Bioregulators mimic the action of plant hormones and are valuable tools in the horticultural management of tree fruit. Different products may induce a broad range of responses in plant growth, but in fact, the same compound can also have completely different effects depending on the rate and timing of its application. For example, the auxin analog naphthaleneacetic acid (NAA) reduces fruit set in apple when applied right after petal fall, increases return bloom when applied during the middle of the growing season, and also reduces fruit abscission when applied shortly before harvest. Many bioregulators have side effects beyond their intended uses and it is important that growers understand all potential outcomes of application of any growth regulator to trees or fruit of a specific cultivar at a given developmental stage.
As with other materials, spray efficacy is a function of complete coverage and the relative absorption of the bioregulator by the plant tissue. Slow-drying conditions (cool temperatures, high humidity, dense canopies, no wind) tend to amplify the effects of the material, while fast-drying conditions (hot temperatures, low humidity, open canopies, wind) can diminish its efficacy. Abundant surface area of young, healthy leaves tends to increase material absorption and trees in this state may exhibit greater responses than trees with fewer, smaller, and/or older leaves. Typically, bioregulators are most effective with thorough coverage and wetting, but do not require excessive drenching; many materials are not translocated within the plant, so direct application to target tissues (leaves, flower clusters, fruit, etc.) is often critical to achieving desired responses. Many bioregulators are sensitive to pH and label directions should be carefully followed regarding spray tank acidity.
Elevated response or injury may be observed from applications of bioregulators under the following conditions:
- high temperatures
- low-vigor / weak trees
- high concentration of active ingredient in spray tank
- use of surfactants
Caution should be exercised when one or more of these conditions are present.