Interactions between<i>Pseudomonas syringae</i>MA-4 and cyprodinil in the control of blue mold and gray mold of apples

Abstract

Blue mold [Penicillium expansum] and gray mold [Botrytis cinerea] are the principal postharvest diseases of apples in Ontario. In an attempt to develop new control measures, the efficacies of two fungicides, cyprodinil and fenhexamid, and a biocontrol agent, Pseudomonas syringae MA-4, were assessed. Surface-disinfested apples were wounded and inoculated with a 20-µL drop containing either P. expansum (1 × 104 conidia/mL) or B. cinerea (3 × 104 conidia/mL), combined with a range of concentrations of either fungicide. Cyprodinil controlled blue mold and gray mold by over 90% at 25 and 5 µg/mL, respectively. Fenhexamid controlled gray mold by 98% at 10 µg/mL but was inactive against blue mold. Cyprodinil was combined with strain MA-4 to control both diseases in separate experiments. Against blue mold, cyprodinil and strain MA-4 provided >90% control at concentrations of 20 µg/mL and 3 × 107 CFU/mL, respectively, on 'Northern Spy' apples. The addition of cyprodinil at concentrations of 2.5 to 20 µg/mL to treatments with strain MA-4 at 3 × 106 CFU/mL significantly increased blue mold control (35–91%) compared with the strain MA-4 alone treatment (11%). However, there were some antagonistic effects between strain MA-4 and cyprodinil at 2.5–10 µg/mL on 'Jonagold' apples. Against gray mold on 'Northern Spy' apples, cyprodinil gave 85% control at 2.5 µg/mL, and strain MA-4 gave 77% control at 3 × 108 CFU/mL. A combination of cyprodinil and strain MA-4 at concentrations of 2.5 µg/mL and 1 × 108 CFU/mL, respectively, controlled gray mold by 100%. Cyprodinil was not bacteriocidal and at concentrations of 5 and 20 µg/mL had no effect on strain MA-4 populations at inoculation sites.