By: GILBERTO OLAYA, Graduate Research Assistant,
GEORGE S. ABAWI, Professor, Department of Plant Pathology,
and JOHN BARNARD,
Computer Services, Cornell University, NYSAES, Geneva, NY 14456
The response of 107 isolates of Rhizoctonia solani AG 2-1, 2-2. 4, and 5, and
binucleate Rhizoctonia collected from roots, petioles, and leaf tissues of table
beets, as well as directly from hymenia of Thanatephorus cucumeris. were tested
in vitro for sensitivity against five fungicides (benomyl, iprodione,
pencycuron, tolclofos-methyl, and fludioxonil [CGA-173506]) each at four
different concentrations. The growth of the isolates was inhibited by
fludioxonil and tolclofosmethyl at I /19 a.i./ml and iprodione at 10 pg a.i./ml,
but the isolates varied considerably in sensitivity to pencycuron and
benomyl even at 100 /ig a.i./ ml. Benomyl. iprodione, tolclofosmethyl, and
fludioxonil were evaluated for control of disease under field conditions. Table
beets were inoculated with soil infested with three highly virulent isolates of
R. solani. In 1991, one spray of each of these fungicides at a rate of 2.2 kg
formulated product per hectare was applied before or after inoculation with R.
solani. The experiment was repeated in 1992,except that a second spray was
applied 2 wk after the first spray. In both years, all fungicide applications
significantly (P < 0.01) reduced the number of infected roots. In 1991,
fludioxonil applied before the inoculation of R. solani reduced the incidence of
infected roots from 21.8 to 3.8% and was the most effective treatment. In 1992,
one spray before inoculation or two sprays (one before inoculation and the
second 2 wk later) of fludioxonil reduced the incidence of infected roots from
14.6 t0 1.2 and 0.7u/o, respectively. Fungicide applications made before
inoculation with R. solani were more effective than those made after inoculation
(l day later).In the field, fungicides should be applied before the first cultivation, at which time a considerable amount of R. solaһi infested soil is thrown on the beet plants
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