Using phosphite fungicides to control sooty blotch and flyspeck on apples

Phosphite fungicides were applied to control sooty blotch and flyspeck (SBFS) in four field trials during the 2006-07 growing seasons. In one trial, ProPhyt (54.5% potassium phosphite), LI-700 (a surfactant and acidifier), and Tactic (a synthetic latex and organosilicone sticker, surfactant, and deposition agent) were applied alone, with captan, or in three-way combinations with thiophanate-methyl (TM) plus captan. Treatments were applied to 'Golden Delicious' trees on 1 Sept. using a handgun sprayer. Fruit were evaluated 33 days later and 96% of unsprayed fruit had flyspeck. ProPhyt applied alone suppressed flyspeck by 50% whereas LI-700 and Tactic did not. Fruit sprayed with TM+captan+Tactic had the least flyspeck (14%), but ProPhyt+captan provided statistically equivalent control with 31% of fruit affected. Captan with LI-700 or Tactic was less effective. Thus, ProPhyt activity against SBFS involves more than spray acidification or surfactant activity. In another trial with 'Honeycrisp', 'Royal Court' and 'Cameo' apples, ProPhyt was applied at two rates either alone or in mixtures with captan, TM, or Pristine (pyraclostrobin plus boscalid). In 10 disease-control data sets from this trial, ProPhyt consistently boosted activity of captan but only occasionally improved activity of TM or Pristine. However, ProPhyt+captan had less residual activity than TM and Pristine and was less effective than TM and Pristine for controlling summer fruit rots. Thus, ProPhyt+captan can be used to control SBFS, but other fungicides may be needed to control fruit rots caused by Botryosphaeria species.