Cause A complex of closely related smuts, the most important of which is Ustilago bullata. The disease is found on a wide range of grass hosts but is most important on bluegrass, wheat grasses, bromegrasses, wild rye, reedgrass, and canarygrass. Head smut appears before harvest as powdery spore masses. Often it is called "loose smut" to distinguish it from kernel smut. Spore masses disperse in wind. Spores infect seed embryos at flowering. 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. Internodes are shortened in many grasses, producing an erect, short stem with a compact panicle (dwarfed). A loss in seed production and in forage production due to stunting 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 (see below), 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 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.