Rational Suppression of Sunflower Rust: Development and Evaluation of an Action Threshold. D. SHTIENBERG, Department of Plant Pathology, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel. Plant Dis. 79:506-510. Accepted for publication 4 August 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0506.

The efficacy of one protectant (maneb) and six systemic fungicides in the suppression of sunflower rust caused by Puccinia helianthi was evaluated in one set of three field trials conducted during 1992. Cyproconazole, hexaconazole, and tebuconazole were highly effective in controlling rust, whereas maneb, fenbuconazole, difenoconazole, and promoconazole were only partially effective. In another set of three field trials during 1992, the injury threshold, i.e., the level of disease intensity at which fungicide should be applied to achieve adequate disease suppression, was found to be an average rust severity of 3%, on the upper four leaves. The relationship between the time at which disease reached the injury threshold and the resulting damage to yield was linear and negative. When the injury threshold was reached at or later than 27 days after flowering, the resulting damage was insignificant. Consequently, the action threshold for management of sunflower rust was defined as the occurrence of disease severity of 3% prior to the 27th day after flowering. The developed action threshold was examined in two field trials during 1993 in which spraying was or was not carried out in accordance with the developed action threshold. Predictions of outcome, based on the developed action threshold, were accurate in all cases.