Downy mildew, caused by Peronospora manshurica, is responsible for yellow, angular spots on emerging or young leaves (Fig. 1). If environmental conditions promote disease development, spots (lesions) enlarge. Plants can be defoliated if the disease becomes severe enough. If humidity is high and temperatures are relatively cool (upper 60s to upper 70s), spore-bearing structures are produced on the underside of leaves. These newly-produced spores are spread by wind and rain to other plant leaves.

Disease resistance increases as leaf tissues age. The disease is rarely a production issue in Minnesota. Fungicide application is not recommended.

Soybean Rust
States in the southern U.S. are reporting increased detections of soybean rust. Alabama, Florida, Georgia, and Mississippi have confirmed the disease on kudzu (a susceptible weedy species) and/or on soybean. To date, the disease has been identified in two commercial soybean fields – one each in Alabama and Florida. Other detections include soybean sentinel plots, research plots, and kudzu patches. Disease development at some sites has been progressing rapidly, resulting in plant defoliation. Researchers and others are scouting on a regular basis to keep local producers, as well as other soybean-producing states, informed about the risk of soybean rust spread. Table 1 lists the dates and U.S. locations of positive soybean rust detections during 2005 and a U.S. map can be found at:

http://www.sbrusa.net/  

Soybean rust has not been detected in Minnesota, and fungicide application is not recommended.

SUNFLOWER
Downy mildew of sunflower
Downy mildew, caused by Plasmopara halstedii, is common in sunflower production fields across the Red River Valley. In the August 4th (2005) issue of NDSU’s Crop & Pest Report, Dr. Marcia McMullen reported that downy mildew incidence (an estimate of the average number of diseased plants per field) ranged from 0% to 17.5% across 24 North Dakota production fields that were surveyed between July 25 and 29. The complete report can be found at:

http://www.ag.ndsu.nodak.edu/aginfo/entomology/ndsucpr/  

Substantial yield loss from the disease can occur if fields have large inoculum loads and cool, wet soils when plants are young. Systematically diseased plants appear stunted with leaves exhibiting a whitish, lacy appearance (Fig. 2).

If the undersides of leaves are examined, fungal spore producing structures become obvious (Fig. 3A).  Spores produced on leaves are spread to healthy leaf tissues by wind and rain. These infections result in localized leaf lesions (Fig. 3B). 

Seed treatments may help to reduce disease losses caused by systemic infection of downy mildew. However, control efficacy varies between seed treatments. Rotating out of sunflower for a minimum of five years, and controlling volunteer and weedy sunflower plants, will reduce in-field inoculum levels. 

Sclerotinia head rot of sunflower
Current hot, dry weather conditions across the Valley support neither infection nor disease development of Sclerotinia head rot of sunflower. While rain is in the forecast, fields of sunflower are either approaching, or at, the susceptible growth stage (flowering) now. 

SMALL GRAINS
Fusarium head blight (FHB, scab) of wheat and barley
Yield and quality losses resulting from scab are not yet known, but are expected to be substantial. Wheat harvest is currently underway at many locations in the Valley. 

Bacterial stripe of wheat
This disease was widespread during the heading growth stages and substantially reduced the functional flag leaf surface area of some varieties at a critical time. Yield losses will likely result where symptoms were widespread and severe. 

Ergot of wheat and barley
Ergot, caused by Claviceps purpurea, can be found in many small grains fields this year. The disease, first described more than 200 years ago, is present at some level every growing season. However, it creates a production issue only periodically. Varietal resistance and efficient seed cleaning equipment have helped in managing this disease. 

Like scab, ergot is initially noted during the grain fill growth stages. Large fungal bodies, or sclerotia, that are black to purplish in color protrude from otherwise healthy-appearing spikelets (Fig. 4A). These sclerotia are harvested with grain, but are often separated during the cleaning process (Fig. 4B). While wind, rain, and insects spread the disease, infection is dependent on cool, wet weather when plants are flowering. Disease management depends on: 

· burying sclerotia by cultivation;

· mowing field edges before wild grasses produce seed heads;

· and rotating out of small grains for at least one year.

Charla Hollingsworth
Extension plant pathologist