Ethylene Management Tool Best Applied to Trees That Are Not Stressed

This is a good topic of discussion for 2007 as growers anticipate earlier harvest dates for many of the apple cultivars that are worthy of long term storage. The earlier dates this year are related to bloom dates and seasonal stresses including moisture deficits, high heat loads in the orchard, excessive soil temperatures and high UV ratings throughout the summer. Indicator cultivars including early maturing apples and tender fruit show that varieties are coming on stream in 2007 much earlier than average and in some cases earlier than ever for growers with good record keeping systems.

It is not that long ago that we did not really have any good way to control ethylene production and ensuing fruit maturity in the canopy of an apple tree in late summer. Crops more or less ripened when they wanted too and growers had to work around those dates. For many growers it meant in some cases watching tons of fruit drop to the ground or lose storage potential as the season progressed. This was especially true with cultivars that produced high levels of ethylene in the orchard like the various McIntosh strains.

ReTain has been used with various degrees of success now for several years in Ontario and in the Great Lakes Region on different cultivars. It has helped growers to manage their harvest operations and stretched out the windows of opportunity during the harvest season. What happens when ReTain is used successfully is that crops are delayed in their maturity, producing fewer drops, and resulting in better postharvest quality (reduced cracking in the stem bowl and factors like watercore are controlled). This response is the result of an inhibition of ethylene production in maturing apples.

When ethylene is managed, crops hang longer and maintain best quality attributes longer in the canopy, allowing the grower to get more quality fruit into the bin on a per acre basis. There are very few apple cultivars out here that hang well naturally without quality degradation on the tree.

Silken does come to mind as one cultivar that has a very wide window of opportunity naturally without the use of any ethylene management tools. Silken is however not considered to be a candidate for long term storage.

Cultivars vary in their sensitivity to ReTain. Gala strains, Honeycrisp and Jonagold are very sensitive to ReTain and higher rates can inhibit the development of characteristic colour and other fruit quality attributes. For this reason, rates are usually cut in half for these cultivars. Growers should use the 4-5 week standard when determining use dates for this product. For a cultivar like Gala it should be used 4 weeks ahead of the anticipated second picking date.

It has been my experience that ReTain will likely not override the effects of treatments used in the orchard that have been shown to accelerate fruit maturity. This would include aggressive treatments of various formulations of calcium in the attempt to combat disorders like bitter pit in cultivars like Honeycrisp.

It has been shown that aggressive use of calcium sprays can put ethylene levels in maturing fruit way over threshold levels several days ahead of control trees.

Chances of getting a response from ReTain on stressed trees or trees with poor foliage health (small, tattered and thin looking canopies) is not good. As with all growth regulators, water volume and coverage are important in getting a response to using ReTain.