Persistence of Systemic Activity for Fungicides Applied to Citrus Trunks to Control Phytophthora Gummosis. M. E. Matheron, Extension Plant Pathologist, Department of Plant Pathology, University of Arizona, Yuma Agricultural Center, Yuma 85364. J. C. Matejka, Research Assistant, Department of Plant Pathology, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 72:170-174. Accepted for publication 21 September 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0170.

The systemic activity of metalaxyl, fosetyl-Al, phosphorous acid (H₃PO₃), oxadixyl, propamocarb, benalaxyl, and ethazol was examined for control of Phytophthora gummosis on citrus. Sections of tree trunks in a 15-yr-old tangelo block were painted with one of the test fungicides. Strips of bark were then removed periodically from within as well as below the treated area and inoculated on the cambium with Phytophthora parasitica and P. citrophthora. For at least 117 days after application, bark tissues treated with metalaxyl, fosetyl-Al, H₃PO₃, or oxadixyl were inhibitory to growth of both fungi. Benalaxyl significantly suppressed the growth of P. parasitica but had no apparent effect on P. citrophthora. Propamocarb and ethazol did not inhibit the growth of either Phytophthora sp. Significant basipetal translocation of fungicide activity was observed at least 15 cm from bark treated with metalaxyl, fosetyl-Al, and H₃PO₃. A trunk paint application of fosetyl-Al to 5-yr-old lemon trees was more inhibitory than a foliar application to growth of P. parasitica and P. citrophthora on bark tissue. One application of metalaxyl, fosetyl-Al, or H₃PO₃ to the trunk significantly reduced canker development on tangelo trees subsequently inoculated with the two Phytophthora spp. Application of fungicides to the tree trunk appears to be an effective procedure for control of Phytophthora gummosis of citrus.