A New Wilt Disease of Lettuce Incited by Fusarium oxysporum f. sp. lactucum forma specialis nov.. J. C. Hubbard, Agricultural Research Service, U.S. Department of Agriculture, 1636 East Alisal St., Salinas, CA 93905. J. S. Gerik, Agricultural Research Service, U.S. Department of Agriculture, 1636 East Alisal St., Salinas, CA 93905. Plant Dis. 77:750-754. Accepted for publication 25 February 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/PD-77-0750.

A wilt disease was observed in the fall of 1990 in one commercial lettuce field (Lactuca sativa ‘Empire’) in Fresno County, California, and in the fall of 1991 in two nearby fields (cvs. Empire and Desert Queen). The disease is characterized by the death of some plants in the seedling stage, accompanied by a red streak through the cortex of the crown and upper root. Older affected heads show a tan-to-yellow tipburn with black streaks in the vascular system of some affected leaves, brown streaks in the vascular system of the crown, and a reddish brown discoloration of the cortex of the crown and upper root. Fusarium oxysporum was readily isolated from all affected plants, and in greenhouse inoculations this fungus caused a wilt disease of lettuce but not of any other crop species onto which it was inoculated. All pathogenic isolates belonged to the same vegetative compatibility group. Because of its apparent specificity for lettuce, the name Fusarium oxysporum Schlechtend.:Fr. emend. W.C. Snyder & H.N. Hans. f. sp. lactucum forma specialis nov. is proposed.