Dr. Duane Greene of the University of Massachusetts provided a brief overview of postbloom thinners and their benefits during tree fruit talks in Grand Rapids, Michigan:
Sevin (carbaryl)—One of the most versatile and safe postbloom thinners. Can be applied almost anytime when the tree is susceptible to thinners, and is the only thinner that is not rate sensitive. Hard to overapply, which is why it is considered "safe". There is concern of the impact of Sevin on mite predators, though some believe predators have built up resistance to carbaryl due to its widespread use. Carbaryl comes in different formulations, but generally, Greene recommends a rate for thinning between .5 to 1.0 pound per 100 gallons of water. If applied at petal fall for insecticidal purposes, 2 pounds per 100 gallons is acceptable. Though a grower favorite, he warns that Europe is in the process of banning the use of Sevin in orchards, which could mean a similar trend in the United States. He believes a replacement chemical is needed.
NAA (napthaleneacetic acid)—One of the most potent thinners, it that can result in overthinning, especially if warm weather follows application. Sometimes, NAA reduces crop load without increasing fruit size. Can result in pygmy fruit in some varieties. NAA is used when aggressive thinning is needed and carbaryl won’t get the job done, Greene said. Rates range from 3 to 15 parts per million. "NAA should be treated with respect, but not feared."
NAAD (napthaleneacetamide)—Similar in structure to NAA, but a milder version. Used less frequently than NAA; more popular as a petal fall spray in early varieties or when fruitlets are in the 7- to 12-millimeter diameter stage. Can sometimes cause leaf wilt. Rates used range from 25 to 50 ppm. "This is a thinner that is underutilized and not usually included in thinning research," he said.
BA (benzyladenine)—Also known as MaxCel, this is the newest thinner on the block. Initially, formulations contained gibberellic acid, but the more recent formulation is BA alone, which has been found to work better, he said. Considered a mild thinner, but when used in combination with carbaryl, turns into a potent thinning combination. Recommended rates range from 5 to 150 ppm. BA increases fruit size by reducing competition, but also by increasing cell division. Generally, is applied later and works better when temperatures are warmer after application. Has been somewhat ineffective when applied near petal fall.
Ethrel (ethephon)—Not a mainstream thinner, but its greatest value is as a "rescue" thinner when fruit is 20 mm or more in diameter. Greene calls it a "Hail Mary" thinner—it has a place when there are no more thinning options. Rates suggested are 250 to 300 ppm when used in combination with carbaryl.
Combination—Greene encourages growers to consider using combinations of thinners, such as BA and NAA, BA and carbaryl, and NAA and carbaryl. Combination sprays are generally applied at lower rates, but they reduce the chance of negative side effects.