Chlorothalonil Fungicides Reduce Vase Life but not Yield of Leatherleaf Fern [Rumohra adiantiformis (Forst.) Ching

Abstract

Established ground beds of leatherleaf fern were sprayed repeatedly with water, a flowable formulation of thiophanate-methyl, or one of four formulations of chlorothalonil on a predominantly weekly schedule. None of the treatments produced visible phytotoxicity symptoms or had any effect on yield (frond number and total fresh mass). However, average masses of fronds from plots treated with a liquid formulation of chlorothalonil were 21% greater than those from control plots. All chlorothalonil formulations left visible residues on the fronds and reduced frond vase life compared to fronds treated with water or thiophanate-methyl. Reduced vase life was due to more rapid desiccation of chlorothalonil-treated fronds. During those months (July—Sept.) when postharvest desiccation is most common, chlorothalonil reduced vase life of fronds by 36% to 62%. Vase life of fronds was generally reduced more by dry chlorothalonil formulations than by liquid ones, probably due to slightly higher application rates of dry formulations. Determination of the mode of action could lead to an understanding of the causes of frond curl syndrome. Until a remedy is found, chlorothalonil should not be used repeatedly on leatherleaf fern. Chemical names used: tetrachlorisophthalonitrile (chlorothalonil); dimethyl [(1,2-phenylene)-bis(iminocarbonothioyl)]bis[carbamate]) (thiophanate-methyl).