The influence of calcium nutrition on grey mold rot caused by Botrytis cinerea Pers. in three tomato cultivars

Due to the declining availability of fungicides there is a need to investigate alternative methods of disease control for use in commercial tomato production. Several theories of disease resistance are associated with an increase in plant tissue calcium (Ca) content which has been shown to increase resistance of tomato seedlings to bacterial wilt and other diseases. Botrytis cinerea Pers. is the major postharvest pathogen of tomatoes, causing losses of tomato fruit in storage. These experiments were conducted to determine if Ca nutrition influences resistance to fungal decay of tomato fruit caused by B. cinerea in commercial tomato cultivars. Two greenhouse tomato experiments containing 54 plants each were performed, one in the fall of 1995 and the other in the spring of 1996. Three tomato cultivars, 'Mountain Supreme', 'Sunrise' and 'Celebrity', were grown in a hydroponic system with perlite as a root support medium. Calcium treatments of 20 mgL^-1, 220 mg L^-1, or 1020 mg L^-1 were added to a commercial fertilizer solution which had an N-P-K ratio of 1.0:0.6:2.5 and contained essential micronutrients. A 3x3 factorial with three tomato cultivars and three Ca treatments in a randomized complete block design was utilized. The mineral content of leaves and fruit was determined by inductively coupled plasma emission spectrometry (ICP). Harvested fruit were inoculated with a 5x10⁵ spore ml^-1 conidial suspension of B. cinerea and the decay lesion diameter measured once daily for six (Spring 1996) or seven (Fall 1995) days.

Across cultivars, leaf Ca content significantly increased as the Ca treatments increased from low to medium (P<0.001) and medium to high (P<0.001; Spring only). Fruit tissue Ca content was significantly increased across cultivars as the Ca treatments increased from low to medium (P<0.001) and medium to high (P<0.07; Spring only). In both the fall and spring experiments, the high calcium treatment significantly (P<0.01 and P<0.04, respectively) reduced the area of decay caused by Botrytis cinerea across all cultivars. The medium Ca treatment reduced the area of decay (P<0.01) in the fall. In the spring, there was a significant difference in the decay area (P<0.01) between the medium and high Ca treatments. There were a few significant cultivar differences for all three variables (leaf Ca, fruit Ca, and decay area) within each treatment level, but those differences were not consistent within or among the two experiments. Significant treatment x cultivar interactions were observed in fruit Ca content. Leaf and fruit tissue Ca concentrations were positively associated with resistance of greenhouse-grown tomatoes to gray mold rot in both experiments, strengthening the hypothesized role of Ca in promoting disease resistance.