See:
Cause A complex of closely related smuts, the most important of which is the fungus Ustilago bullata. The disease is found on a wide range of grass hosts including Agropyron (wheatgrass), Bromus (bromes, cheat grasses), Hordeum (barley and other species), Lolium (ryegrass), and Elymus (squirreltail) species. There is host specificity with different races of U. bullata that infect different grass species. Head smut appears before harvest as powdery spore masses. Often it is called "loose smut" to distinguish it from kernel smut.
Disease is promoted by higher levels of rainfall and warmer temperatures during winter months. Spore masses disperse in wind. Spores infect seed embryos at flowering, causing sterility. Spores may be on receptive floral parts or on the surfaces of nearby flowers. When infected seeds are planted, the fungus grows systemically within the host.
Symptoms Infection can lead to reduced seedling establishment as infected seedlings may die. Internodes are shortened in many grasses, producing an erect, short stem with a compact panicle (dwarfed) and there may be an increase in tillering. Leaves can exhibit light green areas of swelling. A loss in seed production and in forage production due to stunting and less biomass also can occur. At flowering, ovaries in infected plans are converted to masses of spores covered by thin membrane. When the membrane breaks, brown or black spore masses are released.
In kernel smut, spore masses form only in the ovaries, causing smut balls. This usually differentiates the disease from head smut.
Chemical control Treat infested seed used for seed production.
- 42-S Thiram (Group M3) at 8 fl oz/100 lb seed plus a dye. Thiram seed treatment has been satisfactory if infection is light. See label for reentry restrictions.
References Falloon, R.E. 1979. Further studies on the effects of infection by Ustilago bullata on vegetative growth of Bromus catharticus. New Zealand J. Ag. Research 22:621-626. https://doi.org/10.1080/00288233.1979.10417832
Iannone, L.J., Vignale, M.V., Pinget, A.D., Re, A., McCargo, P.D., and Novas, M.V. 2017. Seed-transmitted Epichloe sp. endophyte alleviates the negative effects of head smut of grasses (Ustilago bullata) on Bromus auleticus. Fungal Ecology 29:45-51.