Management of Fusarium head blight (scab)

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

This information is available as a fact sheet on the Field Crops AoE website at: http://fieldcrop.msu.edu/documents/FHB%20fact%20sheetmn.pdf

Fusarium head blight (FHB), commonly called head scab, is caused by several species of fungi in the genus Fusarium. It causes premature bleaching of spikelets and infected spikelets may be sterile. Grain from infected heads may be chalky white to pink, shriveled and light in weight. FHB is capable of causing a loss of grain yield, but the most significant financial losses result from the mycotoxin deoxynivalenol (DON or vomitoxin), which the fungus produces within infected kernels. (view images)

Managing FHB and DON can’t be achieved by any single control measure, but an integrated approach will give the best results.

Weather effects

The severity of FHB depends mostly on weather conditions. Rainfall and moderately warm temperatures at the time of flowering are most conducive to the development of the pathogen and the single best basis for disease prediction. However, rainfall prior to flowering encourages the development of the pathogen’s spores and moist conditions following infection of the flowering heads favors the production of DON. A model predicting the risk of scab can be found at: http://www.wheatscab.psu.edu/. The model uses temperature and relative humidity for the seven day period prior to flowering to predict the risk of an epidemic. Michigan-specific comments relative to wheat development, scab and foliar diseases can be found by clicking on the map icon and selecting Michigan.

Crop rotation and residue management

Residue from corn and wheat is the source of inoculum for FHB, so wheat should not follow a wheat or corn crop. Incorporating these residues into the soil may help reduce the production of inoculum. Varying wheat varieties and planting dates to spread the time of flowering may reduce risk by limiting the exposure of a grower’s entire crop to weather conditions favorable for FHB at this critical stage.

Varietal resistance

Although efforts to develop wheat varieties with resistance to FHB continue, most varieties currently grown in Michigan are moderately to highly susceptible to FHB. There are only a few winter wheat varieties with appreciable scab resistance. However, where available, planting a moderately resistant variety instead of a susceptible one can reduce FHB by 60 percent and DON by 40 percent. Michigan State University’s annual variety trial results for wheat include FHB ratings and can be found at http://www.css.msu.edu/varietytrials/wheat/.

Fungicide applications

Fungicide applications need to be made during the early flowering stage to maximize effectiveness.

A timely and thorough application of tebuconazole has been shown to reduce the severity of FHB by at least 50 percent. However, the application’s impact on DON is less predictable. Experience from commercial fields suggests that the use of tebuconazole may reduce DON by 0.5 to 1 ppm, though research has shown that fungicide’s effect on DON levels can be more significant under severe disease pressure. For 2007, three tebuconazole products are registered in Michigan under a special Section 18 emergency exemption: Folicur 3.6 F, Muscle 3.6 F and Embrace 3.6 L. All three of the labels allow a single application of tebuconazole to be made up to Feekes 10.5.1, the beginning of flowering. In addition to tebuconazole, a new fungicide, Proline (prothioconazole) has been registered in Michigan for use on wheat to control FHB and foliar diseases of wheat. It is labeled for use up to Feekes 10.5.2 (50 percent flowering).

For best results, the fungicides should be applied 1 to 3 days after 75 percent of the wheat is fully headed (the entire head has emerged beyond the flag leaf). Depending on temperature, this timing often corresponds to when 25 percent or less of the heads have begun to flower.

Application technology

When using ground equipment, the boom height should be adjusted to target the wheat heads. Research conducted under the United States Wheat and Barley Scab Initiative suggests that the fungicide should be applied with 10 to 20 gallons of water per acre and a low rate of non-ionic surfactant. Either a single tapered flat fan nozzle oriented forward at 30º from horizontal or a double orifice nozzle (twin jets or twin nozzles) in a forward/backward orientation can be used. They also found that the greatest disease suppression is attained with spray droplet sizes of 275 to 350 microns, which falls between the high end of the fine category to the low end of the medium category droplet size.

White wheat vs. red wheat

Consider local markets. Discount schedules for DON-contaminated wheat varies, not only with current market considerations, but also by wheat type. Soft white wheat is often discounted more than soft red wheat due to milling product requirements. For example, white wheat discounts for white wheat grain might begin when DON levels reach 1 ppm, whereas contaminated red wheat may not be discounted until DON reaches 3 ppm. Therefore, the risk of financial losses from DON is greater for white wheat than red wheat.

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