It is one of the most widely used plant growth regulator in the world. It is used not just in apples, but other crops such as table grapes, oranges, mandarins, pineapples, stonefruit, sugarcane, tomatoes, macadamias, olives, bananas, mangoes and barley to name a few.

Ethephon (2-chloroethylphosphonic acid), which is most commonly sold as Ethrel® 720, is a plant growth regulator first reported in the 1940s.

The chemical, when applied as a foliar spray, will enter the plant where it is subjected to a higher pH converting it to ethylene. It is ethylene that causes the plant’s physiological responses to occur. In this conversion the plant releases chlorides and phosphonates.

Apples naturally produce ethylene throughout the life of the plant and it is involved in regulating ripening, leaf abscission and flowering. Ethylene in its pure form is a gas and there are difficulties with applying gaseous compounds in the field so, instead, growers take advantage of responses to ethylene by using ethephon.

Use in apples

In apples, Ethephon is the most versatile of the plant growth regulators and is effective as an early and later thinner, it can reduce biennial bearing, increase return bloom, reduce excess vigour, enhance colour development and bring forward maturity.

Ethephon is fully registered by the Australian Pesticides and Veterinary Medicines Authority (APVMA) for apples for all uses just mentioned including the advancement of maturity and enhancement of red colour of apples. In Australia, the maximum residue limit was first set in 1975 when ethephon was subsequently registered. The original research and recommendations around this product tends to be on older varieties no longer grown and tends to be hard to come by and less research would be available for the main varieties now grown such as Gala and Cripps Pink.

Ethephon as a thinner

Ethephon is a product of high industry importance mainly due to its use as a thinner. It is at its highest activity at pink bud stage and this activity diminishes as flowering advances to the point where little effect is reached at petal fall. As a thinner, it is most commonly used at early stages of bloom near balloon blossom stages of flowering.

Another later opportunity exists to use ethephon as a thinner when fruitlets are 20 to 30mm in size. This later use is less reliable however and not recommended as part of a normal thinning strategy. The negatives are that it is less reliable, it does not improve fruit size and it can cause harvest effects such as yellowing of fruit.

In circumstances when growers may have been hit by hail or any other event and would prefer not to salvage any fruit for the season, then Ethephon can be used to remove the crop. This use will encourage a strong return bloom but will hold back excessive vegetative growth and a general biennial bearing habit will likely commence.

The amount of thinning that will occur depends heavily on temperature making it difficult to give set recommendations and, added with the complexities of fruit set and varieties, growers are urged to get local agronomic advice and follow the label. Ethephon is just one of the many thinners available and used in combination with other products.

When used as a thinner, it has little to no effect on the storability of the fruit.

Enhancing colour and advancing maturity

While not very commonly used, ethephon can also be applied to enhance colour and advance maturity. The application of Ethephon accelerates the ripening process by slowly releasing ethylene, which stimulates anthocyanin accumulation (reddening) in apples. The main motivator for using it is for its colour enhancement and researchers have yet to find a good way to eliminate the ripening effect.

When used for colour enhancement on earlier varieties, Ethephon is applied 2-3 weeks before the usual harvest timing. Depending on the variety, the effect can be quite rapid and quite visual within a week, which can bring forward the first pick by about one week. Applications to early varieties can cause more problems mainly due to heat and because there is a higher risk of fruit maturity outturn problems that are worsened if the fruit is stored, or not sold quickly.

In later maturing varieties, Ethephon can be applied further out from normal harvest timing by up to four weeks. The exact application practices vary slightly from grower to grower, but it’s generally a single application at a higher rate or two or more applications at a lower rate. The expected effect would be to harvest one week earlier.

A further point is that harvesting earlier can potentially reduce fruit size and yield. It would be expected that fruit size in Galas would be more effected than for Cripps Pink.

Currently, the weekly growth rate of Cripps Pink at harvest is down to just 0.9mm/week whereas Gala, when they are picked in February, are still growing at a rate of above 2mm/week at harvest.

Fruit that is treated with Ethephon should be treated differently. as the storage life may be reduced. The reduced storage life of Ethephon-treated fruit may come from: using rates that are too high; too large an interval between application and picking; high temperatures after application; and failure to cool fruit in sufficient time for storage.

A side effect that can be observed is increases in skin waxiness, which may lead to difficulties achieving a high gloss when applying wax.Trees that are weak or under stress should not be treated at all with Ethephon.

Better alternatives

For colour enhancement and maturity advancement, Crawford suggests that the more common agronomic methods used by industry are those that improve light distribution within the tree’s canopy, such as summer pruning, leaf plucking and reflective mulches (SunUp, Extenday™). He adds that these are more expensive, but often more reliable than plant growth regulators that depend on the correct rate, weather conditions, tree conditions, variety and other factors being well managed.

Another option is the use of ReTain® that can work with colour in delaying harvest into cooler weather more suited for colour development. ReTain works by suppressing ethylene production making it effectively the opposite of Ethephon.

SmartFresh™ used after harvest shuts down the effects of ethylene production by interacting with the receptors of ethylene. Ethephon used in conjunction with SmartFresh has not been recommended as no one can guarantee that SmartFresh will have the same effect in extending storability and maintaining quality in Ethephon-treated fruit as on fruit that has not been treated.

For growers, it is extremely important that we produce apples that look good with an attractive colour. Colour development depends on many environmental and physiological factors including the right varieties, management of nutrition and experiencing favourable weather. Ethephon can be part of crop management to improve colour, but it shouldn’t relied on, alone. Getting more light exposure in the canopy is probably the most critical factor for achieving good fruit colour.

Proper understanding of Ethephon is vital to ensure growers can keep using it in its many potential ways in the growing season without any unwanted side effects. Poor quality apples seen by retailers can be caused by any number of factors. Nevertheless, the industry needs to be on full alert when considering using Ethephon close to harvest to avoid any quality issues.

Source: Angus Crawford, APAL Technical Manager