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Verticillium Wilt of Velvetleaf (Abutilon theophrasti). S. M. Sickinger, Graduate Research Assistant, Departments of Agronomy and Plant Pathology, University of Wisconsin-Madison 53706. C. R. Grau, and R. G. Harvey. Associate Professor, Department of Plant Pathology, and Professor, Department of Agronomy, University of Wisconsin-Madison 53706. Plant Dis. 71:415-418. Accepted for publication 17 November 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0415.

Since 1975, symptoms typical of a vascular wilt disease have been observed on velvetleaf (Abutilon theophrasti) within a weed nursery at the University of Wisconsin Experimental Farm, near Arlington. Verticillium dahliae was recovered from symptomatic plants, and Koch’s postulates were completed to determine the causal agent. Leaves show mottling initially, and irregular chlorotic areas coalesce and progressively become necrotic; interveinal necrosis becomes pronounced and major veinal tissues remain green. In symptomatic plants, there is a darkening of the steele of the roots and stems. Plants with advanced symptoms of the disease were observed to have reduced seed production and a general decline in height and vigor. Field experiments showed that the indigenous soil population of V. dahliae was specific for velvetleaf and failed to induce symptoms in and was not isolated from pea, potato, tomato, sunflower, tobacco, soybean, cotton, cabbage, pepper, peppermint, or alfalfa. V. dahliae was isolated from 92% of the velvetleaf plants sampled by the end of the growing season (2 September). The number of free microsclerotia per gram of soil varied from 2,300 to 13,400 and 800 to 7,800 for total Verticillium spp. and V. dahliae, respectively. About 59% of the Verticillium species recovered from the Arlington Plano silt loam soil were identified as V. dahliae. V. dahliae was not isolated from seeds of naturally infected or artificially inoculated velvetleaf plants.