MODE OF INFECTION AND FACTORS AFFECTING DISEASE INCIDENCE OF LOOSE SMUT OF CRABGRASS

Ustilago syntherismae, which causes loose smut of crabgrass, infects i ts host systemically and reduces seed production. In greenhouse experi ments to evaluate its utility as a biological control agent, the fungu s infected southern crabgrass, Digitaria ciliaris, both when applied t o soil and when applied directly to seed. Disease incidence was high ( up to 96%) in some treatments. When spores were added to the potting m ixture, high smut incidences were obtained only when time between spor e application and sowing was short (4 days or less). Under greenhouse conditions, the first panicles of infected plants emerged about 4.5 we eks later than those of healthy plants, which explains why the disease does not appear in the field until late in the season. Southern crabg rass developed smut when inoculated with spores collected from large c rabgrass, D. sanguinalis, but not with spores from smooth crabgrass, D . ischaemum. Conversely, smooth crabgrass became infected only by spor es from smooth crabgrass, not by spores from large crabgrass. (C) 1997 Academic Press.