Year in and year out, diseases of wheat caused by the fungi Septoria tritici and S. nodorum are among the most troublesome to Kentucky wheat producers. In fact, diseases caused by Septoria spp. are economically important in nearly all wheat-producing areas of the world, destroying nearly 2% of the world's wheat annually.
S. tritici is principally a foliar pathogen, but it can infect wheat heads, while S. nodorum commonly affects both foliage and heads. Both fungi can, and frequently do, occur within the same field and on the same plants. The diseases they cause are often difficult to distinguish from one another without the aid of a good hand lens and, frequently, microscopic examination of diseased tissue. Consequently, the foliar diseases are often referred to as the septoria leaf blotch complex or septoria complex. When one or both fungi infect the heads, the disease is called glume blotch.
Yield reductions resulting from infection by the Septoria fungi can approach 10-20% where fungicides are not used for disease control. Reduced grain yield and test weights are the result of reduced tillering of plants, reduced seed set, poor grain fill and loss of shriveled grain with chaff during harvest. Losses are greatest when epidemics develop prior to heading for septoria leaf blotch complex and in the early stages of head development for glume blotch.
Other grass crops are also affected by Septoria spp. S. tritici can attack triticale and rye during disease-favorable conditions and can also be found on barley and certain oat species. S. nodorum can cause serious losses in triticale and barley and infects leaves of rye, but not the heads. The grasses Poa and Agrostis spp. are also susceptible, but to only a limited extent.

Symptoms & Signs
Foliage
Lesions of S. tritici start as small yellow flecks, usually on lower leaves that are in contact with the soil. Flecks expand into red-brown rectangular lesions 1/16 - 1/8 in. wide by 1/4 - 3/4 in. long, with regular to irregular margins. Lesions will have numerous black pycnidia dispersed throughout. Pycnidia are spore-producing structures of the fungus which, in the case of S. tritici, are visible to the naked eye. Pycnidia are most noticeable following a period of dew or rain.
S. nodorum infections start as small dark brown to black flecks which eventually expand into lens-shaped lesions, 1/8 - 1/4 in. wide by 1/4 - 3/4 in. long. Lesions are usually surrounded by a distinct yellow halo and are initially red-brown, but may develop gray-brown centers as they age. Lesions will have pycnidia, but these are difficult to see except by using a 20X lens and looking at lesions by holding diseased leaves up to the light. The difficulty in seeing pycnidia is the result of their light color and the fact that they are buried in the leaf tissue rather than being superficially imbedded, as is the case with S. tritici.
Pycnidia of S. nodorum are honey-brown and are almost perfect circles. Mature pycnidia will have a dark brown hole (ostiole) in the center and may appear dark brown around the edges. In contrast, pycnidia of S. tritici are black, appear as slightly compressed circles (but of varying shapes) and are generally about one-third smaller than pycnidia of S. nodorum. Individual lesions frequently have pycnidia of both fungi present.
Lesions of one or both Septoria fungi can rapidly expand and kill portions of leaves or entire leaves. Lesions frequently begin from the middle to the tips of leaves. This is especially true when leaves are damaged due to freezing temperatures or some other physical or chemical factor.

Head
When one or both Septoria fungi invade the heads of wheat, the disease is known as glume blotch. Infected glumes will show irregular gray-brown lesions with a purplish border. The infection starts at the tips of the glumes and works its way toward the base. Pycnidia, a diagnostic sign of the disease, will be visible in the lesions using a 10X lens. Entire heads may become dark, and kernels from diseased heads are usually shriveled and lightweight.
When attempting to diagnose glume blotch in the field, take care not to confuse this disease with genetic discoloration of heads of some wheat varieties, with superficial molds growing on over-mature wheat, or with black chaff, a bacterial disease. The latter disease is characterized as a purplish streaking of the veins at the tips of glumes which is quite distinct from the glume lesions of glume blotch.

Disease Development
As previously stated, septoria leaf blotch complex is the result of two distinct diseases caused by two different species of Septoria. The two diseases may occur individually, especially in either early or late season, but they frequently occur on the same plant at some point in the season and form a complex. Infection by S. tritici is highly temperature-dependent and requires rather cool, wet conditions in order to occur. If conditions are favorable, infection will occur at any stage of plant development. However, because of the temperature and moisture requirements, infections by S. tritici are most common in early to mid-season when temperatures are coolest. Infections usually start in the lowest portions of plants and move upward until, and if, high temperatures become limiting.
In contrast, infections by S. nodorum occur over a wide temperature range but are highly dependent on plant stage of development. Specifically, infections tend to become most evident about the time that plants are in the "boot" stage (before head emergence) and get increasingly more severe as the season progresses. Foliar infections serve as a springboard to head infection, but serious glume blotch can occur with only light to moderate foliar infection. This is due to the fact that infectious spores of Septoria are readily dispersed within the crop canopy by wind and splashing rain.
The fungi survive from year to year (overwinter) in diseased wheat straw of previous crops, volunteer wheat, diseased seed, and other susceptible grasses. In seed, the fungi can remain viable for a year or more. In infested straw, the fungi can remain viable for as long as three years. Overwintered sources of the fungi provide spores (inoculum) for infection of the next wheat crop grown. Spores that are produced from overwintered sources, as well as secondary spores produced in active lesions, are relatively resistant to adverse (dry) conditions because of a protective coating. Spores of S. tritici can remain dormant for months at temperatures of 37-50°F, and those of S. nodorum can tolerate temperatures above this range. Thus, spores can be produced, blown, or splashed to wheat and remain dormant on plant tissue until conditions become favorable for infection.
Infection requires at least 6 hrs of wetness (up to 16 hrs for S. nodorum), and secondary spores are produced in lesions within 10-20 days. Development of septoria leaf blotch complex is optimal between 59-77°, but can occur between 41-95°F. Activity of S. tritici is favored by temperatures between 59-68°F, whereas S. nodorum is most active at 68-81°. Because of these specific temperature and moisture requirements, wet, windy weather favors epidemics of leaf blotch, while dry weather tends to slow or completely halt disease development.

Control
Crop Rotation and Tillage
Because the Septoria fungi overwinter in infested wheat residue, any practice that reduces the amount of residue in a field may also help reduce the number of Septoria spores available to infect the next wheat crop. Generally, cropping sequences in which wheat or other cereals appear no sooner than once every third year will help to reduce the quantity of residue in a field that may harbor Septoria. In addition, plowing under wheat residue after wheat harvest or following harvest of double-crop soybeans will:
1.help to speed residue decomposition and
2.limit the dispersal of Septoria spores from the residue before decomposition. Farms and fields in which wheat is planted every other year, especially where wheat and other crops are planted using no-till or reduced tillage methods, are at the highest risk for infection by Septoria.

Canopy Management
Because spore production and infection by Septoria are highly dependent on moisture, planting wheat in rows, rather than broadcasting seed, and using adequate, but not excessive, levels of nitrogen may help in reducing infection by Septoria. These practices somewhat limit canopy density which, in turn, promote air circulation and light penetration into the crop canopy. As a result, leaves tend to dry out more rapidly, providing less favorable conditions for infection by Septoria.

Seed Source
With the knowledge that Septoria can be brought in with seed, it is important to plant only high quality, certified seed. As opposed to "bin run" seed and many other seed sources, certified seed will be well-cleaned and generally free of diseased and infested seed. Treating seed with fungicides containing carboxin or thiram, among others, may also help in reducing levels and activity of Septoria under field conditions.

Resistant Varieties
Wheat varieties differ widely in susceptibilty to both species of Septoria. Differences in susceptibility also exist relative to foliage and head infection by the same species of Septoria. Generally, resistance to septoria leaf blotch complex is less developed than is resistance to glume blotch. In addition, no varieties have a high level of resistance to either leaf or glume blotch. Nonetheless, where Septoria has been a problem, it is important to plant varieties with the highest available level of resistance to foliar and/or glume infection, depending on the disease history of a farm.
For a listing of disease reactions of the most common wheat varieties grown in Kentucky, consult the Kentucky Small Grains Variety Trial bulletin, published by the University of Kentucky's Department of Agronomy, and available at your local county Extension office.

Foliar Fungicides
When varieties susceptible or moderately susceptible to septoria leaf blotch complex and/or glume blotch are grown, foliar fungicides, when properly applied, can slow or even halt disease development. Fungicides are most effective when applied in the early stages of an epidemic. Waiting too late to apply fungicides may negate their value altogether. On the other hand, applying fungicides too early in disease development may also be of limited or no value. Timing of fungicide application, thus, plays an important role in determining the level of disease control.
Other factors to consider when using foliar fungicide are the use of appropriate rate of fungicide, using spray additives when necessary, and achieving good coverage of plants with fungicide.
Protective fungicides, such as the mancozeb products, must be applied before infection occurs. These products will have no effect on infections that have already occurred. Systemic products, such as propiconazole (i.e., Tilt®) are able to eradicate infections to a certain degree. They also reduce the secondary development of Septoria after infections are well-established. Both mancozeb and propiconazole will give good results against Septoria when used properly and according to label directions.
Fungicide use decisions are complex and involve many factors such as crop yield potential, variety, crop stage of development, disease development, current and forecasted weather, and production practices, among many others. For help in making fungicide use decisions, consult PPA-36 Fungicide Use in High Yielding Wheat available at your county Extension office.
Depending on the season, more than one fungicide application may be necessary in order to achieve control of septoria leaf blotch complex and glume blotch. However, when producers are unwilling to make more than a single application in a season, it is usually best to make that application in the early heading stages rather than at flag leaf expansion.
While this approach allows early disease to develop, late-season disease will be controlled. This almost always results in higher crop yields than in the converse situation where early disease is controlled, but late-season Septoria is allowed to develop unchecked. The advantage, however, may not be as great as where two fungicide applications were made. The risk to this approach is in waiting too long to make the application and allowing too much damage to occur before acting.
The main goal in using fungicides is to keep the top two leaves of plants (and heads) relatively free of disease. Once the upper leaves or heads become heavily infected and are showing symptoms, yields will have already been seriously affected and fungicides will be of no value. Thus, when scouting wheat, a decision to spray is usually based on disease development on the second (F-I) and third (F-2) leaves down on the plant. If the F-I leaf is showing 0-5% Septoria and/or the F-2 leaf 10-20%, then fungicide use should be considered as a means of protecting the upper leaves and heads.
For glume blotch control, best results are achieved when sprays are delayed until partial head emergence through the stage at which the heads are completely emerged. However, it would be unwise to delay making a fungicide application until this time if the upper two leaves are being threatened before the heads completely emerge. This is a judgment call which requires some experience and regular field scouting.
It must be stated that, even where fungicides are used properly to control Septoria and other foliar diseases, serious yield loss can still occur due to the development of other diseases for which fungicides offer no control, or because of the development of other yield-limiting situations. Thus, fungicide use is not a panacea, but it must be considered as a major part of wheat production if the highest economic yields are to be reached.

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