Rind distortion of lemon caused by Botrytis cinerea Pers.

The infection process by Botrytis cinerea Pers. on lemon fruits (Citrus limon (L.) Burm.), and development of rind distortion are described. Infections occur on juvenile fruitlets soon after petal fall from hyphae growing over the surface of the fruit from adjacent colonised flower debris. The hyphae form compact infection cushions at their tips. There is a collapse of epidermal cells and several layers of underlying cells in the vicinity of the infection cushion, leading to the formation of small necrotic pits on the surface of the fruit. As the fruitlets grow, there is generalised hyperplasia in a zone up to 20 cell layers deep in the region of the necrotic pits, leading to conspicuous outgrowths on the fruit surface. The initial wound becomes stretched, flattened, and corky as the fruit enlarges. A cambial layer is located below the area of damaged tissue, fluorescence in the vicinity of the cork cambium indicates the presence of suberin in the cambium. There was no evidence of suberin or lignin deposition in the region of the original necrotic pit. The necrotic pits are characteristic of a non-pathogenic reaction between the fungus and the host, with fungal hyphae failing to become established in the necrotic tissues of the pit. The hyperplasia leading to the rind distortions is considered to be a generalised reaction to the initial wound and is commonly observed in lemon following other types of injury to the rind. The principal sources of inoculum in orchards are Botrytis-colonised floral debris caught up in flower clusters or adhering directly to the fruit surface. The etiology of the disorder helps explain why control by conventional fungicides is seldom successful. Not only are there physical limitations to providing a protective cover on very small fruit, but also the extended period over which flowering occurs in lemon imposes limits on the number of applications able to be economically applied. In the absence of effective control systems, growers are advised to modify husbandry practices such as lowering the density of shelter belts, opening lemon canopies to assist drying out of the flowers, and use of air blast orchard sprayers to physically remove floral debris from the vicinity of young fruit.