Mode of Infection and Factors Affecting Disease Incidence of Loose Smut of Crabgrass

Ustilago syntherismae,which causes loose smut of crabgrass, infects its host systemically and reduces seed production. In greenhouse experiments to evaluate its utility as a biological control agent, the fungus infected southern crabgrass,Digitaria ciliaris,both when applied to soil and when applied directly to seed. Disease incidence was high (up to 96%) in some treatments. When spores were added to the potting mixture, high smut incidences were obtained only when time between spore application and sowing was short (4 days or less). Under greenhouse conditions, the first panicles of infected plants emerged about 4.5 weeks later than those of healthy plants, which explains why the disease does not appear in the field until late in the season. Southern crabgrass developed smut when inoculated with spores collected from large crabgrass,D. sanguinalis,but not with spores from smooth crabgrass,D. ischaemum.Conversely, smooth crabgrass became infected only by spores from smooth crabgrass, not by spores from large crabgrass.