Control of tree growth in low-chill stonefruit using paclobutrazol: A synopsis of research findings over the past 5 years

Abstract

Under warm subtropical climates, tree growth rates of low-chill temperate fruit species are often very high, resulting in excessive shading, loss of fruiting wood and poor fruit quality. Various techniques used to control tree vigour in low-chill cultivars include cincturing, pruning and use of growth retardants. Of these, the growth retardant paclobutrazol has been the most successful in controlling tree vigour, and reducing the need for sequential summer pruning. Eight experiments to evaluate the effects of soil-applied paclobutrazol were conducted over 5 years. A synopsis of the findings of these experiments are presented. Paclobutrazol reduced tree size and shoot growth by as much as 60% and increased fruit size by as much as 48%. In most of the experiments paclobutrazol advanced fruit maturity by about 10 days. There appear to be synergistic responses between paclobutrazol and many other management technique; for example, when paclobutrazol was combined with post-harvest topping yield efficiency was doubled, and, when combined with higher rates of nitrogen, by 60%. The efficacy of stub-thinning as a rapid method of fruit thinning and potassium nitrate as a rest-release chemical were improved when combined with paclobutrazol. The use of plastic cloches to advance fruit maturity and to improve fruit quality was only beneficial when combined with paclobutrazol to control tree growth. The rate of response to soil applied paclobutrazol is soil-type dependant, and the level of response to the concentration applied. Rates need to be adjusted according to tree shoot growth, with a general reduction in rates in subsequent years of application. Split applications (spring and mid-summer) appear to be more effective than a single application in controlling growth and offer a greater margin of safety.