By Laura Sweets, University of Missouri
The 2010 growing season is already presenting challenges for Missouri producers. A wet fall and delayed harvest in many parts of the state lead to a significant decrease in the number of acres of winter wheat planted in Missouri. For wheat that was planted, planting dates ranged from near normal to much later than normal depending on weather conditions last fall.
As a result there is a wide range in stage of growth of the wheat crop now. Some fields are finally beginning to green up and take off. Some fields in southeast Missouri are already in the boot stage. So far reports of foliage diseases on wheat in Missouri have been minimal. However, as the wheat begins to move towards boot and flowering stages of growth, it is important to scout fields for foliage diseases.
There are definitely foliage diseases that can cause losses on winter wheat in Missouri. Leaf rust, stripe rust and Septoria leaf blight are the three most likely to cause losses on soft red winter wheat grown in Missouri. Powdery mildew can be a problem on hard red winter wheat and, under the correct environmental conditions, may also cause losses on soft red winter wheat. The incidence and severity of these foliage diseases will depend on the weather conditions during the growing season, the susceptibility of the variety to each of these diseases and the amount of inoculum in the field or area.
There have been reports of leaf rust developing on wheat in southern states recently. However, there have not yet been any reports of leaf rust or stripe rust on winter wheat in Missouri. 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 lead 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.
Lesions of Septoria leaf blotch 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.
The initial symptoms of tan spot 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 which causes tan spot, Pyrenophora tritici-repentis, 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. Although tan spot can occur in Missouri, it is not usually a problem in the state.
Leaf rust lesions appear primarily on the upper leaf surfaces and leaf sheaths. Initially, lesions are small, yellow to lightgreen flecks. Eventually, leaf rust appears as small, circular to oval shaped, orange-red 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. 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 yellowishred 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.
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.
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 this 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 that are 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, 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 tables for additional information on wheat fungicides).