The effect of phosphite on mycorrhiza formation in American chestnut (Castanea dentata)

One of the primary hindrances to Castanea dentata restoration in the Southeast is the root rot disease caused by the fungus-like microorganism Phytophthora cinnamomi. Root rot can be combated by the application of mono- and di-potassium salts of phosphorous acid, which are marketed as phosphite fungicides. Despite its value in preventing infection by P. cinnamomi it is also thought that phosphite may impede root colonization by beneficial, ectomycorrhizal fungi. I hypothesized that plants given a routine application of phosphite will display fewer mycorrhizas in the root tips than those plants that were not treated with potassium phosphite. Therefore I attempted to elucidate this potential problem by inoculating C. dentata roots with three species of ectomycorrhizal fungi (Pisolithus tinctorius, Scleroderma geaster, and Scleroderma citrinum) in greenhouse and nursery settings. Spores of each species of ectomycorrhizal fungus were inoculated into two groups of twenty plants each. For each fungus species one experimental group was treated with potassium phosphite while the second was given no potassium phosphite, serving as a control. Potassium phosphite was administered in an aqueous solution sprayed directly onto the potting medium in the manufacturer recommended concentration of 2.4 g phosphite L-1. The effect of biweekly potassium phosphite application on mycorrhiza formation was studied by measuring the degree of fungal colonization of root tips. Observably, trees given a routine phosphite treatment exhibited a lesser degree of mycorrhizal formation. Statistical tests supported this observation; mycorrhizas are negatively affected by phosphite when applied in the manufacturer-recommended dosage