Pathogenesis of Plasmopara lactucae-radicis, a Systemic Root Pathogen of Cultivated Lettuce. M. E. Stanghellini, Department of Plant Pathology, University of Arizona, Tucson 85721. J. E. Adaskaveg, and S. L. Rasmussen. Department of Plant Pathology, University of Arizona, Tucson 85721. Plant Dis. 74:173-178. Accepted for publication 20 July 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0173.

Plasmopara lactucae-radicis, a recently described causal agent of downy mildew, is reported for the first time as a root pathogen of cultivated lettuce. This unique fungus, which only colonizes roots, caused yield reductions of about 50% in greenhouse pathogenicity tests. Direct penetration of roots by zoospore cysts, systemic invasion of roots by intercellular hyphae and intracellular haustoria, and sporulation on infected roots occurred at root temperatures between 18 and 26 C. No infection occurred at root temperatures of 12, 14, and 30 C. Lettuce cultivars Ostinata and Salina are susceptible and Sitonia is resistant.

Keyword(s): disease development, histology, ultrastructure, zoospores.