Issue link: http://read.dmtmag.com/i/117824
THINNING rules I n an article in Good Fruit Grower in 2009, Cornell University���s Dr. Terence Robinson laid out some general rules for fruit thinning: ��� Dark, cloudy weather of more than one day reduces the carbohydrate supply, resulting in greater natural drop and greater response to chemical thinning. ��� Nighttime temperatures above 65��F increase carbohydrate demand, thereby increasing natural drop and chemical thinning response. ��� Daytime temperatures above 85��F increase carbohydrate demand and result in excessive thinning. ��� Cool daytime temperatures, below 65��F, reduce fruit carbohydrate demand, resulting in poor thinning response. He rates McIntosh, Paulared, Honeycrisp, Spartan, and Stayman as moderately sensitive to thinners. Spur Delicious, Gala, Fuji, Rome, Golden Delicious, and Cameo are hard to thin, he said. For the easy-to-thin varieties, he recommends lower rates of thinners and a single application when fruitlets are 10 to 12 millimeters in diameter. For moderate varieties, he recommends two applications of medium rates of thinners in a tank mix with carbaryl, starting at petal fall if bloom is heavy. For hard-to-thin varieties, he recommends multiple applications and aggressive thinning, starting with blossom or petal fall thinners, using the highest label rates and adding oil and carbaryl at the 10-12 mm size, and following with a third spray of ethephon or carbaryl if needed. The basic theory of fruitset says that it���s all about stress and the supply of and demand for carbohydrates. ���Carbohydrates come from plant reserves and from this year���s photosynthesis,��� Schwallier said. ���About the time fruitlets reach ten millimeters in size, a Throwing 30 million bushels of small apples into the market this year will not be cure for the problems of last year, he said. The freezes last year and the absence of crop have set the stage for prolific flowering this spring and the setting of an excessive crop. That will mean a small crop again in 2014 and an ongoing biennial bearing cycle. Some varieties, such as Golden Delicious, Fuji, Paulared, and Honeycrisp, have a strong tendency to bear biennially. Special attention must be paid to them, Schwallier said. Rome, Jonagold, Red Delicious, Empire, and McIntosh are mildly biennial, and Gala, Granny Smith, Ginger Gold, and Idared have low tendency to bear biennially. Growers have tools to address the problem. The first is pruning intensity, and he advised them to prune hard this winter to adjust crop potential. Then follow with chemical thinning during the best thinning windows, and follow that with rescue thinning when fruits reach 20 millimeters, if needed. If all fails, hand thinning may be required. If the set is still too heavy, return bloom sprays can help adjust crop potential for 2014. Variety differences Apples vary in their sensitivity to thinners. Schwallier rates Ginger Gold, Empire, Cortland, Idared, Mutsu, Jonagold, nonspur Rome, and Jonathan as easy to thin. NAA rates Easy to thin 10 ppm Intermediate 15 ppm Difficult 20 ppm Very difficult 20 ppm + oil or multiple sprays at at petal fall and 10 mm or Sevin + 15 ppm + oil If adding Sevin to naphthaleneacetic acid, reduce NAA 50%. www.goodfruit.com GOOD FRUIT GROWER April 1, 2013 9