Wheat Scouting Update and Disease Thresholds

Shawn Conley

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.