Rind distortion of lemon caused by Botrytis cinerea Pers.

The infection process by Botrytis cinerea Pers. on lemon fruits (Citrus lim on (L.) Burm.), and development of rind distortion are described. Infection s occur on juvenile fruitlets soon after petal fall from hyphae growing ove r the surface of the fruit from adjacent colonised flower debris. The hypha e form compact infection cushions at their tips. There is a collapse of epi dermal cells and several layers of underlying cells in the vicinity of the infection cushion, leading to the formation of small necrotic pits on the s urface of the fruit. As the fruitlets grow, there is generalised hyperplasi a in a zone up to 20 cell layers deep in the region of the necrotic pits, l eading to conspicuous outgrowths on the fruit surface. The initial wound be comes stretched, flattened, and corky as the fruit enlarges. A cambial laye r is located below the area of damaged tissue, fluorescence in the vicinity of the cork cambium indicates the presence of suberin in the cambium. Ther e was no evidence of suberin or lignin deposition in the region of the orig inal necrotic pit. The necrotic pits are characteristic of a non-pathogenic reaction between the fungus and the host, with fungal hyphae failing to be come established in the necrotic tissues of the pit. The hyperplasia leadin g 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 i njury to the rind. The principal sources of inoculum in orchards are Botryt is-colonised floral debris caught up in flower clusters or adhering directl y to the fruit surface. The etiology of the disorder helps explain why cont rol by conventional fungicides is seldom successful. Not only are there phy sical limitations to providing a protective cover on very small fruit, but also the extended period over which flowering occurs in lemon imposes limit s on the number of applications able to be economically applied. In the abs ence of effective control systems, growers are advised to modify husbandry practices such as lowering the density of shelter belts, opening lemon cano pies to assist drying out of the flowers, and use of air blast orchard spra yers to physically remove floral debris from the vicinity of young fruit.