Foliar diseases significantly reduce yields of wheat in years when weather conditions favor their occurrence and spread. When diseases occur early in the season on susceptible varieties, yield losses can be as high as 25 to 30%. Powdery mildew (Table 3) and Stagonospora leaf blotch have about the same potential for yield loss, whereas leaf rust can have a greater impact on yield with similar levels of leaf damage.
| Table 3. Predicted Percentage Yield Loss to Powdery Mildew Based on the Level of Resistance in the Variety When 1% Disease* Is Detected on Different Leaves at Head Emergence. | |||
| Variety Resistance | Flag Leaf** | Leaf Two | Leaf Three |
| Susceptible | 25%*** | 16% | 6% |
| Mod. Susceptible | 16% | 10% | 4% |
| Mod. Resistant | 8% | 5% | 2% |
| Resistant | — | — | — |
|
* 1% disease = 2 to 3 powdery mildew lesions per leaf. ** Flag leaf is the top leaf; leaf two is the leaf below flag; and leaf three is below leaf two. *** Percentage yield loss based on six varieties over three years. |
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Disease management strategies have been based primarily on use of resistant varieties, proper crop rotation, and planting dates. In recent years, some producers have used higher seeding rates, earlier planting, and higher rates of nitrogen in an attempt to increase yields. Each of these factors has increased the amount of disease in wheat fields.
Research has shown that early planting (prior to the Hessian fly-safe date) and higher seeding rates have not increased yields. Spring nitrogen application, at the recommended rates, has significantly improved the yield of wheat on a statewide basis. Additionally, many of the newer, higher-yielding varieties are susceptible to one or more of the diseases present in Ohio. There are highly effective fungicides available for use on wheat, but the economics of wheat production has been the limiting factor affecting their use.
The most important factors affecting the use of fungicides on wheat are:
Foliar fungicides are not recommended for all wheat fields. Fields with yield potentials less than 60 bushels per acre should not be considered for fungicide applications unless the producer knows that yield potential has been limited to this level by disease rather than agro-nomic factors. In cases where good stands of wheat with high yield potential have received a well-timed application of fungicide, yield increases have ranged from 10 to 30%.
The level of disease in fields varies with weather conditions. Powdery mildew is important in mild seasons with high relative humidity. Stagonospora leaf blotch is common in years with two to three days of rain each week during May. Thus, the diseases that are important in fields this year may not be present next year. Monitoring fields for diseases and following weather reports in May and early June will help determine the need for fungicide applications. Specific information on important diseases and overall disease management strategies can be obtained on the Crop Observation and Recommendation Network web site at http://corn.osu.edu or on the Ohio Field Crop Disease web site at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/.