Issue link: https://read.dmtmag.com/i/307101
10 MAY 15, 2014 GOOD FRUIT GROWER www.goodfruit.com That model shows that when apple trees are stressed by weather conditions that result in the tree producing less car- bohydrate than growing fruit demands, some of that fruit will fall off the tree. Add- ing more stress, like a chemical thinner, increases thinning. Weather monitoring allows prediction of periods when fruit is most susceptible to thinning, so that tim- ing and rate of chemical application can be determined. Thinning windows As Greene explained during the fruit school, growers have several "windows" during which to thin apples. The first opportunity is during bloom, when caustic materials like lime sulfur and fish oil will damage flowers, reduce pollination, and lower fruit set. The second is from petal fall to when fruits are 6 millimeters (mm) in diameter. Then comes the "ideal" window when fruits are 7 to 15 mm in size. A third window is when fruits are 18 to 25 mm. When fruits are larger than 25 mm, thinning is very difficult. Hand thinning, and the labor bill that goes with it, are the final determiners of how effective chemical thinning was. "Chemical thinning is a process that by its very nature provides different opportuni- ties," Greene said. "We must understand the underlying fundamentals of each to take full advantage of fruit vulnerability at different periods." Carbohydrate demand Chemical thinning can be done during about 28 days after petal fall, during which time fruits grow to about 25 mm in diameter, or just under one inch. When fruit is small, from petal fall to 6 mm, some thinning is possible, he said. But since the fruit is small and growth is relatively slow, carbohydrate demand is not great. Fruit is more difficult to thin unless fruit is stressed. Sevin (carbaryl) and NAA (naphtha- leneacetic acid) are thinners of choice, although NAD (naphthaleneacetamide) is useful on some varieties at this time. Trees might become extremely stressed if temperatures are high, increasing carbohy- drate demand, but light conditions are poor, resulting in lower photosynthesis. Exces- sive thinning can occur when nights are warm and days are cloudy. The carbohydrate model monitors these conditions and rates the potential for carbohydrate deficiency. "Chemical thinning is most successful when fruits are from 7 to 14 mm," Greene said. "Fruit growth is proceeding rapidly. At high temperatures and/or low light levels, excessive thinning is possible." Growers may need to lower chemical rates to prevent overthinning. Chemicals used at this time include NAA, carbaryl, and BA, usually in combination. Thinning effects are additive, he said. Greene said of the 18 to 25 mm size, "This is the period when it is very difficult to thin, even when there is a carbohydrate deficit. We have very few thinners that work well and consistently at this fruit size." Fruit is physiologically different at this size, he said. Starch accumulation is going up, and ethylene production is going down. Because it is so hard to thin at this fruit size, Greene has concentrated his attention there, hoping to give growers a workable alternative to hand thinning. Greene's experiments showed that larger fruits are harder to thin because they have more carbohydrate reserves and are less vulnerable to stresses and because the seeds in the apples produce auxins that prevent abscission. So, he believes, fruit thinning may be possible if auxin movement from the seeds to the stem, where abscission occurs, can be restricted. Alternatively, ethylene increases auxin destruction. Ethephon has been used as a rescue thinner, mixed with carbaryl, because it will knock larger apples off trees, but it carries some risk and uncertainty, he said. Greene is working with some newer chemicals like metamitron, a herbicide that inhibits photosynthesis, 1-aminocyclopropane-1-carboxylic acid (ACC), which pro- duces ethylene, and abscisic acid (ABA), which causes abscission. All of these potentially could make chemical thinning effective on larger apples. Fruit growth model Greene is best known for his work on the fruit growth model. "To thin effectively, more than one thinner application is generally required," he said. "We have had no way to tell if or how well thinners worked until after the thinning window of opportunity has passed. This model was developed to serve as a tool to assess the effects of previous thinners, usually within seven days of application." The fruitlet model is based on two observations, he said. "The first observation is that fruit that persist will start to grow rapidly a few days after fertilization, and their growth will continue somewhat regularly and without interruption throughout the season." The second observation is that fruit destined to fall off would grow slower well in advance of the time they actually fall off. "Under most circumstances, measuring the reduction in fruit growth between four and seven days after thinner application has proved sufficient to determine if a fruit will continue to grow or to abscise. All fruit that slow to a growth rate of 50 percent or less of the growth rate of the fruit that persist to harvest will ultimately stop growth and abscise." The measuring procedure was described in an article in the April 15, 2010, issue of Good Fruit Grower. Here is a brief summary: • Select 10 to 20 spurs per tree on 5 to 10 trees (50 to 80 spurs). Mark and identify individual fruits on each tagged spur. Greene uses a permanent marker to write a number on each fruitlet. • Using a caliper (digital readout is handy), measure each fruitlet starting no earlier than when it reaches 6 to 7 millimeters diameter and record each fruit's size. • After thinner application, measure fruit. As few as two measurements may be enough—one starting four days after application and another three to four days later. • Predict which fruitlets will drop off—those failing to grow at least half as fast as the fastest growing. • Fruit growth after thinner application Fruitlets destined to fall off grow noticeably more slowly within 3 to 4 days of the thinning application. 0 2 4 6 8 10 12 14 Days after application of NAA Fruit diameter (mm) 22 20 18 16 14 12 10 8 Persist Abscise SOURCE: Duane Greene, University of Massachusetts CHEMICAL THINNING expert D r. Duane Greene has been a professor at the University of Massachusetts at Amherst for more than 40 years. He earned his doctorate in 1968 from Michigan State University, where he was mentored by Dr. M. John Bukovac, who was doing pioneering work with thinners and other plant growth regulators on apples and cherries. At the University of Massachusetts, he conducted physiology studies on tree fruit hormones and plant growth regulators. Over the years, he has studied vegetative growth control, flower bud formation, fruit set, fruit abscission, and documented the effects of growth regulators on postharvest fruit condition. Chemical thinning is an area he has emphasized for more than 30 years. Initially, this involved traditional thinning studies, but that evolved into more fundamental studies of why fruit abscise. He developed his fruitlet model that helps growers predict how effective chemical thinning applications will be soon enough for them to make repeat applications during the time frame when fruits can be thinned. He has also worked to find solutions to the problem of preharvest fruit drop. Greene has been active in pomological societies. In March, he received the Outstanding Research Career Award from the Environmental Physiology of Fruit Crops Working Group of the International Society of Horticultural Scientists. —R. Lehnert Duane Greene