How to Manage Pests
UC Pest Management Guidelines
Fruit Spray Thinning
(Reviewed 3/14, updated 3/14)
In this Guideline:
Olive trees are alternate bearing: under normal conditions, they produce heavy crops one year and a light one the next. Alternate bearing creates problems for growers; thinning is recommended to produce a more consistent yearly crop, larger and higher quality fruit, earlier maturity dates, and lower harvest costs.
The greatest obstacle to chemically thinning is variable thinning responses related to temperatures following application. Thinning response can vary from very little with unseasonably cool temperatures to almost complete crop removal with excessive temperatures. The benefits, however, outweigh potential risks. To evaluate the effectiveness of the spray thinning, it is a good idea to always leave some unsprayed trees.
SPRAY THINNING WITH NAA
The synthetic plant growth regulator, naphthalene acetic acid (NAA), is used to effectively thin fruit. It is absorbed into the leaves and fruit and then translocated to the fruit stems where an abscission layer forms within 2 weeks of application, causing some of the fruit to drop.
There are two NAA ammonium salt products currently available for use on olive in California: Fruit Fix Concentrate 800 (AMVAC Corp.) and Liqui-Stik Concentrate (Platte Chemical Co.). Fruit Fix is formulated with 800 grams of active ingredient (a.i.) per gallon, whereas Liqui-Stik has 200 grams.
Precautions When Using NAA
If used as directed and at moderate temperatures, NAA will not damage fruit or retard fruit growth. Label registrations for NAA cover the period from full bloom to 2.5 weeks after bloom. NAA applications after that point are both illegal and ineffective. Too early an application can overthin; too late an application will yield unsatisfactory results. An application during bloom can eliminate the crop.
Sometimes NAA does kill or curl young, tender tip growth on some new shoots, but this has no lasting effect. The effects of NAA depend upon dose, temperature at time of application and for about one week after, and tree condition.
Complete Fruit Removal
Occasionally it is desirable to remove the entire crop, as in the case of ornamental plantings of olives and unused ripe olives that would otherwise drop and create a nuisance and contribute to the buildup of olive fly populations. Crop removal may also be beneficial for increasing next year's yield potential by reducing competition between the energy requirement of an economically questionable crop load on small trees (2-year old trees in super-high-density plantings) and that of the current year's shoot growth.
For complete crop removal, a solution of 150 parts per million (ppm) NAA is applied in two sprays, the first 2 to 3 days before full bloom and the second a week later. If the bloom period is short, a second spray may not be necessary. A single spray may be applied at full bloom, but often crop removal is incomplete. For large trees, a power sprayer is required; 5 to 10 gallons of solution per tree may be required to give good coverage. Although NAA is not normally used to thin Sevillano variety, treatments as described will result in complete or near complete fruit removal of this variety as well as the other common varieties.
Note that spraying with NAA when the temperature exceeds 100°F (38°C) may injure new growth and may also cause some leaf drop. Tender ornamentals nearby should be covered, and drift should be avoided by spraying only under calm conditions and by using moderate pressure to apply a coarse spray.
Timing is critical to the effectiveness of a thinning spray. Treatments are applied between 12 and 18 days after full bloom. There are two methods of spray timing: one based on the fruit size and the other based on full bloom date. Both methods are acceptable under normal springtime weather conditions, but if abnormally cool weather delays fruit growth, use the fruit-size method. With both methods, treatments are applied as dilute sprays (300-500 gallons per acre.)
When the average size of young fruit is between 1/8 and 3/16 inch in diameter, apply 150 ppm NAA. With this method, the time of application will vary from 12 to 18 days after full bloom, depending on weather. Include a wetting agent or spreader-sticker, according to the NAA manufacturer's recommendation, or a spray oil.
Concentration to use
If you use a spray oil, mix 100 ppm NAA with a light or light-medium summer oil emulsion at a rate of 1.5 gallons of oil per 100 gallons of spray mix. Do not use a wetting agent if you use oil. Do not use oil with NAA when daytime temperatures are 90°F or higher or when soil moisture is low. Failure to observe these precautions may result in leaf and shoot burn, defoliation, fruit injury, and excessive thinning.
Days after full bloom method
Determine the date of full bloom, which is defined as the time when 80% of the flowers are open, 10% of the flowers are not open, and 10% of the flowers have petals falling. Petals are white (not brown), and pollen is shedding. To see whether pollen is shedding, you can run your hand down the full length of a shoot in bloom. Yellow pollen in your hand indicates shedding. Record the date of full bloom and apply NAA 12 to 18 days after that date. This method is appropriate if weather and rate of fruit development are normal between bloom and the spray date.
Concentration to use
To be effective, the spray must cover the undersides of leaves on fruiting branches, but a heavy drenching application such as is used for scale control is usually unnecessary and wasteful. NOTE: For effective thinning do not apply less than 72 ounces of Liqui-Stik Concentrate (200 grams a.i. per gallon) per acre or 18 ounces of Fruit Fix 800 (800 grams a.i. per gallon) per acre. See Table 1 for the amount of NAA to mix in various amounts of water per acre to obtain various spray concentrations.
Apply treatments with either an air-blast sprayer or a high-pressure hand-gun sprayer. When most trees bear a heavy crop, an air-blast sprayer will do the best job of covering the whole tree. For good spray distribution, drive a properly adjusted air-blast sprayer at 1.5 to 2 miles per hour. If only some trees in an orchard are overloaded with fruit, use a high-pressure handgun sprayer with number 8 discs at 200 to 400 pounds per square inch (psi) pressure.
If only certain limbs in each tree require thinning, spot spraying may be effective. It is a good idea to leave some unsprayed check areas to help gauge the effectiveness of spray thinning.
UC IPM Pest Management Guidelines: Olive
Fruit Spray Thinning
W. H. Krueger, UC Cooperative Extension, Glenn County