Bacterial streak and black chaff in wheat
Figure 1. Bacterial streak on triticale in a breeding nursery at the UNL Havelock Research Farm on May 18. Last week bacterial streak, also known as black chaff, was observed in winter wheat fields in south central and southeast Nebraska.
Bacterial streak and black chaff are two phases of the same disease on wheat and other small grain cereals. As the name implies, it is caused by a bacterium, Xanthomonas campestris pathovar undulosa. It causes necrotic (tissue dying and turning brown) streaks on leaves (bacterial streak). These streaks can coalesce to affect large areas (Figures 1 and 2) and are most noticeable after heading. Bacterial streak symptoms can appear suddenly on upper leaves without appearing to progress from lower leaves, and can be confused with those of Septoria tritici blotch.
Figure 2. Bacterial streak on a wheat leaf in south central Nebraska on June 11. On the head, symptoms are black, longitudinal stripes on the glumes and purplish black areas on the peduncle (part of the stem that supports the head) and rachis (main axis of the head) (Figures 3 and 4). These symptoms can be confused with those of a fungal disease known as glume blotch and a physiological condition known as pseudo-black chaff.
The bacterium is mainly seedborne. Survival on crop debris and alternate hosts is possible, but is not considered an important source of inoculum when the disease occurs at epidemic levels. The bacterium is spread by splashing or wind-driven rain and infects through stomata and wounds.
click image to zoomFigure 3. Black chaff on heads of a susceptible wheat line at the UNL Agricultural Research and Development Center near Mead on June 13. Note that the bacterium has killed all the leaves on one of the two plants in the foreground. This year bacterial streak and black chaff are more prevalent than has been observed in previous years, especially in the eastern half of the state. This may be due to a more favorable environment (prolonged rainfall) during this growing season.
Figure 4. Close-up of wheat heads of a susceptible breeding line affected by black chaff at the UNL Agricultural Research and Development Center near Mead on June 13. Bacterial streak and black chaff can be managed by avoiding use of infested seed. Seed lots can be assayed to determine how much they are infested. There is no commercial seed treatment to reduce infestation in affected seed lots. Cultivars known to be highly susceptible should be avoided. In irrigated wheat, irrigation should be managed to avoid prolonged periods of wetness.