MSU Extension

Ethephon is a plant growth regulator (PGR) and results from its use vary with chemical concentration and time of application. As with many PGRs, ethephon has systemic properties that allow it to penetrate plant tissue and is eventually decomposed to produce ethylene. In cherry systems, ethephon is used to promote fruit loosening to assist with mechanical harvest of fruit. Ethephon, sold under the trade name Ethrel, is a standard management practice in tart and sweet cherry harvest.

Ethephon releases ethylene, which penetrates plant cells and binds to receptors that affect expression of various genes. In the case of cherries, ethephon affects the gene that controls the synthesis/activation of cell wall loosening enzymes, thus dissolving the pectins between cells in the abscission layer. This chain-like reaction leads to cell separation in the developmentally-programmed abscission zone between pedicel and fruit or pedicel and spur. In short, ethephon loosens the cherries from the stem, which results in a gentler “shaking” of the tree to remove the fruit.

Michigan State University Extension reminds growers that in years past, we have observed ethephon-induced damage in hot and dry weather, especially in sweet cherries. Ethephon can have excessive activity under hot and dry conditions, which can result in tree injury. Of sweet cherry varieties, Golds were observed to be the most sensitive. If temperatures are in the high 70s to mid- or upper 80s and sunny during the 72 hours following application, this weather could be conducive for causing Ethrel damage; the magnitude of ethephon response is increased at higher temperatures following application.

Tree vigor also influences the degree of response achieved by an ethephon application. Trees low in vigor or under stress due to drought, cold damage, San Jose scale infestation, disease, virus, phytotoxic injury, etc. will respond to a greater extent, and gumming and leaf abscission may result. Hence, growers may choose to reduce rates in orchards that are stressed, particularly if temperatures will be higher with the potential to cause injury.

Crop load is also a factor when determining what rate to use. Heavy crop loads are typically more difficult to loosen compared with light crop loads. Many orchards have a heavy crop load this season, and these growers may need to use a higher rate or leave extra time to achieve optimal loosening. Again, an increased rate could cause injury if temperatures are high following the application. Furthermore, growers should be prepared that if an orchard is taking a longer time to loosen, then the orchard may need to be treated with an insecticide that is effective against spotted wing drosophila to prevent larvae in fruit. Please review MSU’s “2016 Fruit Management Guide” and “Managing Spotted Wing Drosophila in Michigan Cherry” bulletin, as well as insecticide labels, for additional information on efficacious insecticides and pre-harvest application intervals. Balancing spotted wing Drosophila management and harvest will take increased consideration at the grower level and good communication between growers and processors.

The following recommendations should be used when applying ethephon to cherries:

• Rate. Vary the rate depending on anticipated temperatures for 72 hours after application, days before harvest, tree stress and past experience. Lower rates decrease the likelihood of tree injury. If temperatures 72 hours after application are predicted to be in the 80s, growers should reduce the Ethrel rates.
  • Light sweets. When applied concentrate (80 gallons water per acre or less), 1 to 2 pints per acre applied 10-14 days before anticipated harvest should provide adequate loosening. Rates up to 2.5 pints per acre may be necessary for harvesting in less than 10 days. When applied dilute, use no more than 0.75 pint per 100 gallons or 3 pints per acre. Reducing rates in light sweet cherries, particularly Golds, is recommended if predicted temperatures are in the 80s after application.
  • Dark sweets. When applied concentrate, use 1.5 to 2.5 pints per acre applied 10-14 days prior to anticipated harvest. Rates up to 3 pints per acre may be necessary for harvesting in less than 10 days. When applied dilute, use no more than 1 pint per 100 gallons or 4 pints per acre.
  • Tart cherries. When applied concentrate, use 0.5 to 1 pint per acre applied seven to 14 days prior to anticipated harvest. When applied dilute, apply no more than one-third pint per 100 gallons or 1 pint per acre.
• Time of application. Apply approximately seven to 14 days before anticipated harvest. Do not harvest within seven days of application (Ethrel has a seven-day pre-harvest interval).
• Temperature. Avoid application when high temperatures are expected to exceed 80 F or remain below 60 F for the 72-hour period after application. Growers should use lower than normal rates when highs are expected in the 80s.
• Tree stress. Do not spray trees that are low in vigor or under stress conditions.
• Do not spray trees that had serious gumming the previous year.
• Crop load. Heavy crop loads (i.e., low leaf to fruit ratio) are more difficult to loosen than lighter crops. There is a heavy crop load in many orchards this season, and growers may need to use relatively higher rates or expect a longer time to achieve desired loosening. In trees with a light crop, reduced rates are recommended and rate reductions in light blocks will still achieve adequate loosening while minimizing the potential for injury in hot conditions this season.
• Concentrate spraying. Applying ethephon with concentrate sprayers (i.e., 80 gallons of water per acre or less) achieves the same level of loosening at lower rates per acre than does dilute applications. Uniform coverage is important.
• Tree size. Suggested rates per acre are based on full-sized trees. Adjust rates downward when treating blocks with smaller trees.

Growers should pay particular attention to the temperatures after the time of ethephon application. As evident from past experiences, hot temperatures can do damage to cherry trees. Growers that have had problems in the past years should reduce rates, especially if the trees showed serious gumming and leaf loss.

Dr. Rothwell’s work is funded in part by MSU’s AgBioResearch.