could have occurred in ND winter wheat, especially in the more droughty areas of the state. However, recent rains and cool conditions would certainly slow development and spread of the disease. I have not heard any reports of WSMV in ND, but winter wheat growers should be scouting their fields for this potentially destructive disease.
Extension Plant Pathologist
PROTECT SEEDS DURING EARLY-SEASON SOYBEAN PLANTING
Even though it is probably a little early, planters are rolling, and soybean seeds have been going into the cool soil. Although the minimum temperature for soybean seed germination is approximately 50 degrees, germination is slow, which means seeds could sit in the soil for a longer period of time than under optimum conditions. While a soil temperature of 50 degrees (or less) is not optimum for soybean germination, it is optimum for a pathogen known as Pythium, which can cause seed and seedling blight, damping-off, and root rot of soybean. Two fungicide seed treatments that work very well against Pythium are metalaxyl (Allegiance, Gustafson) and Apron XL (mefenoxam, Syngenta). Use of one of these seed treatments or a seed treatment that contains either metalaxyl or mefenoxam such as ApronMaxx RTA (Syngenta), Warden RTA (Agriliance), or SoyGard (Gustafson) is recommended to help ensure good stands when planting into less than optimal conditions.
Other factors to consider when to use fungicide seed treatments on soybean seed include:
*Cropping history: To reduce the inoculum levels of root rot pathogens, rotation with small grains or corn is necessary. However, if planting soybean after a broadleaf crop (especially soybean or dry bean), seed treatment with a fungicide may be required.
*Tillage practice: In fields where reduced tillage practices are used, soil takes longer to warm up and dry out. This cool and moist environment is conducive for seed and seedling blight.
*Seed quality: Most seed quality problems can be avoided by purchasing quality-assured or certified seed; however, bin-run seed can harbor several seedborne pathogens that can cause seed rot, or adult plant diseases such as charcoal rot, downy mildew, stem canker, and white mold. If used, bin-run seed should be cleaned and treated with a fungicide.
*Inoculants: Older seed treatments such as captan and PCNB may harm Bradyrhizobia inoculants, and should be avoided if inoculating. No matter what seed treatment product is used, always allow the fungicide to completely dry before using inoculants, and inoculate as close to planting time as possible. Use of granular inoculants may help avoid any antagonism with seed treatments.
Extension Plant Pathologist
PYTHIUM DISEASE OF SUGARBEET
Pythium occurs in all fields and current wet conditions is favorable for infection of seeds or seedlings. Pythium infect and rot seeds before or as they germinate. Sugarbeet seeds treated with Apron and Thiram provides protection from Pythium. However, Pythium can cause seed rot when conditions slow or delay emergence. These conditions include cold weather, deep planting, and excessive soil moisture. Under extremely wet conditions, Pythium also causes damping-off, usually within the first week of emergence. Symptoms of Pythium include a brown, water-soaked discoloration of the seedling before or just after emergence. Pythium infects seeds at the same temperatures (40°-95° F) that favor germination of sugarbeet seed. Shallow planting at 3/4 inch depth encourages emergence and reduces disease in wet, early-seeded fields.
SECTION 18 LABEL FOR EMINENT ON SUGARBEET APPROVED
Cercospora leaf spot is one of the most devastating foliar disease of sugarbeet. In 2002, the strobilurins Headline (pyraclostrobin) and Gem (trifloxystrobin) were approved for use on sugarbeet to control Cercospora leaf spot. Growers also have Tin (triphenyltin hydroxide) and Topsin (thiophanate methyl) for Cercospora control. However, because of Cercospora tolerance to Tin and resistance to benzimidazoles (Topsin), a section 18 label was requested for Eminent, a triazole, to serve as an alternating partner for the currently available fungicides. EPA has approved the request and Eminent will be available to sugarbeet growers in North Dakota and Minnesota for the 2003 growing season. Growers can not apply more than two Eminent applications during the growing season.
Extension Sugarbeet Specialist
SUBMITTING SAMPLES TO THE PLANT DIAGNOSTIC LAB (PDL)
The quality of the samples sent to the PDL for evaluation is directly related to the accuracy and specificity of the diagnosis and response. A high quality sample mailed to the lab will include the plant or plant parts (appropriately packaged), a name, address, and contact information for the person requesting the testing (or another individual who is familiar with the sample), and background information on the situation. Questions to consider for background information include: how many plants are affected (how much of the field), what is the primary concern or perhaps what was the first observation that indicated a problem, is there a pattern either in a field or on a single plant, were pesticides (herbicides, insecticides, fungicides) used, was fertilizer applied, is there a topographical correlation with symptoms, what plant or crop was planted in that site previously, recent weather conditions; and include a written description of the symptoms of concern. A very low quality sample will come in a box with a piece of paper saying, "please diagnose."
Sample packaging may vary depending on the problem. If the plants in question show foliar symptoms, sending several leaves that show the symptom or range of symptoms layered in dry paper toweling or between sheets of paper is often sufficient. If there are symptoms throughout the plant, or whenever it is reasonable, it is nice to get a whole plant or two. In this case, the root ball can be wrapped or enclosed in plastic but the foliage should be left unwrapped, and the plant(s) placed in a box to be mailed. As long as the plants fit snugly and do not shift, it is fine to send them without packing material. If the plant material may shift significantly in the box or padded envelope, some newspaper or other packing material would be helpful. Plastic bags, in general, are not a good idea; however the exception to this is as stated above - root balls may be wrapped in plastic. For trees or other woody plant species, snipping off a branch or twig may be appropriate as long as it shows the symptoms of concern. In this case, it is often helpful to also see a branch or twig that appears "normal." If a "normal" sample is not included, the background and description are even more important. If a root problem is suspected, dig plants and include some of the attached soil. Sending only material that is completely dead is rarely sufficient.
If you are interested in an insect identification, please do not send live insects. Put them in a sealed container with rubbing or denatured alcohol. It is helpful to get several insects. The same type of information listed above is useful. For plant identifications, send as much of the plant as possible, and try to lay leaves flat between sheets of paper or paper toweling. Both scanners and digital cameras may provide adequate views of these for a diagnostic identification. The key to getting a specific, accurate identification from digital samples is focus. Take several shots, from different angles and showing different sides, and be sure to view them before sending them. Again, background information is useful - where did you find this insect or plant, are they numerous, when were they first noticed, and are they always present or do they appear at certain times of the year.
If you send a diagnostic lab form with the sample, be sure to fill it out as completely as possible. If you have any questions about how to send or package a sample, or if you would like some diagnostic lab forms to keep on hand for sending samples, feel free to call or email the lab.
NDSU Plant Diagnostician