Factors Associated with Excessive Fruit Drop of Navel Orange1

Two distinct periods of fruit drop, summer (mid-June to mid-August) and summer-fall (late August to late October), of navel sweet orange [ Citrus sinensis (L.) Osbeck] were observed during the period between the end of the June drop (mid-June) and legal maturity (late October) in the 1978-79 and 1979-80 seasons. Summer drop reduced fruit number in 1979 by as much as 16.5% and was mostly caused by a yellowing and subsequent decay of the secondary fruit. A separation zone was found between the central axis and the secondary fruit of those with yellow navels. Summer-fall drop was responsible for an additional 14.5% reduction in fruit number based on catching frame counts. It was caused primarily by stylar-end decay, dieback of branches, splitting and brown rot ( Phytophthora spp.). Causes of stylar-end decay, branch dieback and splitting were not identified. Incidence of stylar-end decay and splitting was greatest on fruit with a large (> 12 mm) stylar-end aperture (navel) of irregular shape, and adjacent thinner than usual peel. Stylar-end decay was associated with protrusions of rind-like tissue from the secondary fruit into segments of the primary fruit.