D.S. Tustin, G.A. Dayatilake, K.C. Breen , M.J. Oliver
Growth of individual apple fruit during their early development occurs in an environment of intense competition among individuals within a spur as well as among adjacent sinks. To increase fruit size and quality, chemical and hand thinning are used to reduce fruit numbers on trees. Thinning is essential but expensive and chemical thinning responses are often unpredictably variable. Thinning can also be considered to be wasteful of dry matter resources because many young fruit are removed from the tree. This paper examines the concept of regulating floral bud distributions within the tree, in order to manipulate fruit set and early fruit development to more optimally use dry matter resources at this time of limited supply and intense compete¬tion. In a randomized block layout, artificial bud extinction was used, just prior to bud break, to set floral bud densities in a range from 2 to 6 buds per cm2 branch cross-sectional area on tall spindle trees of the heavy-flowering apple cultivar ‘Scifresh’ (6 years old on M.9 rootstock). Equivalent unmodified trees (controls) were thinned by hand after final drop to fruit numbers equal to those calculated for bud extinction treatments. No chemical thinning was used. Fruit set and return bloom were measured annually during three successive years of treatment. The patterns of fruit set showed that the set of individual buds was highly dependent on the total density of floral buds present and that responses were repeatable annually. In conditions of very high floral bud density, fruit set of individual buds was low and almost 60% of buds failed to set any fruit. Reducing floral bud density by artificial bud extinction increased the proportion of floral buds that set fruit and increased the number of fruit set on individual buds. Annual bud extinction treatments induced a high proportion of buds into return bloom each spring. These fruit set and return bloom responses show artificial bud extinction could be a valuable physiological tool for optimizing fruit development and crop load. Bud extinction prior to growth resumption in spring largely sets the target fruit number and controls the number of competing floral sites during the early season when reserves and current resources are most limited. In this way the process appears to facilitate allocation of resources at times of limited supply to sites whose fruit are already selected for development to maturity.
Tustin, D.S., Dayatilake, G.A., Breen , K.C. and Oliver, M.J. (2012). FRUIT SET RESPONSES TO CHANGES IN FLORAL BUD LOAD - A NEW CONCEPT FOR CROP LOAD REGULATION . Acta Hortic. 932, 195-202
DOI: 10.17660/ActaHortic.2012.932.28
Malus × domestica Borkh., flowering, bud extinction, growth allocation, thinning

Acta Horticulturae