Management of foliar wheat diseases, Part 1

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

The most important leaf diseases of wheat in Michigan include powdery mildew, Stagonospora leaf and glume blotch, Septoria leaf blotch, and leaf rust. Depending on conditions, a serious level of any one of these fungal diseases can cause a significant yield loss. Part 1 of this article on wheat foliar diseases covers powdery mildew, Septoria leaf blotch, and Stagonospora leaf and glume blotch. Part 2, which will appear in the April 26 issue of the Field Crop CAT Alert, will cover leaf rust, stripe rust and fungicides for managing diseases. A fact sheet, Management of Foliar Wheat Diseases, is available at: http://fieldcrop.msu.edu/documents/wheat%20foliar%20disease-final.pdf

Leaf diseases affect yield by reducing the amount of green leaf area available to capture sunlight and make carbohydrates used during grain filling. The flag leaf is the most important in determining grain yield and quality, but heavy infections as early as the fully-tillered stage have been shown to lead to modest yield declines.

Varietal resistance
is the first line of defense in managing foliar disease of wheat. Michigan State University’s annual variety trial results for red and white winter wheat (back to 1997) include disease resistance ratings, and can be found at the website: www.css.msu.edu/varietytrials/wheat/.

Use the tables to view field trial ratings and determine variety performance with respect to susceptibility or resistance to a particular disease.

Cultural practices
, particularly crop rotation, can serve to prevent or minimize disease development. Wheat should never follow wheat or other cereals, if at all possible. In addition, growers should avoid heavy fertilization with nitrogen as overly dense, lush stands tend to encourage leaf diseases. Planting wheat prior to the Hessian Fly Free Date may encourage the establishment of diseases such as powdery mildew and Septoria, and is an additional reason to delay planting.

Fungicide application
may be profitable when there is severe disease pressure, and a susceptible variety has been planted. Fungicide research trials have shown that under low to moderate disease pressure, many wheat varieties may not show an economic return when treated with fungicides. Foliar fungicides protect yield when disease pressure is severe. The best candidates for fungicide treatment are fields with the highest yield potential. Before using a foliar fungicide, growers should determine the yield potential of the wheat crop, and the straw, if it is to be marketed, and determine whether applying a fungicide is likely to be profitable. Management practices targeted toward specific diseases are listed under the specific disease headings.

Scouting
individual fields is critical to making sound management decisions. Inspect fields for disease before making a decision to apply fungicides. Check about 30 to 50 tillers randomly in the field to get a clear picture of the extent of disease present; avoid looking in just one or two locations.

Powdery mildew (view images)

Cause: On wheat- Erysiphe graminis f.s tritici (fungus). On barley- Erysiphe graminis f.s. hordei (fungus). These fungi are host specific.

Symptoms:
Powdery, white to light gray patches may appear on leaves and stems (especially upper leaf surfaces) any time after seedlings emerge. Black specks containing spores may form in the patches of mildew as the season progresses.
Powdery mildew overwinters as resting spores on straw, stubble, volunteer or overwintering wheat.

Conditions favoring the disease:
Cool temperatures (59 to 72ºF) and high humidity (greater than 85%) are optimal for the development of the disease. Heavy nitrogen fertilization also enhances disease development.

Management:
Select resistant varieties. Avoid heavy amounts of nitrogen, which can stimulate rapid growth. Determine the need for fungicide treatment by scouting for powdery mildew at flag leaf emergence and the boot stage. The threshold is an average two to three spots per leaf (averaged over 30 to 50 leaves) on the leaf below the flag leaf.

Septoria leaf blotch, Stagonospora leaf and glume blotch (view images)

Causes: Septoria tritici and Stagonospora nodorum. (fungi).

Symptoms:
The first symptoms are tiny yellow flecks on the lower leaves. Septoria expands to angular, tan to brown lesions containing black, pinpoint specks (pycnidia, which produce spores). It does not infect the glumes. Stagonospora lesions are lens-shaped with yellow halos and may contain brown pinpoint specks (pycnidia) within the lesions. Stagonospora affects both leaves and glumes. On wheat heads, it starts as gray-brown spots on the chaff that become dark brown blotches with grayish-white centers on the glume.

Conditions favoring the disease:
These fungi overwinter on straw, living plants or seed. Spores are present in late summer and fall, and can germinate over a wide temperature range. Spores are produced during periods of wet weather and can cause infections throughout the growing season.

Barley is generally less susceptible than wheat. Wet and windy weather favors the development of the disease. Septoria is more prevalent earlier in the season (at temperatures around 50-68°F), during the period from stem elongation to flag leaf emergence.Stagonospora tends to appear around heading (temperatures are in the 68-81°F range). Cool, wet weather during flag leaf emergence provides favorable conditions for severe outbreaks of this disease. Planting small grains as successive crops allows inoculum to build up in the field, especially under no till or minimum tillage.

Management:
Select varieties with resistance. Use certified seed. Seed treatment may help limit seedborne disease. Rotate out of small grains for two years. Avoid planting into wheat stubble. Determine the need for fungicide treatment by scouting at flag leaf emergence and the boot stage. The threshold is an average one to two lesions per leaf (averaged over 30-50 leaves) on the leaf below the flag leaf.

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