The effects of different concentrations of phosphite on ectomycorrhiza formation by Pisolithus tinctorius in Castanea dentata

Phosphites, salts of phosphorous acid, are used to combat the devastating root-rot diseases of American chestnut (Castanea dentata) and other woody plants caused by the Oomycete Phytophthora cinnamomi. However, phosphite treatments may negatively affect the plant’s ability to form ectomycorrhizas, the mutualistic symbioses between tree roots and certain soil fungi, and the phosphites themselves may be phytotoxic at high doses. Container-grown chestnut seedlings were treated with six phosphite solutions ranging from 0.0g/l to 12g/l both with and without a spore inoculation of the ectomycorrhizal fungus Pisolithus tinctorius. Seedling survival, height, root collar diameter, and ectomycorrhizal colonization were measured during one growing season. Spore inoculation significantly improved seedling survival across all phosphite doses. Phosphite applications of 1.5 g/l (the lower boundary of the manufacturer’s recommendation) enhanced seedling growth and did not limit mycorrhiza formation significantly. Trees treated with highest doses of phosphite had lower survival, fewer mycorrhizas, and were significantly smaller.