Ustilsgo syntherismae (Schwein.) Peck, which causes
loose smut of crabgrass, infects its host systemically and
destroys seed production. Greenhouse experiments were
carried out to investigate the mode of infection, the
disease incidence that can be obtained by artificial
inoculation and the conditions necessary for optimum
infection, cross-infection on southern and smooth
crabgrass, and the reasons for the late observance of the
disease. U. syntherismse infected southern crabgrass,
Digitaria ciliaris (Retz.) Koeler, by both seed-borne and
soil-borne teliospore inoculum. Disease incidence was high
(84-91%) in some treatments. Higher rates of teliospore
application resulted in larger percentages of smutted
plants. When infesting the potting mixture, the highest
percentages of smutted plants were obtained when
infestation and planting were done at the same time.
Earlier or later planting resulted in fewer smutted plants.
When planting at various depths, greater numbers of smutted
plants were obtained when inoculated seed were planted 0.5
to 1.0 cm below the surface versus planting on the surface
or at greater depths. The appearance of 50% of infected
plants' first panicles (in greenhouse experiments) was
delayed by 4.5 weeks compared to the emergence (50%) of
first panicles from healthy plants. In cross inoculation
experiments, southern and smooth crabgrass plants developed
smut only when inoculated with spores collected from large
and smooth crabgrass~ respectively.
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