In the absence of carbaryl, growers will need a palette of chemical thinning options and strategies.
Several new fruit thinning products are on the horizon that might serve as replacements for carbaryl, but none are close to commercialization in the United States.
Dr. Duane Greene, horticulturist at the University of Massachusetts, has been testing alternative products and strategies on the assumption that carbaryl (sold under the brand name Sevin) will not be available to U.S. growers for much longer because of regulatory concerns. It is no longer registered in Europe, and production has stopped in the United States.
Greene said growers should not expect to see another product that is a direct substitute for carbaryl. They will need a full palette of chemical options in order to adjust their crops to the appropriate levels.
On the horizon
A product that might someday be added to the thinning repertoire is metamitron, a photosynthesis inhibitor.
A theory about how thinners work is that young fruitlets grow at a very high rate after pollination, greatly increasing the tree’s need for carbohydrates, which are produced through photosynthesis. When photosynthesis is limited, this can result in the tree shedding fruit.
Metamitron is used as an herbicide in sugar beets in Europe. It has been extensively tested as a thinner in Europe, and is undergoing registration for that use. The product creates a photosynthetic deficit in the tree that lasts for up to 12 days.
Greene said if its efficacy can be demonstrated in the United States, it might be a suitable product for Washington State where high light levels normally result in high levels of photosynthesis.
In his tests last year on CandyCrisp apples, metamitron at 50 parts per million (the lowest rate he tested) gave similar results to a combination of MaxCel (100 ppm) and carbaryl (1 pound). The applications were made at the 15-millimeter stage.
“It appears to have promise,” he said. “It shows strong thinner activity and appears to have no unfavorable fruit effects, although you can get phytotoxic effects at higher rates.”
Another potential thinning tool is abscisic acid (ABA). Greene said he’s been testing it since 2003, but had inconsistent results. He found that ABA applied at relatively high rates did thin, but questions remain, such as whether it needs to be combined with MaxCel to thin well and whether the rates needed for good results are economical.
Dr. Greg Clarke, senior field development scientist with Valent Biosciences, said ABA is a natural hormone found in a number of plants, including apples and cherries. It can cause the stomates on a plant to close, controlling water release and photosynthesis. It is also involved in adaptation to stress, promotion and inhibition of growth, maturation and ripening, senescence, abscission, and dormancy. Scientists are just beginning to discover some of the practical uses it could have in agriculture.
One of the reasons it has not been used in agriculture until recently was the high cost of the pure chemical, but Valent Biosciences has developed a way to produce the molecule at a lower cost.
Two commercial abscisic acid products are on the market: ProTone, which enhances red color in table grapes, and Contego Pro, which is designed to reduce drought stress in ornamental plants. Clarke said ABA can also help plants that are sensitive to chilling injury, such as basil, to withstand the cold.
A better understanding of how it might cause apple thinning and the reason for variable results will be needed before the company can go ahead and develop the product as an apple thinner, Clarke said.
Some studies with ABA, both by Greene and other scientists, have shown a good thinning effect, while others haven’t. “It’s kind of frustrating,” Clarke said. “There’s some exciting things here, but we haven’t got to the point where we fully understand what ABA is doing for thinning, and the variability is driving us a little nuts. We’re trying to find ways to make better sense of this. It’s interesting, it shows some potential, but we haven’t worked out all the kinks yet.”
Clarke said it’s not being explored for color enhancement in apples because the biochemical systems for color development in apples and grapes are not the same. In tests on apples, applications three to four weeks before harvest removed 30 to 40 percent of the crop.
“It’s called abscisic acid for a reason,” Clarke noted. “It’s active in grapes and works pretty well, but even on paper, with what we know about the physiology of apples, it was a long shot.”
Greene said he has tested ABA as a thinner on cherries, applying it at bloom and petal fall. As a blossom thinner, it worked fairly well, he reported. “It was good enough it would be worthwhile looking at,” he added, noting that there would need to be more work to find out the best rate.
Another possibility is ACC (1-aminocyclopropane-1-carboxylic acid), a naturally occurring product that is a precursor to ethylene in the biosynthesis pathway. Greene tested it at two timings—10-mm and 20-mm fruitlet diameter—and found it worked best at the later timing with an increase in fruit size. “I think it may be a viable thinning option,” he said.
Clarke said this compound is of academic interest, but a long way from being a commercial product. Tests show that it can be a strong thinner in apples, with a wide window of application from 10- to 20-mm fruitlet size. “If there’s an opportunity for a rescue thinner, this and metamitron would be serious contenders,” he said. ACC also shows promise in combination with MaxCel, but its down side is that it causes significant leaf drop. Up to 30 percent of the leaves can be lost.
In tests, it has been effective as a postbloom thinner of peaches, which would be of great interest to growers in major peach producing regions, Clarke said. “A postbloom peach thinner is the Holy Grail in some ways, but what makes it unacceptable is the amount of leaf drop.”
The product has also given variable results, depending on the variety of peach.
Greene concluded that while there are no silver bullets for thinning, there are options, and U.S. apple growers are fortunate that they still can use carbaryl for a little longer while alternatives are explored.