Figure 1 (left). Bacterial streak on wheat at the ARDC near Mead on May 27. Figure 2 (middle). Bacterial streak on triticale at Havelock Farm in Lincoln on May 27. Figure 3. Bacterial streak on oats at the ARDC near Mead on May 27.

A survey of wheat (and other small grain) research plots at UNL's Havelock Farm in Lincoln and at the UNL Agricultural Research and Development Center (ARDC) near Mead on May 27 found the predominant disease to be bacterial leaf streak, also known as black chaff when it occurs on heads. It was present on wheat (Figure 1), triticale (Figure 2), and oats (Figure 3).  Bacterial leaf streak has also been seen on oats in southwest Nebraska.

Figure 4. Loose smut on wheat at the ARDC near Mead on May 27.

 

Figure 5. Fusarium head blight (scab) at Havelock Farm in Lincoln on May 27. Only trace levels of the disease were seen.

Figure 6. A wheat plant severely stunted by wheat streak mosaic virus at the ARDC near Mead on May 27.

At Havelock Farm and the ARDC on May 27, only trace levels of fungal foliar diseases (Septoria tritici blotch, tan spot) were found. Other diseases seen at trace levels were loose smut (Figure 4), Fusarium head blight (scab, Figure 5), and wheat streak mosaic (Figure 6).

No rust diseases were seen at Havelock Farm and the ARDC on May 27. A report from Kansas on May 27 stated that a hot spot of stripe rust was found in research plots near Manhattan, but nowhere else in the state.  Today, May 28, leaf rust was confirmed in a wheat field in Nuckolls County in south central Nebraska.  It was just starting to develop.  The Nebraska Wheat Crop Report released today is reporting sightings of stripe rust and leaf rust in select areas in the northern part of southwest Nebraska.  Overall, the risk for development of damaging levels of leaf rust or stripe rust remains low. Continue scouting wheat fields for detection of rust, leaf spots, and powdery mildew. The decision to apply a fungicide should be made now since most of the wheat crop is headed or flowering.

Management

Bacterial leaf streak cannot be controlled once it occurs. It is a bacterial disease and therefore fungicides have no effect on it. Because it can be confused with Septoria tritici blotch, which can be controlled with a spray fungicide, correct identification is essential. The bacterium that causes bacterial leaf streak is mainly seedborne.  Therefore, the most effective strategy for managing the disease is to plant certified, pathogen-free seed.

Virus diseases, like bacterial diseases, cannot be controlled by applying a fungicide.  The most effective strategy for managing wheat streak mosaic virus (and Triticum mosaic virus) is to control post-harvest volunteer wheat before planting wheat in the fall.  Post-harvest volunteer wheat usually is heavily infested with wheat curl mites which transmit wheat streak mosaic virus and Triticum mosaic virus to newly emerged fall-sown winter wheat.  Volunteer wheat should be completely dead at least two weeks before planting wheat in the fall.

Loose smut can be controlled with fungicide seed treatment before planting.  Fungal leaf spot diseases and rusts are effectively controlled by foliar fungicide application which should be timed to protect the flag leaf. This year these diseases have been slow to develop. Given the current low levels of fungal leaf spot diseases and the low risk for rust diseases, with wheat already headed and flowering, it may not be necessary to apply a fungicide.

If a fungicide is applied, it is best to time it at early flowering to target both foliar fungal diseases and Fusarium head blight (FHB).The FHB Risk Assessment Tool (http://www.wheatscab.psu.edu/) is currently indicating mostly a low risk in Nebraska.  However, with the recent rainfall over the Memorial Day weekend and its timing (wheat heading or starting to flower), the risk of Fusarium head blight may be elevated in localized areas. 

Beware of pre-harvest intervals when applying any fungicide to wheat at the flowering growth stage.

Stephen Wegulo
Extension Plant Pathologist