The maturity of our early apple varieties, Gala and Honeycrisp, seems to be more variable than usual in some parts of Michigan. Researchers in Minnesota have also reported similar variability.
This trend was noted in last week's maturity samples in the Michigan State University Extension apple maturity reports and has continued to be evident this week. In this case, multiple harvests may be needed to minimize variation in fruit maturity to optimize fruit storage potential.
The relationship between ethylene and starch index has been pretty good in the samples we have been evaluating from growers orchards so far this season.
See the simple graph for some Honeycrisp samples from east and west Michigan.
Importantly, this variability seems to have been suppressed in those blocks receiving ReTain treatments a few weeks back. The early suppression of ethylene formation in the weeks preceding harvest likely minimizes some of the orchard and canopy factors that would otherwise lead to greater variability.
Provided color development is not negatively affected and a timely harvest can be managed, the outlook is good for storability of ReTain-treated fruit.
Even so, there have been some reports of ReTain treatments lacking full effectiveness. This could be a function of the environmental conditions at the time of application.
The longer the drying time for polar compounds like acids (the active ingredient in ReTain is an amino acid), the better the penetration. Penetration of polar compounds is always a challenge given the waxy (non-polar) coating of the apple fruit. So, application during the more humid times of the day or during cloudy periods can benefit uptake of the active compound.
Interestingly, the ethylene action inhibitor 1-MCP (Harvista) does not have this limitation. Its route for penetration into the apple fruit is thought to be through the lenticels as a gas. Harvista/1-MCP, like ReTain, has been shown to reduce variability in fruit maturation as well and can be effectively applied closer to harvest.
|Relationship between starch index and ethylene in Honeycrisp samples.|