ISSUE 7 June 14, 2001
WHEAT TAN SPOT
Tan spot continues to be the most common disease of wheat observed by IPM crop scouts .
The highest incidences of tan spot recorded by the scouts runs along a transect from the NW corner towards the SE corner of the state. These early season tan spot observations are following several patterns: where rainfall has been favorable for infection and where previous crop was wheat. The crop scouts have recorded previous crop in the survey, and much higher incidences of early season tan spot are associated with fields that had wheat as last year’s crop. Continuous rains across the state may make tan spot infections more widespread. The NDSU wheat disease forecasting model indicates a majority of the days in the past 12 days have been favorable for tan spot infection in most locations monitored by the model.
Jerry Schneider, IPM Crop Scout working out of the Carrington REC, found a trace amount of wheat leaf rust in a winter wheat field in Benson County this past week. Because of the isolated occurrence of the rust, it is possible that the rust overwintered at this site on the winter wheat. No other rust has been observed by the field scouts at this time.
No symptoms of leaf rust, stem rust, or stripe rust were observed by Dr. Jim Miller, USDA Cereal Rust Pathologist, in his survey of winter wheat varieties at Casselton on June 12. The winter wheat varieties were in the boot to heading stage. Dr. Miller also examined winter rye for rust and found none.
Wheat stripe rust was observed in South Dakota, Nebraska, and Colorado last week on winter wheats. A few leaf rust pustules also were observed by Dr. Yue Jin at Brookings, SD, on winter wheat. Recent winds and rains may bring rust infections northward into our state. We will need to be vigilant about rust detections and severity levels, as the severity of rust diseases can escalate rapidly if environmental conditions are favorable. Unusually cool conditions in late May and the first week of June allowed stripe rust to become severe in some wheats in Kansas.
BACTERIAL BLIGHT IN SMALL GRAINS
Recent rain storms have favored some bacterial blight infections in wheat and barley. My observations of our barley fungicide plots on June 12 indicated some minor watersoaking and browning of wheat leaves, and more severe watersoaking and brown streaking on barley. Many of the top leaves of the barley crop had brown streaking and watersoaking and some tattering or shredding of the leaf tips, with some yellowing, as well. These conditions shouldn’t cause any severe problems, but sunny, drier days are needed for the infections to dry out, and for the new leaves to remain healthy.
DOWNY MILDEW OF SUNFLOWER
Downy mildew infections may appear in sunflower in the next few weeks in areas where heavy rains and saturated soils coincide with sunflower germination and emergence. The downy mildew fungus is a "water mold" and thus is favored by recent weather. Systemic infections by the downy mildew fungus result in stunted plants with yellowing along the main veins on the upper leaf surface, and a downy white growth along the main veins on the lower leaf surface. After the systemically infected plants appear, secondary spread may occur if wet weather reoccurs.
Once downy mildew is present, nothing can be done to halt development. Seed treatment with Apron used to be a very effective means of control, until the occurrence of races resistant to this class of fungicide.
The amount of downy mildew present in any field depends on what percent of the downy mildew population in that field is resistant to Apron seed treatment, how wet the soil was before emergence, and how long the soil was wet. Sunflower seed generally is still treated with Apron or similar product, but the percent of the downy mildew population that is Apron resistant varies considerably among fields, as shown by Dr. Tom Gulya, USDA sunflower pathologist. Satisfactory substitutes for Apron seed treatment for control of downy mildew are still not available, but Dr. Gulya continues to seek alternatives. He has four seed treatment fungicide trials on sunflower this year, located at Whapeton, Prosper, and two north of Moorhead. (Information provided by Dr. Art Lamey)
POTATO LATE BLIGHT HOT LINE
The Potato Late Blight Hot Line as of June 13 indicated that late blight severity values did not increase significantly over the previous reports in non-irrigated fields, but some substantial increases occurred in irrigated areas of McLeod and Oakes. Late blight has been confirmed in a field in Wisconsin. The appearance of the disease this early in the season indicates that Late Blight pressure could be high this year. With recent rains and scattered thundershowers, growers are urged to destroy all cull piles and to scout for seed-borne late blight in low lying, wetter areas. If found, infected plants and volunteers should be removed and destroyed. Growers are reminded to begin fungicide application as row closure approaches 50% and to continually check the hot line at 1-888-482-7286.
Extension Plant Pathologist
APHANOMYCES ROOT ROT OF SUGARBEET
Sugarbeet plants that are in warm, wet soils may be affected by Aphanomyces root rot. Optimum condition for infection occurs in wet soils at temperatures of 72 - 82EF. Aphanomyces is most common in southern Minnesota and the Southern areas of the Red River Valley. Aphanomyces can be devastating in the seedling stage, and can also cause serious root rot later in the season. Infected plants turn a sickly yellow green and tend to wilt in the afternoons of hot and sunny days. Assessment of fields should therefore be done on hot sunny days. Some plants may die; those that survive have their roots easily dislodged at harvesting. Plants that survive infection have reduced root yield, lower sucrose content, and higher impurities. Diseased roots have much higher respiration rates compared to healthy roots. As a result, the quality of storage piles can be reduced when diseased roots are stored with healthy roots.
Aphanomyces can be managed by using tolerant varieties; using Tachigaren pelleted seeds; planting early when possible; keeping the soil dry by cultivation and enhanced drainage; and avoid spreading of contaminated soil from infected fields to disease free fields. Fields with a history of Aphanomyces should be planted with tolerant varieties approved for the particular factory district. These seeds should be treated with Tachigaren to provide additional protection, especially since North Dakota and Minnesota are in a wet cycle, with conditions favorable for Aphanomyces occurring annually.
Dr. Mohamed F. Khan
NDSU Extension Sugarbeet Specialist
PLANT DIAGNOSTIC LAB SAMPLE SUMMARY AND HERBICIDE INJURY ASSESSMENT
Aphanomyces, as well as Rhizoctonia seedling blight, have been confirmed on two sugarbeet samples from the southern end of the RRV this week. One wheat sample from the eastern side of ND was infected with Cochliobolus sativus, the common root rot pathogen. Pythium was also detected in this sample. While not considered the primary pathogen causing common root rot, Pythium can contribute to the root rot complex, decreased plant vigor, and cause yield loss. Rotation is the best way to manage both common root rot and Pythium root rot in wheat.
Herbicide injury complaints are starting again this year. There have been several calls expressing concern about RoundUp. Regular scouting and crop monitoring are important for assessing plant problems in general but can be very important in diagnosing injury from herbicide misapplication. Test results for RoundUp residues will only be reliable and conclusive if samples are taken in within a 2-4 week window of potential exposure. As a reminder, include a "control," or unaffected plant of the same physiologic maturity for best results, for RoundUp testing or any herbicide diagnosis. Include the whole plant if possible in the sample. Pack or wrap the sample in paper and send in an appropriate envelope or box. It is very important to include any herbicides used on the crop in the current year as well as which herbicides may have been applied to what crop on that field the previous year. The other important piece of information is the pattern and distribution of symptoms in the field. Sketches or diagrams of the field, where the injury plants are and something of the topography of the field are very useful for documenting and diagnosing possible misapplication problems. Without enough information and documentation, we cannot make a justifiable diagnosis. One additional note on herbicide injury diagnosis, especially on trees - most herbicide injury symptoms are on foliage or root systems. Please do not send bare branches or stems for assessment of herbicide damage.