By Stephen Wegulo and Randy Pryor
During wheat disease surveys in Saline, Jefferson and Perkins counties in late May, stripe rust was confirmed for the first time in Nebraska this growing season. The disease was found in a grower's field in Perkins County on May 30. Incidence was low, and the severity was trace.
In the same field, septoria tritici blotch was present at a high incidence and average-to-high severity on the lower leaves. In a different field in Perkins County, tan spot was present at trace-to-low levels on the lower leaves.
All fields surveyed looked lush green on the surface of the canopy. Growth stage ranged from boot to starting to head in Perkins County.
In Saline and Jefferson counties, most fields were headed or flowering and had trace-to-low levels of fungal leaf spots in the lower canopy, except for one field in Saline County that had powdery mildew in the lower and midcanopy — in addition to fungal leaf spots.
Favorable conditions for disease
Conditions are conducive for the development of fungal leaf spots and stripe rust where it is present. All fields surveyed had soils saturated with moisture, and areas within fields with standing water were common.
These conditions result in prolonged leaf wetness at night and continuous high humidity in the crop canopy. Coupled with cool night temperatures, these conditions are conducive for rapid development of stripe rust.
In addition, recent heavy and frequent rains that occurred before flowering and continued into the flowering period have increased the risk of fusarium head blight (scab), which is favored by the high humidity present in wheat fields.
Because many fields are headed or flowering, the best strategy is to apply a fungicide that will suppress scab and at the same time control stripe rust, leaf rust and the fungal leaf spots, such as septoria and tan spot.
The recommended fungicides for suppression of scab and control of foliar fungal diseases are Prosaro, Caramba and Miravis Ace. Each of these fungicides contains a triazole as an active ingredient. Triazoles have good efficacy on scab and good-to-excellent efficacy on powdery mildew, rusts and fungal leaf spots. They also have residual effectiveness lasting 21 to 28 days and have both curative and preventive activity.
Fungicides containing a strobilurin as an active ingredient have good-to-excellent efficacy on powdery mildew, rusts and fungal leaf spots. They also have residual effectiveness lasting 21 to 28 days.
Their activity is mostly preventive, and maximum effectiveness is achieved when they are applied before disease starts to develop. They delay leaf senescence, which lengthens the period of grain fill, resulting in higher yields.
However, strobilurin fungicides are not recommended for scab suppression because they have been shown to elevate levels of vomitoxin (deoxynivalenol or DON) in grain and have been shown to be less effective than triazoles in suppressing scab.
Wegulo is a Nebraska Extension plant pathologist, and Pryor is a Nebraska Extension educator.