The Effect of Calcium on Botrytis Blight of Petunia Flowers

During greenhouse production, petunia plants are frequently exposed to conditions optimal for botrytis blight, caused by Botrytis cinerea. Once plants are flowering and ready to be shipped to retail, they are irrigated and tightly packed onto shipping carts, creating a humid environment conducive for Botrytis growth. At the retail environment, petunia flowers exhibit symptoms of botrytis blight, e.g., tan necrotic spots that coalesce making the plants unsellable. Fungicide applications are the primary strategy for Botrytis management; however, fungicide resistance is a common phenomenon that results in lack of fungicide efficacy. Alternative Botrytis management strategies are necessary. The goal of this thesis was to explore the potential of calcium (Ca) to reduce botrytis blight on petunia flowers. In the first part of this study, increasing Ca concentrations in a calcium chloride spray solution reduced botrytis blight severity up to 93%. In Chapter 1, three factors that may have been the source of the disease reduction were explored: Ca, the chloride ion (Cl), and/or the electrical conductivity (EC) of the solution. The results demonstrated that Ca was the sole active ingredient in calcium chloride causing the reduction in botrytis blight severity. In Chapter 2, two methods for delivering Ca to plant tissues were compared, namely fertigation and spray applications. Calcium fertigation applications were ineffective for increasing Ca concentration in flower petals and for reducing botrytis blight severity. In contrast, Ca spray applications increased Ca concentration in flower petal tissue and reduced botrytis blight severity in the flowers. In Chapter 3, six calcium sources were evaluated for their potential for reducing botrytis blight while not causing spray damage. Calcium chloride, either lab grade or commercial grade, provided the highest disease reduction with the lowest spray damage potential at 1250 mg·L 1. Higher rates (2000 mg·L 1 Ca) did not improve the reduction in disease infection and did cause spray damage to flowers; however, flower buds were not damaged by the 2000 mg·L 1 Ca treatments. We conclude from this study that spray applications of 1250 mg∙L-1 Ca from CaCl2 are safe to spray on both flower buds and open flowers without risk of spray damage for maximum botrytis blight efficacy. The results from this research project demonstrate the effectiveness of Ca spray applications as an additional management strategy for decreasing flower susceptibility to botrytis blight