Influence of Orchard Ground Cover Management on the Development of Phytophthora Crown and Root Rots of Apple. I. A. Merwin, Assistant Professor, Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853. W. F. Wilcox, and W. C. Stiles. Associate Professor, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; and Professor, Department of Fruit and Vegetable Science, Cornell University, Ithaca 14853. Plant Dis. 76:199-205. Accepted for publication 11 September 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0199.

Eight tree-row ground cover vegetation management systems—including a crown vetch “living mulch,” close-mowed and chemically growth-regulated sod grasses, pre- and postemergence herbicide strips, a straw mulch, and monthly rototillage—were established in a newly replanted apple (Malus domestica) orchard. After 4 yr, Phytophthora crown or root rots (PCRR) had developed on 35% of the trees in straw mulch plots, whereas disease incidence was only 0–6% in other treatments. Phytophthora cactorum, P. megasperma, and P. cambivora were isolated from diseased trees. Stepwise regression and principal component analyses of soil physical and edaphic variables indicated that prolonged soil saturation and high soil K concentrations were closely associated with both straw mulch and PCRR incidence, although soil K was not considered to be a functionally causal or predisposing factor for PCRR. Soil temperature and bulk density varied significantly among the vegetation management systems but appeared not to be significantly correlated with PCRR occurrence. MM.111 clonal apple rootstocks were very susceptible to PCRR in this site. Trees in the sod grass and crown vetch plots remained free of symptoms of PCRR during the 4 yr of observation.