MSU Extension

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.    

Fusarium head blight (FHB), commonly called scab, continues to be the single most important disease of wheat and, unfortunately, the most difficult to prevent. FHB is capable of causing some loss to grain yield, but the most significant financial losses stem from a mycotoxin created by the fungus within the infected grain called deoxynivalenol (DON or vomitoxin). More information can be found at http://www.scabusa.org.

Weather has the greatest influence on disease development. Rainfall and moderately warm temperatures at the time of flowering are most conducive to the pathogen. However, the disease is also favored by above normal rainfall several days prior to flowering, as humidity and precipitation encourage spore numbers and dissemination. Likewise, wet conditions following flowering can compound the problem as it favors disease development and production of DON. A risk model based on local weather can be found at http://www.wheatscab.psu.edu/riskTool_2010.html and a sign-up to receive in-season scab alerts can be accessed at http://scabusa.org/fhb_alert.php.

Varietal resistance offers limited protection from FHB, as the highest yielding varieties currently grown in Michigan are moderately to highly susceptible. However, breeders are beginning to release less susceptible varieties that could help reduce DON levels. Michigan State University’s annual variety trial results report a FHB index for all varieties (http://www.css.msu.edu/varietytrials/wheat/).

Soft white and soft red wheat, as sub-classes, are generally comparable in their susceptibility to FHB, though white wheat is often more apt to accumulate DON. Soft white wheat is also disadvantaged by being more likely than soft red wheat to incur discounts because of their different end-uses. While market discounts vary somewhat, soft white wheat growers may experience discounts beginning at 1 ppm of DON, whereas discounts for soft red grain may not be imposed until 2 or 3 ppm.

Crop rotations matter, as residues from the previously infected crop can harbor the Fusarium that causes FHB. Residues that represent the greatest risk are those from corn, followed by wheat and barley. Hay sods can also pose a significant risk based on industry experience in Michigan. Using tillage to incorporate infected residues will reduce disease risk, but will not completely mitigate the threat these rotations pose.

Fungicide use can reduce the severity of FHB by as much as 50 percent. The same might be said for a reduction in DON, but the effect is less predictable. Growers and researchers have also found that fungicides targeting FHB may result in improved grain quality or yield even when FHB does not develop. This is attributed to the fungicides’ contribution toward the control of foliar diseases that may threaten the upper plant canopy. Successful applications of fungicides against FHB depend on the following points.

1) Recommended fungicides (Table 1). To date, the most effective products are Prosaro and Caramba. Folicur (and other brands of tebuconazole) are less effective but, because of less cost, might be considered where the risk of FHB is low and foliar diseases threaten the flag leaf or threaten to spread to the head.

2) Proper application timing. Applications should be made while the wheat heads are in the early stages of flowering (or when anthers can be seen on 25 to 50 percent of heads). This level of flowering usually occurs, depending on air temperatures, one to four days after the majority of the heads have extended beyond the collar of the flag leaf.

3) Application methods including adjustment of boom height to target the wheat heads; angling the nozzles forward to be 30 degrees from horizontal; use of a non-ionic surfactant (refer to product label); 10 to 20 gallons of water per acre; and a spray droplet size that is between the “fine” and “medium” categories (275 to 350 microns).

Table 1. Efficacy of fungicides for FHB management based on appropriate application timing (Click to view pdf)
(source: North Central Region Committee (NCERA-184)