The Effect of Cover Crops and Fertilization with Ammonium Nitrate on Corky Root of Lettuce. Ariena H. C. Van Bruggen, Assistant Professor, Department of Plant Pathology, University of California, Davis 95616. Philip R. Brown, Carol Shennan, and Arthur S. Greathead. Post Graduate Research Assistant, Department of Plant Pathology, University of California, Davis 95616; Assistant Professor, Department of Vegetable Crops, University of California, Davis 95616; and Former Farm Advisor, University of California, Agricultural Extension, 118 Wilgart Way, Salinas, CA 93901. Plant Dis. 74:584-589. Accepted for publication 16 January 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0584.

Rye (Secale cereale) planted as a winter cover crop slightly reduced the severity of corky root early in the growing season of a subsequent lettuce crop, but at harvest the suppressive effect was observed in only one of four lettuce crops. Lettuce yields were not affected by cover crop use. Side-dressing with ammonium nitrate increased corky root severity in most plots and reduced yield in all plots. There were no significant interactions between cover crops and nitrogen fertilization with respect to lettuce yield. Nitrate concentrations tended to be lower in lettuce plants grown in the previously fallowed plots than in those grown in plots that had been cover-cropped, and higher in the plants grown in plots that received a side-dressing with ammonium nitrate. The soil moisture content at depths of 75 and 90 cm was lower in plots planted to rye than in those planted to broad bean or left fallow. The reduction in corky root after a rye cover crop might be related to a reduction in soil moisture or improvement in soil structure rather than to nitrate content in soil or lettuce tissue. The inoculum potential in soil, determined with a lettuce bioassay, was not affected by cover crop.