Effects of Calcium Salts on Apple Bitter Rot Caused by Two Colletotrichum spp

The effects of three calcium salts on conidial germination, germ tube elongation, growth in vitro, and infectivity in the laboratory and field were studied for the apple bitter rot pathogens, Colletotrichum gloeosporioides and C. acutatum. Calcium chloride, calcium propionate, and calcium silicate at 1,000 μg of calcium per ml had no effect on conidial germination; however, calcium chloride and calcium propionate inhibited germ tube growth by 41 and 50% relative to the control, respectively. All three calcium salts reduced fungal dry weight in liquid culture media. When calcium salt solutions were applied to wounded apples prior to inoculation, fruit treated with calcium chloride and calcium propionate exhibited 30% smaller lesions than those treated with calcium silicate or the control, which were similar. Fruit treated with calcium chloride and calcium propionate exhibited delayed formation of acervuli relative to the control and calcium silicate, which were similar. When fruit were inoculated with varying concentrations of conidia, from 1 × 10³ to 1 × 10⁵ conidia per ml, fruit treated with calcium chloride exhibited reduced incidence of infection after inoculations with 1 × 10³ conidia per ml. In all tests at 1 ×10⁴ and 1 × 10⁵ conidia per ml, the control and calcium salt treatments resulted in similar incidences of infection after inoculations. In six field trials, where fruit received three weekly dilute applications of calcium solutions, fruit treated with calcium salts and then inoculated with conidia of either C. gloeosporioides or C. acutatum exhibited lower incidences of infection when compared with control fruit. These experiments demonstrate that calcium salts have suppressive activity against the bitter rot pathogens and could be used as part of a disease management program.