Effect of Fungicide Applications on Populations of Phytophthora parasitica and on Feeder Root Densities and Fruit Yields of Citrus Trees. H. A. Sandler, Assistant in Plant Pathology, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. L. W. Timmer, J. H. Graham, and S. E. Zitko. Professor, Associate Professor, and Biological Scientist II, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 73:902-906. Accepted for publication 14 June 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0902.

Fungicide treatments for control of feeder root rot of citrus caused by Phytophthora parasitica were made for 3 yr in four Florida orchards. Treatments consisted of foliar applications of fosetyl-Al at high (FOS-H) and low (FOS-L) frequency and soil applications of metalaxyl (MET). Propagule densities were positively correlated with feeder root densities in some orchards, especially in plots treated with fosetyl-Al. Expression of propagule densities on a root weight rather than a soil volume basis permitted more meaningful comparisons of fungicide effects on soil populations of the fungus. Propagule densities of P. parasitica were lower in the metalaxyl treatments and in some cases were lower in the fosetyl-Al treatments than in the untreated controls. Feeder root densities in the four orchards over all 3 yr increased by an average of 29.6, 9.4, and 43.5% above the untreated controls in the FOS-H, FOS-L, and MET treatments. Total fruit and juice yield was increased by the FOS-H and MET treatments in one orchard and by the MET treatment in another orchard, and average fruit weight was increased by all fungicide treatments in a third orchard. Tree appearance was significantly improved by the treatments in three of the four orchards. Feeder root loss in citrus due to infection by P. parasitica appears to be substantial and is corrected by applications of fosetyl-Al or metalaxyl. However, the lack of consistent tree response to treatment indicates that fungicide applications may not be economically justifiable in many cases.