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 determined. 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 PDA amended with calcium propionate. Calcium hydroxide, calcium oxide, calcium silicate, and calcium pyrophosphate reduced growth by approximately 65% compared with the control. Substances that were inhibitory on amended PDA were also inhibitory in potato-dextrose broth (PDB) but varied in effectiveness depending on whether PDB or PDA was used. The correlation 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. Greatest inhibition of PG was associated with calcium propionate followed by calcium sulfate, tribasic calcium phosphate, calcium gluconate, and 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 less than or equal to 0.04) but was not correlated with growth in PDB. When 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 containing 1,200 mg of calcium per liter, brown rot severity was least for fruit treated with calcium oxide and calcium hydroxide. For nonwounded fruit and drop inoculations, calcium hydroxide was the most effective in reducing brown rot incidence, and all salts reduced rot severity similarly. There were significant correlations between growth in PDB and disease incidence and severity 4 days after inoculation at both calcium levels (600 and 1,000 mg per liter). Furthermore, disease incidence and severity at both levels of calcium were also correlated significantly with PG activity. Future investigations on brown rot and calcium salts should utilize PDB for in vitro investigations and spray mist inoculations for fruit studies.