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Preventive and Curative Trunk Treatments for Control of Phytophthora Foot Rot of Citrus. L. W. Timmer, Texas A&I University Citrus Center, Weslaco, TX 78596; Phytopathology 67:1149-1154. Accepted for publication 7 March 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1149.

To evaluate the effectiveness of fungicidal trunk paints in preventing infection by Phytophthora parasitica, trunks of young citrus trees were treated, and fungicidal activity was determined in a bioassay using P. parasitica zoospores. To attain 100% inhibition of Phytophthora, 60 mg/ml of captan and copper fungicides were required, whereas only 0.6 mg/ml of captafol was needed (amounts as active ingredient or metallic Cu). In longevity tests on exposed trunks painted with fungicide at 3 mg/ml, captafol was the only material that was still active after 2 wk. At 60 mg/ml, copper ammonium carbonate (CAC), cupric hydroxide (CH), and captafol were active for at least 33 wk, but captan and pyroxychlor were active for less than 17 wk. In tests on trees wrapped with the polyurethane mats presently used for freeze protection of young trees in Texas, captafol, CAC, CH, and copper salts of fatty and rosin acids at 40-60 mg/ml retained some activity for 52 wk. The standard trunk paint in current use (2.5% captan + 2.5% copper fungicide in bentonite) was ineffective at 0.12 g/ml, but was active through 29 wk at 0.48 g/ml. On trees infected by Phytophthora, excision of affected tissue and painting with a copper fungicide slightly improved tree recovery. Pyroxychlor applied to trunks at 240 mg/ml remained active on the bark surface for at least 8 days, penetrated the bark to the cambium, but was not translocated away from the treated area. Pyroxychlor applied as a trunk paint, but not as a soil drench, limited the expansion of foot rot lesions.

Additional keywords: fungicides, grapefruit.