Research conducted in the USA in the late 1980s showed that thinning of apple trees could be enhanced by shading the trees (Byers et al., 1991). Severe shading for brief periods during the development of apple fruitlets can induce fruit drop.
Attempts were made to verify the USA trials in further work conducted in Switzerland (Bertschinger, et al., 1998) where structures used to support hail netting were used to create shade within orchards.
- The shade was created using polypropylene sheeting and 50% and 100% shade levels were imposed for 5 days at either 14 or 28 days after full bloom to ten‑year-old trees of Arlet and Jonagold on M.9 rootstocks.
- The ideal level of fruit set was achieved using the 100% shade for 5 days commencing 14 days after full bloom.
- This fruit set was equivalent to that obtained by hand thinning after June drop.
- Fruit quality and yield productivity were acceptable with this treatment.
- Application of 100% shading 28 days after full bloom caused 100% fruit drop.
Although successful in the Swiss trial, shading as a means of fruitlet thinning does carry some inherent risks.
- It is known that shading reduces relative growth rates of fruitlets in the period of 1 to 5 weeks after full bloom.
- This is the phase of cell division and other studies have shown that reductions in growth in this period may result in reductions in yield (Lakso et al., 1989).
- Similarly, it has been shown in Italian studies that moderate shading (to 40% normal light) of extension shoots three weeks after full bloom suppresses the movement of carbohydrates to fruits.
- Further studies are needed before this non-chemical strategy for inducing fruit drop can be recommended.