And what are best practices for use. By Karen Theron, Schalk Reynolds, and Schalk van der Merwe.
Most fruit trees set far more fruit than they can support. Trees respond naturally by shedding fruitlets. In apples and some pear cultivars, this response is usually too weak to produce fruit of the size growers want. We therefore apply chemical thinners to stimulate fruit drop.
There are several thinning chemicals registered for use on apples and pears in South Africa: 6-benzyladenine (6-BA); 6-benzyladenine plus gibberellic acid (6-BA plus GA); 1-naphthaleneacetamide (NAD); 1-naphthylacetic acid (NAA); metamitron; and carbaryl.
These are available in different formulations under various trade names. Different formulations are registered for different cultivars, and each has its own instructions for use. The summary below is a broad overview: always study product labels carefully for the details!
Current thinning agents at a glance
Trade names: Bapsol, Exilis, MaxCel.
6-BA can be used as a single application on Bon Chrétien, Golden Delicious, Granny Smith, Cripps Pink, and Royal Gala — apply when king fruitlets are 8–12 mm in diameter. It can also be used in combination with NAA on Fuji, Golden Delicious, Cripps Pink and Royal Gala. Apply as a tank mix at petal drop, or when king fruitlets are 10–12 mm in diameter if using after NAA. Temperatures should remain above 18 °C for two to three days following application, but temperatures above 28 °C can lead to excessive thinning.
6-benzyladenine plus gibberellic acid (6-BA plus GA)
Trade names: Elppa, Gibbalin, Perlan, Promalin.
6-BA plus GA is registered for thinning of Golden Delicious, Granny Smith and Royal Gala. 6-BA plus GA is usually combined with NAD, separately or as a tank mix, with the specific programme depending on cultivar and formulation. It can also be used alone in two applications: the first at 85 % full bloom and another 14 days later.
1-naphthylacetic acid (NAA)
Trade name: PoMaxa.
NAA is registered as a single application for easy-to-thin cultivars including Granny Smith, standard red cultivars, and young, vigorous Royal Gala types. Spray between petal fall and a maximum fruitlet diameter of 5 mm. NAA can result in overthinning in Granny Smith and must not be used in combination with other thinners in easy-to-thin cultivars.
In difficult-to-thin cultivars —red spur types, Braeburn, Cripps Red, Cripps Pink, full-bearing Royal Gala types, Golden Delicious and Fuji — application of NAA can be followed by 6-BA when fruitlets reach 8–12 mm diameter. Alternatively, NAA can be combined with 6-BA in a tank mixture and sprayed between petal fall and when fruitlets reach 12 mm in diameter. Later application can cause pygmy fruit in susceptible cultivars such as Fuji.
Trade name: Amid-Thin W.
NAD is used as a single application on Golden Delicious and Royal Gala two to six days after full bloom. It is also registered for use with 6-BA plus GA in Golden Delicious, Granny Smith and Royal Gala. NAD has lost ground to better and safer thinners.
Trade name: Nevis 150 SC.
Metamitron is a photosynthetic inhibitor and used as a single application only. It is registered for use in Abate Fetel, Cripps Pink, Early Bon Chrétien, Forelle, Fuji, Golden Delicious, Granny Smith and Royal Gala. Spray from the end of flowering up to an average fruitlet size of 10 mm in diameter. Consult a specialist if a follow-up application of another agent is considered necessary.
Trade names: Carbaryl 850 WP, Karba 850 WP, Karbacure, Sevin XLR Plus.
Carbaryl is best known as an insecticide, but it is registered as a thinner for apples, including difficult-to-thin cultivars like red spur types, Braeburn, Cripps Red, Cripps Pink, full-bearing Royal Gala types, Golden Delicious and Fuji. The dose is adjusted according to cultivar and a lower dose used when conditions favour thinning. Carbaryl is no longer allowed in countries such as the United Kingdom and Germany. South Africa will probably follow this trend.
What’s next for thinners?
Researchers are always working to produce new agrochemicals. One exciting development is the evaluation of S-abscisic acid (S-ABA) on Forelle pears. S-ABA showed promise when applied at the 8–12 mm fruitlet stage. An increase in fruit size was observed in addition to thinning. S-ABA is already registered in South Africa for other applications, which should expedite registration as a thinning agent.
1-Aminocyclopropane-carboxylic acid (ACC) is a precursor of ethylene occurring naturally in plants. It is an effective chemical thinner of Japanese plums and some peach cultivars and is currently being evaluated on Cripps Red, Fuji, and Royal Gala apples. Both S-ABA and ACC may eventually be permitted in organic farming.
Factors influencing the efficacy of chemical thinners
Cultivars can be divided into easy, difficult, and very difficult to thin. The easy cultivars are Granny Smith, Starking and Top Red. Braeburn and Cripps Pink are difficult, and Fuji, Cripps Red, Golden Delicious and Royal Gala are very difficult. The general approach with a very difficult cultivar is to use a programme that involves multiple products and applications.
Less vigorous trees — those with low nitrogen or other reserves; moisture or other stress; shading; cold damage; or any condition that inhibits normal growth and fruit set — are more susceptible to chemical thinning. Weak spurs in the lower, inner parts of the tree thin more easily. Young trees thin more easily than older trees. In general, chemical thinners should only be applied to healthy, actively growing trees.
Chemical thinning is optimal at 20–27 °C. Cool temperatures, high humidity or rain, and low irradiance increase uptake of thinning agents by leaves, leading to more severe thinning. Spur leaves exposed to frost before flowering absorb chemicals more readily. Frost also damages flowers and lessens pollen tube growth and fertilisation, thereby increasing fruit abscission. Sunny weather directly after a cool period stresses the tree resulting in more severe thinning by chemical agents.
Heavy flowering and fruit set, especially in successive years, facilitates chemical thinning. Thinning is easier when fruit set in clusters rather than singly.
Image supplied by Elise-Marie Steenkamp | Hortgro.