Influence of Drip and Furrow Irrigation on Phytophthora Root Rot of Citrus Under Field and Greenhouse Conditions. S. J. Feld, Department of Plant Pathology, University of California, Riverside 92521. J. A. Menge, and L. H. Stolzy. Department of Plant Pathology, and Department of Soil and Environmental Sciences, University of California, Riverside 92521. Plant Dis. 74:21-27. Accepted for publication 7 July 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0021.

The influence of drip and furrow irrigation on the distribution of populations of Phytophthora parasitica and citrus roots in soil was examined under field conditions and in the greenhouse and lathhouse. During a 2-yr field study in a 70-yr-old citrus orchard, the soil population of P. parasitica was significantly lower under drip irrigation than under furrow irrigation; however, this difference was evident only during the summer months when the fungus was most active. The abundance of citrus roots under furrow irrigation increased with distance from the furrow center. Citrus roots under drip irrigation were uniformly distributed in the area wetted by the emitter. Distribution of P. parasitica propagules correlated directly with the distribution of citrus feeder roots. In both greenhouse and lathhouse, Phytophthora root rot of citrus seedlings was most severe where seedlings were watered by drip irrigation or kept constantly moist by furrow irrigation. Seedlings that received water by furrow irrigation that allowed the soil to dry sufficiently between irrigations were able to produce more feeder roots than seedlings grown under drip irrigation. Data obtained from greenhouse and lathhouse studies indicate that the commonly recommended method of furrow irrigation that allows the soil to dry to –50 to –70 cb Ψm between irrigations favors the host plant and reduces damage by P. parasitica more than the other irrigation methods investigated.