Looking Ahead - Early Scab Forecast and Fungicide Labels

Winter wheat in southern Wisconsin is starting to head (Feekes 10.1) indicating we are ~ 7 days away from anthesis or flowering (weather dependent of course). Calendar wise this puts the WI wheat crop developmentally about 3 weeks ahead of normal. Anthesis is the last critical wheat management growth stage and accurate identification of this stage is important due to fungicide label restrictions as well as timing of fungicides for management of fusarium head scab. To better understand and identify the flowering growth stage, please check here for an UWEX YouTube Video.

Most commercial fungicide products are only labeled through Feekes 10.5 (full heading) and others have restrictions based on days to harvest. A partial summary of these products as well as those labels for head scab please access them here. Not all products may be listed and not all products (especially some generic forms of tebuconazole) may be approved in WI. It is important to always check the label for specific use requirements.

Our prediction for Fusarium head scab is based on the Fusarium Head Blight Risk Assessment Tool (please see below). In terms of Fusarium head scab risk as of today (May 8), pockets of medium to high risk are showing up in the western portion of the state, as well as a small pocket in the eastern portion of Winnebago and Outagamie Counties. The current increase in risk is most likely a function of the weather events of the past several days. Interestingly, when we checked the May 7 forecast map and did a 24 hour prediction, it did pick up the areas in the western portion of the state, although today's risk is somewhat higher than the forecast was. There is a chance for rain today around the state before some drying patterns move in. The risk for FHB may remain higher over the next several days. However, since we are approximately a week from flowering in the southern portion of WI, these conditions can, and will change, so we will continue to monitory and update the risk information, and management recommendations as we near flowering. 

Wheat Scouting Update and Disease Thresholds

A check of our research studies found that the wheat crop is at the Feekes 6 and 7 growth stages (first and second nodes) across the state. This is an important time to be out in the wheat and scouting to determine if the use of a foliar fungicide will be needed as we move into flag leaf emergence (Feekes 8). The flag leaf is the most important leaf, accounting for upwards of 50% or more of the final yield and a goal with the use of a foliar fungicide is to protect this leaf. Most of the questions to date have been about powdery mildew. Susceptible varieties are showing significant disease pressure as warm March temperatures contributed to lush vegetative growth and in some cases pre-jointing lodging.

Looking ahead, there exist some thresholds as you scout at Feekes 7 and 8 to determine if a foliar fungicide may be warranted.

At Feekes 7, the thresholds are:

Powdery mildew: check the uppermost leaf - the threshold is an average of five pustules per leaf.

Wheat leaf rust: check any leaf - the threshold is an average of one pustule per leaf.

Septoria leaf blotch: check the uppermost leaf - the threshold is 25% of the leaves having expanding blotches

At Feekes 8, the thresholds are:

Powdery mildew: check from the flag-2 leaf (2nd leaf below the flag leaf) and upward - the threshold is an average of five pustules per leaf on the flag-2 leaf.

Wheat leaf rust: check from the flag-3 (3rd leaf below the flag leaf) and upward - the threshold is an average of one pustule per leaf on the flag-3 leaf.

Septoria leaf blotch: check from the flag-2 and upward - the threshold is 25% of the leaves having blotches

If you note disease in the lower canopy but not on any of the leaves discussed above, consider increasing the frequency of your scouting to determine if there is evidence of new pustules or blotches moving into the upper canopy.

For further information regarding factors to consider for determining the need for a foliar fungicide recommendation, please check here.

For information regarding the efficacy of several foliar fungicides against different wheat diseases, please check here.

Results across our different studies the past few years have indicated that the best response to a foliar fungicide application has occurred when powdery mildew was the target and the wheat variety was susceptible. When the wheat variety was resistant to powdery mildew, we have not seen a consistent response, if at all.