University of Illinois

No. 5/April 24, 1998

Wheat Diseases Expected Soon

Several wheat diseases are likely to develop in Illinois in the next month. These include Septoria leaf blotch (S. tritici), Septoria glume blotch (S. nordorum), powdery mildew, and leaf rust. All these diseases can be managed with fungicides, as indicated in Table 12.

Table 12. Fungicides registered for control of leaf diseases of wheat.

Disease controla
Leafrust Remarksb
Mancozeb Limit of 3 applications/season. 26-days-to-harvest restriction.
Dithane M-45 80WP 2.0 lb G G
Dithane F-45 1.6 qt G G
Manzate 200 80WP2.0 lb G G
Manzate 200 75DF2.0 lb G G
Mancozeb 4L 1.7 qt G G
Manzate 200 75DF 2.0 lb G G
Penncozeb 80WP 2.0 lb G G
Bayleton 50DF or WP2 to 6 oz
4 to 8 oz
EG G Limit of 16 oz material/season. 21-days-to-harvest restriction.
Tilt 3.6EC 4.0 fl ozE E E Limit of 1 application per season. Do not apply after flag leaf emergence (Feekes 8).
Dithane M-45 80WP + Bayleton 50DF or WP1-1/2 to 2 lb + 2 to 8 ozc E G G Same as listed above for Mancozeb and Bayleton.
Dithane F-45 + Bayleton 50DF orWP 1-1/2 to 2 lb + 2 to 8 ozc E G G Same as listed above for Mancozeb and Bayleton.
Manzate 200 80WP 75DF+ Bayleton 50DP or WP1-1/2 to 2 lb + 2 to 8 ozc E G G Same as listed abovefor Mancozeb and Bayleton.
Benlate 50WP or DF + Manzate 200 80WP or DF 1/4 to 1/2 lb + 1 to 2 lb G G G Same as listed above for Mancozeb.

aE = excellent, F = fair, G = good, P = poor.
bDue to possible changes in federal labels, be sure to consult the current label and follow restrictions listed on it.
cHigher rates are more effective.

Be sure that you are aware of the differences between a protectant fungicide (must be applied before infection takes place) and an eradicant/systemic fungicide (may be applied after infection) in terms of disease control. Scout fields regularly and select a material that will control the diseases that are developing on the uppermost leaves. Not all materials control all diseases, so it is important to identify the diseases accurately.

Fungicides also have different recommended or mandatory times of application, so be aware of when specific products need to be applied, and schedule applications accordingly. For instance, the fungicide propicanazol, sold as Tilt, must be applied no later than growth stage 8 (flag leaf emergence). The development of Septoria leaf blight and glume blotch, powdery mildew, and leaf rust are all favored by high humidity and temperatures between 60 and 75 degrees F. Septoria glume blotch can continue to develop at temperatures above 75 degrees F, but Septoria leaf blight stops developing at higher temperatures. The Septoria diseases are spread by splashing and wind-blown rain. Some fields in southern Illinois are already showing symptoms of Septoria infection. Inoculum of leaf rust and powdery mildew overwinter on wheat growing in Central Ameria and the southern United States, and the spores are spread by air currents.

All three diseases start as chlorotic flecks on the leaves. The symptoms of septoria and powdery mildew appear first on the lower leaves, while rust usually starts on the upper leaves. With Septoria, the chlorotic flecks expand into small irregular, rectangular, or lens-shaped lesions. Initially, the lesions have a water-soaked appearance, but they soon become dry and yellow, and eventually turn reddish brown. Lesions at the base of a blade may kill the entire leaf. The centers of the lesions turn grey with age; and, under favorable conditions, small black structures (pycnidia) develop.

The first diagnostic symptoms of powdery mildew are patches of white-to-grey cottony growth on the leaf surfaces. The corresponding tissue on the opposite side of the leaf becomes chlorotic. With time, small borown-to-black spherical structures (cleistothecia) develop in the cottony growth.

With leaf rust, the chlorotic flecks turn into small, raised pustules or blisters. These pustules eventually break open to expose masses of reddish orange spores. As the season progresses, the reddish orange spores are replaced by black spores. It is possible for more than one disease to be present on a single plant, so initial symptoms can be confusing.

Septoria and powdery mildew are favored by dense growth and high nitrogen levels, so wide row spacing and a moderate and balanced fertilizer program help reduce disease severity. With all three of these diseases, it is important to remember that the most significant yield losses result from infections on the flag leaf and the leaf just below (flag minus one). Diseases may be present on the lower leaves without affecting yield. Therefore, it is important to monitor weather conditions as the plants approach flag-leaf emergence (growth stage 8). If conditions remain favorable (that is, wet, with temperatures between 60 and 75 degrees F) and diseases appear to be spreading onto the upper leaves, then a fungicide application may be warranted. If, however, conditions become warm and dry, then a fungicide is not needed, even with disease symptoms on the lower leaves.

H. Walker Kirby (kirbyw@mail.aces., Extension Plant Pathology, (217)333-8414