Figure 1. Flowering wheat heads.	Figure 2. Bleaching of wheat heads by the scab fungus usually begins in the middle of the head, but can begin anywhere on the head.
	Figure 3. (left)Bleaching of wheat heads by the scab fungus can progress until most of the head or the entire head is bleached.

May 27, 2009

Scout Your Fields Now

Although the risk of Fusarium head blight (scab) of wheat in Nebraska has been low due to an extended period of dry weather during the last few weeks (see story in May 22 Crop Watch), wheat that is flowering (Figure 1) or is yet to flower is at risk of scab infection if heavy rain occurs. Wheat that is long past flowering is at less risk.

Figure 4. Scab is manifested as bleached or whitened heads which appear suddenly and are randomly scattered throughout the entire field or large areas of the field.

Figure 5. As of May 26, the national Fusarium head blight risk assessment tool showed a moderate risk of scab in some areas in Nebraska.

Symptoms

Fusarium head blight is recognized by the partial bleaching or whitening of premature wheat heads. Bleaching usually starts in the middle of the head (Figure 2), but can start anywhere on the head. The bleaching can progress until most or all of the head is whitened (Figure 3).

The white heads usually appear suddenly and are distributed randomly throughout large areas of the field or the entire field (Figure 4).

Potential Losses and Thresholds

Economic losses result from reduced grain yield and discounting of grain at the elevator due to the presence of deoxynivalenol (DON, vomitoxin), a toxin produced by the scab fungus.

Producers in scab-prone areas (south central and eastern Nebraska) should consider applying a fungicide to suppress scab if heavy rain falls when wheat is in the full heading to about 50% flowering growth stages. As of May 26, the national Fusarium Head Blight Risk Assessment Tool showed a moderate risk in areas of north central and western Nebraska (Figure 5).

Currently, most wheat fields in scab-prone areas are heading, fully headed, or flowering. If heavy rain is forecast and the probability of rainfall is high, apply fungicide now to suppress scab.

Treatments

Two fungicides effective against scab are Prosaro (Bayer CropScience) and Caramba (BASF Ag Products). Be aware of pre-harvest intervals when applying a fungicide (see story in May 22 Crop Watch). Prosaro can be applied up to mid flowering. This is when 75%-100% of wheat heads on the main stem are fully emerged and when 50% of the heads on the main stem are in flower. Caramba can be applied at early flowering. The pre-harvest interval for Caramba is 30 days.

Several cultural practices also can help manage wheat scab:

 

• Avoid planting wheat after corn or wheat.
• Avoid cultivars that are known to be highly susceptible to wheat scab.
• Plant several cultivars that flower at different dates to increase the probability that some will escape scab infections.

Stephen Wegulo
Extension Plant Pathologist