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Inheritance of Resistance in Lettuce to Plasmopara lactucae-radicis. G. J. Vandemark, Department of Plant Pathology, University of Arizona, Tucson 85721; M. E. Stanghellini(2), S. L. Rasmussen(3), and R. W. Michelmore(4). (2)(3)Department of Plant Pathology, University of Arizona, Tucson 85721; and (4)Department of Vegetable Crops, University of California, Davis 95616. Phytopathology 82:272-274. Accepted for publication 5 August 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-82-272.

Plasmopara lactucae-radicis recently has been described as the causal agent of downy mildew on roots of cultivated lettuce. This fungus is the only known causal agent of downy mildew that is restricted to the roots of its host. Thirty-seven cultivars of Lactuca sativa were screened for resistance to P. lactucae-radicis. Two-week-old lettuce plants grown under hydroponic conditions were challenged with this fungus and evaluated 2 and 3 wk after inoculation for resistance. Root necrosis and production of sporangia on roots was considered a susceptible reaction. Five cultivars were determined resistant to P. lactucae-radicis. Resistant cultivars were colonized by this fungus, but the fungus was unable to sporulate on infected roots. Data from F2 and F3 progenies demonstrated that resistance in the cultivar Cobham Green was conferred by a recessive allele at a single locus (plr). Fungal infection of susceptible plants by P. lactucae-radicis resulted in significant decreases in fresh root and shoot weights and leaf number compared with decreases in these characters associated with infection of resistant plants.

Additional keywords: cultivar screen.