Search IPCM Newsletter Archives

Certainly the big question with this year's winter wheat crop still is the extent of damage from the unusually low temperatures the first week in April. In many fields it is becoming much more evident which plants have been killed and which may survive. However, there are still some fields in which it is diffi cult to know how much of the stand has been affected or what the yield potential will be. There have also been a few questions on which foliage diseases are showing up and if any will be more severe because of the cold temperatures. There have been a few reports of leaf rust and stripe rust on wheat in southern states over the last few weeks. However, there have not yet been any reports of leaf rust or stripe rust on winter wheat in Missouri. Wheat fields in various areas of the state are showing scattered Septoria leaf blotch lesions in the lower canopy but, thus far, the incidence of Septoria leaf blotch is quite low. Powdery mildew has been reported in fields in the southern third to half of the state. Powdery mildew may be more prevalent in lodged wheat. It does not require actual moisture on the leaf tissue to develop but does prefer high relative humidity. The appearance of powdery mildew in fields may be related to lodging due to low temperature damage to stems. The development of foliage diseases on wheat and their severity this season will depend to a large degree on the weather conditions the rest of the season. Most wheat foliage diseases are favored by warm, wet conditions. Frequent light rains, heavy dews, high relative humidity and warm temperatures would be ideal for the buildup of the foliage diseases. The buildup of foliage diseases prior to flowering can led to yield losses, especially if weather conditions remain favorable for disease development during and after flowering. It is important to scout wheat fields for foliage diseases, especially if there are scattered periods of precipitation as the temperatures warm up. There are a number of foliar fungicides labeled for use on winter wheat. This year in particular, it will be important to evaluate fields for stand and yield potential as well as for incidence and severity of foliage diseases before making a decision on foliar fungicide application.

Septoria leaf blotch
Lesions begin as light yellow flecks or streaks. These flecks expand into yellow to reddish-brown, irregularly shaped blotches. Dark brown specks (fruiting bodies or pycnida of the causal fungus, Septoria tritici) may be scattered within the centers of mature lesions. Lesions may coalesce killing larger areas of leaf tissue.

Stagonospora glume blotch
Formerly called Septoria glume blotch; may also begin as light yellow flecks or streaks on leaves. The lesions also turn yellow to reddish-brown but usually have a more oval to lens-shaped appearance than those of Septoria leaf blotch. Again, the dark brown specks or fungal fruiting bodies of the causal fungus Stagonospora nodorum may be evident within the lesions. Symptoms of Stagonospora glume blotch are more common on heads than foliage of wheat. Infected heads will have dark blotches on the glumes.

Tan Spot
The initial symptoms are small tan to brown flecks on the leaves. These expand into tan to light brown elliptical lesions, which often have yellow borders. The centers of mature tan spot lesions may have a dark brown region caused by outgrowth of the fungus. But the fungus that causes tan spot, Pyrenophora triticirepentis, does not produce pycnidia or fruiting bodies as the Septoria fungus does. So mature tan spot lesions do not have the distinct dark brown specks scattered throughout the centers of the lesions as do Septoria leaf blotch lesions.

Leaf Rust
Lesions appear primarily on the upper leaf surfaces and leaf sheaths. Initially, lesions are small yellow to light-green flecks. Eventually, leaf rust appears as small, circular to oval shaped, orangered pustules. These pustules break open to release masses of orange-red spores of Puccinia recondita. The edges of the open pustules tend to be smooth without the tattered appearance of stem rust pustules. Heavily rusted leaves may yellow and die prematurely.

Stem Rust
Caused by the fungus Puccinia graminis f. sp. tritici, is most common on stems and leaf sheaths of wheat plants, but may develop on any of the above ground portions of the plant, including both upper and lower leaf surfaces and glumes and awns. Stem rust pustules are small, oval and reddish-brown. The ruptured pustules tend to have more ragged edges than leaf rust pustules. Frequently, both leaf rust and stem rust occur on the same plant and both types of pustules may develop on an individual leaf.

Stripe Rust
Caused by the fungus Puccinia striiformis, has become more prevalent in Missouri over the last few years. Stripe rust may develop earlier in the season than leaf rust or stem rust. The pustules of stripe rust are yellow or yellowish-red and occur in obvious stripes or streaks running lengthwise on the wheat leaves. This disease is more commonly associated with cooler temperatures, especially cooler night temperatures.

Powdery mildew
Infections begin as light-green to yellow flecks on the leaf surface. As powdery mildew develops, the leaf surfaces become covered with patches of cottony white mold growth of Erysiphe graminis f. sp. tritici, the causal fungus. These patches eventually turn a grayish white to grayish brown in color and small black fungal fruiting bodies may be visible within the patches of mildew growth.

The fungi which cause most of these wheat foliage diseases survive in infested wheat residues left on the soil surface. The next growing season spores are produced during moist periods and are carried by wind currents to susceptible wheat leaves where infection may begin. Disease problems tend to be more severe when wheat is planted in fields with infested wheat residue left on the soil surface. Eventually spores that are produced in the initial lesions on plants are wind blown to other leaves or other plants causing secondary infection.

Leaf rust, stem rust and stripe rust are exceptions to this simplified explanation of disease development. The rust fungi do not survive in infested residue left in a field. Rather, the rust fungi are reintroduced into the area each season when spores are carried up on air currents from the southern United States.

Most of the foliage diseases of wheat are favored by warm, wet or humid weather. Frequently, infection begins on the lower portion of the plant. If weather conditions are favorable for disease development, the disease may move up through the plant. Severely infected leaves may yellow and die prematurely. Yield losses tend to be highest when the flag leaves are heavily infected.

There are several fungicides labeled for use on wheat to control fungal foliage diseases. It is important to scout wheat fields and determine which leaf diseases are occurring as well as the level of their severity before making a decision to apply a foliar fungicide. In particular, be on the lookout for Septoria leaf blotch, Stagonospora glume blotch, tan spot, leaf rust and stripe rust. When scouting fields, try to identify the disease or diseases which are present, determine the average percent of infection on a leaf and the number of leaves showing infection and determine the stage of growth of the crop. Generally, the profitable use of foliar fungicides on wheat depends on a number of factors including varietal resistance, disease severity, effectiveness of the specific fungicides and timing of fungicide application. The greatest increases in yield are usually obtained when fungicides are applied to disease susceptible varieties with high yield potential at the early boot to head emergence growth stage when the flag leaf is in danger of severe infection. Fungicide applications are seldom beneficial if applied after flowering or after the flag leaf is already severely infected. It is also important to read the fungicide label for specific information on rates, recommended timing of application, frequency of applications, preharvest intervals and grazing restrictions.

A management program for foliage diseases of wheat should include the following steps:

• Plant disease free seed of varieties with resistance to diseases likely to occur in your area.
• Rotate with non-host crops for one or more years.
• Manage residues- if tillage system is a conservation tillage system, particular care should be given to rotation and variety selection.
• Maintain good plant vigor with adequate fertility.
• Control volunteer wheat.
• Use foliar fungicides if warranted ( see accompanying table of foliar fungicides labeled for use on winter wheat).

Laura Sweets
Plant Pathologist
SweetsL@missouri.edu