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Issue No. 7, Article 6/May 6, 2005

Understanding and Managing Fusarium Head Blight (Scab) of Wheat

Wheat made it through the winter in good to fair condition in most areas of Illinois. Several diseases can adversely affect yield and quality of wheat, but Fusarium head blight (also called scab) can be one of the most significant. Although scab does not become a problem until the wheat begins flowering, it is important to understand this disease and the options for disease management.

Scab can be a damaging wheat disease anywhere in Illinois. Where it occurs depends on weather conditions in the week before and when the wheat is flowering. Scab was a major problem in the southern half of Illinois in 2003 and in northern Illinois in 2004. This disease can reduce yields, market grade, and quality. Frequently, scab in Illinois reduces grain value because of the production of the mycotoxin deoxnavalinol (also called DON, or vomitoxin) in the grain.

Scab is caused by the fungal pathogen Fusarium graminearum. Another form of this pathogen causes Gibberella stalk and ear rot of corn. This fungus overwinters on wheat, corn, and grass residues. Thus, it is favored by large amounts of corn or wheat residue on the soil surface. Fungal spores produced on these residues are spread by wind and rain to wheat heads, where infection occurs only during flowering in wet weather. Symptoms can develop within 3 to 4 days after infection, when temperatures are favorable for infection (70° to 85°F).

It is important to remember that this disease has a narrow, exclusive time frame for infection-only at flowering, and only when conditions are wet, humid, and warm at that time. The symptom of scab that is easiest to recognize is premature bleaching of several or all spikelets on a head of wheat. Orange masses of spores or small black specks may also develop at the base of infected spikelets. The fungus can produce high levels of DON in heads that do not appear to be heavily infected.

The following steps can be taken to reduce the threat of scab development:

  1. Avoid planting wheat after corn.
  2. Use scab resistance as one factor in variety selection.
  3. Plant varieties with different maturities.
  4. Consider application of a foliar fungicide when wheat begins flowering. Although fungicide application is the only management option available this year, it may be helpful to compare scab resistance in different varieties and to consider rotational plans for fields prior to planting wheat again next fall.

A temporary Section 18 label has been approved in 2005 for use of the foliar fungicide Folicur to manage scab on wheat in Illinois. Folicur 3.6F (tebuconazole) fungicide, manufactured by Bayer Crop Science, received a Section 18 temporary use exception for the first time in Illinois in 2004 for restricted use to manage scab. The Section 18 label requires that specific guidelines be followed for use of this product.

Results from many trials indicate that Folicur frequently suppresses but does not eliminate scab and the accumulation of the mycotoxon deoxynivalenol (DON) in wheat. Data from multiple years and several states suggest that on average Folicur may suppress scab severity and DON up to about 30%. Greater and lesser effects on disease severity and DON concentrations have been reported. A key point is that Folicur can be expected to suppress scab and DON but not eliminate them.

Application timing is critical for effective use of Folicur to manage scab. Wheat should be closely monitored on a daily basis for flowering 1 to 3 days after heads begin to emerge to determine when to apply Folicur. The flowers (anthers) are about 1/8 inch long and pale yellow green. If weather conditions favor disease development, Folicur should be applied at early flowering when about 25% of the primary heads have started to flower. Applications will be much less effective if they made after 50% to 70% of the heads are flowering.

Symptoms of premature bleaching on wheat heads infected with Fusarium head blight (scab).

Folicur is authorized to be used only when weather conditions favor scab, and it should be used in combination with other management tactics. Only one application may be made using ground or aerial equipment per year. Folicur may be applied up to the beginning of flowering only (Feeke's growth stage 10.51). Application may not be made within 30 days of harvest. The label and more specific guidelines for the application and use of Folicur are available through Bayer Crop Science representatives.

A forecasting system has been developed for predicting the likelihood of scab developing in wheat. This system may be valuable in helping to determine if and when Folicur should be applied for scab management. The forecast is based on rainfall, temperature, and relative humidity obtained from NOAA weather data that are collected on a 20-kilometer grid. However, it is important to understand that this forecasting system is experimental and is still being tested in Illinois. This system, developed by plant pathologists from several universities, was first available for Illinois in 2004. The forecasting system had mixed results last year, and data from Illinois and other states from 2004 were used to improve the forecasting system for this year. For more information and to see the forecast for your area, go to the Web site http://www.wheatscab.psu.edu/, especially when the crop is approaching flowering.

Wheat scab is a challenging disease to manage. Folicur and the forecasting system provide promising tools to assist producers in managing scab. Using all available disease management tactics will improve management of scab, thus reducing DON contamination of grain and increasing yields to the benefit of producers and the milling industry.--Dean Malvick

Dean Malvick

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