Authors
D. S.
Mueller
,
Department of Crop Sciences, University of Illinois, Urbana 61801
;
A. E.
Dorrance
,
Department of Plant Pathology
,
R. C.
Derksen
,
USDA/ARS
, and
E.
Ozkan
,
Department of Food Agriculture and Biological Engineering, Ohio State University, Wooster 44691
;
J. E.
Kurle
,
Department of Plant Pathology, University of Minnesota, St. Paul 55108
;
C. R.
Grau
,
Department of Plant Pathology
, and
J. M.
Gaska
,
Department of Agronomy, University of Wisconsin, Madison 53706
;
G. L.
Hartman
,
USDA/ARS
, and
C. A.
Bradley
and
W. L.
Pedersen
,
Department of Crop Sciences, University of Illinois, Urbana 61801
ABSTRACT
Sclerotinia stem rot of soybean, caused by Sclerotinia sclerotiorum, is a major disease in the north central region of the United States. One approach to managing Sclerotinia stem rot on soybean is the use of fungicides. S. sclerotiorum was assayed for sensitivity to benomyl, tebuconazole, thiophanate methyl, and vinclozolin in pure cultures on agar medium, inoculated soybean seedlings, detached inoculated leaves, and in experimental field plots. To evaluate the inhibitory effect of four fungicides on growth of S. sclerotiorum in vitro, potato dextrose agar (PDA) was amended with the fungicides at six concentrations. Based on measurements of fungal radial growth, vinclozolin was the most effective in inhibiting S. sclerotiorum mycelial growth at 1.0 μg a.i./ml of PDA. Ranges of reduction of radial growth of 91 isolates of S. sclerotiorum on PDA amended with thiophanate methyl and vinclozolin were 18 to 93% and 93 to 99%, respectively, when compared with the nonamended agar control. Benomyl, thiophanate methyl, and vinclozolin applied to greenhouse-grown seedlings prevented S. sclerotiorum from expressing symptoms or signs on leaf tissue. Detached leaves sprayed with thiophanate methyl and then inoculated with mycelial plugs of S. sclerotiorum did not express symptoms or signs. Of 13 different environments in Illinois, Indiana, Ohio, and Wisconsin from 1995 through 2000, six had low Sclerotinia stem rot incidence (<1%), three environments had low to moderate Sclerotinia stem rot incidence (5 to 25%), and four environments had high Sclerotinia stem rot incidence (>25%). When disease incidence was high, no consistent control of Sclerotinia stem rot was observed with benomyl or thiophanate methyl using different application systems. However, under low disease incidence, spray systems that were able to penetrate the canopy reduced the incidence of Sclerotinia stem rot an average of 50%.
