Apple Bloom Thinning
Research has shown that materials which damage sensitive flower parts (stigmas, styles, pollen) and/or induce whole-tree stress can reduce fruit set. Programs which have shown promise in experimental settings include caustic salts, weak acids, lime sulfur, and combinations of spray oils and lime sulfur. Lime sulfur programs not only damage floral anatomy but temporarily depress plant photosynthesis, inducing apple trees to abort some fruitlets which may have already been fertilized. Because their success is not solely reliant on damaging recently exposed organs in unpollinated flowers, lime sulfur-based thinning programs have shown more of a “kickback” effect than caustic salts in research studies. Sequential applications of lime sulfur or oil + lime sulfur can have a cumulative effect on plant stress and typically increase levels of thinning. Growers might improve their chances of hitting chemical thinning objectives with the use of pollen tube growth models to time their bloom thinning applications; these models may be accessed on WSU's AgWeatherNet system (weather.wsu.edu).