First Report of Web Blight Caused by Thanatephorus cucumeris on Sophora tomenlosa in the United States.. R. T. McMillan, Department of Plant Pathology, University of Florida, Tropical Research and Education Center, Homestead, FL 33031. Jr., H. Vande Hei, and W. R. Graves. Department of Plant Pathology, University of Florida, Tropical Research and Education Center, Homestead, FL 33031. Plant Dis. 78:03171. Accepted for publication 20 December 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0317E.
Sophora tomenlosa L., commonly known as necklace pod or silverbush, is a popular subtropical landscape tree grown only in Dade County, Florida. Water-soaked leaf spots that increased rapidly in size and became light tan to brown necrolic areas were observed on necklace pod in commercial nurseries during periods of cloudy, rainy weather from July through September of 1991 and 1992. Severe infections resulted in defoliation and in some cases in death of plants. A Rhizoctonia-like fungus was consistently isolated from infected leaves and stems and was identified as Thanatephorus cucumeris (A.B. Frank) Donk. with Rhizoctonia solani Kuhn as its anamorph. The identity of isolate IMI 354462 was verified as T. cucumeris by the International Mycological Institute, Kew, England. The AG grouping could not be determined because of a lack of availability of tester isolates. Six-month-old plants of 5. tomenlosa were inoculated with a 3-day-old hyphal suspension of T. cucumeris, grown on potato-dextrose broth in a growth chamber at 22 C,, placed in a dew chamber at 27 C for 7 days, and observed for symptoms after 10 days. Foliar web blight symptoms on inoculated plants were identical in all respects to the blight resulting from natural infection. Tests were repeated several times with similar results. Koch’s postulates were fulfilled by reisolation of the pathogen. This appears to be the first occurrence of T. cucumeris on S. tomenlosa in the United Stales.