EFFECTS OF CALCIUM SALTS ON GROWTH, POLYGALACTURONASE ACTIVITY, AND INFECTION OF PEACH FRUIT BY MONILINIA-FRUCTICOLA

The effects of several calcium salts on growth, polygalacturonase (PG) activity, and infection of peach fruit by Monilinia fructicola were d etermined. All salts except calcium formate, calcium pantothenate, and dibasic calcium phosphate reduced growth of M. fructicola on amended potato-dextrose agar (PDA) after 7 days. Minimal growth occurred on PD A amended with calcium propionate. Calcium hydroxide, calcium oxide, c alcium silicate, and calcium pyrophosphate reduced growth by approxima tely 65% compared with the control. Substances that were inhibitory on amended PDA were also inhibitory in potato-dextrose broth (PDB) but v aried in effectiveness depending on whether PDB or PDA was used. The c orrelation between the amount of growth on PDA versus that in PDB was not significant. Fungal PG activity was inhibited by all salts used in this study except dibasic calcium phosphate and calcium tartrate. Gre atest inhibition of PG was associated with calcium propionate followed by calcium sulfate, tribasic calcium phosphate, calcium gluconate, an d calcium succinate. The activity of calcium salts was not affected by pH. PG activity was correlated with growth on PDA (r(s) = 0.48, P les s than or equal to 0.04) but was not correlated with growth in PDB. Wh en inoculum was sprayed on detached fruit, the incidence and severity of brown rot were least on fruit that had been dipped in solutions of calcium propionate or calcium silicate. When inoculum was applied as a localized drop to wounded fruit that had been dipped in a solution co ntaining 1,200 mg of calcium per liter, brown rot severity was least f or fruit treated with calcium oxide and calcium hydroxide. For nonwoun ded fruit and drop inoculations, calcium hydroxide was the most effect ive in reducing brown rot incidence, and all salts reduced rot severit y similarly. There were significant correlations between growth in PDB and disease incidence and severity 4 days after inoculation at both c alcium levels (600 and 1,000 mg per liter). Furthermore, disease incid ence and severity at both levels of calcium were also correlated signi ficantly with PG activity. Future investigations on brown rot and calc ium salts should utilize PDB for in vitro investigations and spray mis t inoculations for fruit studies.