Comparative Ability of Six Fungicides to Inhibit Development of Phytophthora Gummosis on Citrus

The activity of the registered fungicides fosetyl-Al and metalaxyl (subsequently replaced with mefenoxam by the manufacturer) was compared with other potentially useful compounds, azoxystrobin, dimethomorph, fluazinam, and zoxamide, for suppression of canker development on citrus bark after inoculation with Phytophthora citrophthora or P. nicotianae. The number of sweet orange trees on which cankers developed after inoculation with P. citrophthora and the average size of cankers when present were lower on plants treated with dimethomorph, fosetyl-Al, or metalaxyl compared with nontreated trees and those treated with azoxystrobin or fluazinam. When bark removed from treated trees was inoculated with P. citrophthora on the cambium surface at 5, 30, or 60 days after treatment (DAT), inhibition of lesion development on bark strips treated with dimethomorph, fosetyl-Al, or metalaxyl was significantly greater than that detected on bark treated with azoxystrobin, fluazinam, or zoxamide. When inoculated with P. nicotianae at 5 or 30 DAT, reduction of lesion size on bark strips treated with dimethomorph, fosetyl-Al, or metalaxyl was significantly greater than that detected on bark treated with azoxystrobin or fluazinam. Inhibition of lesion development by zoxamide was significantly less than that observed with metalaxyl at 5 DAT on bark inoculated with P. nicotianae; however, at 30, 60, and 90 DAT there was no significant difference in the performance of either fungicide. Reduction of lesion growth on the cambium surface compared with outer bark surface, when inoculated with P. citrophthora, did not differ significantly from 5 to 30 DAT for bark tissue treated with azoxystrobin, dimethomorph, fosetyl-Al, or metalaxyl. Among the nonregistered fungicides tested, dimethomorph provided the best level of Phytophthora gummosis control on citrus.