MSU Extension

Apogee is a plant growth regulator composed of prohexadione-calcium that can be used in apples with significant advantages to the grower. Prohexidione-calcium reduces terminal growth by inhibiting important enzymes that help form growth-specific gibberellins. This group of plant hormones is primarily responsible for regulating shoot elongation in apple trees. In laymen’s terms, Apogee helps control tree vigor. Controlling vigor can reduce the amount or intensity of pruning, decrease internal shading – a major proponent to properly color apples – and reduce canopy density for thorough pesticide coverage.

This product has also been a reliable tool for minimizing impacts of shoot blight caused by the fire blight pathogen Erwinia amylovora. Shoots that have less growth are not as susceptible to fire blight, and Michigan State University Extension specialist George Sundin’s work has shown that Apogee greatly reduces the potential for shoot blight. When applying Apogee to apples, growers should consider the following: timing, rate per acre, thinning relationships, and compatibility with other chemistries in the tank.

Timing

Apogee should be applied when vegetative shoot growth is less than 3 inches. To best time the application, there is a seven- to 10-day window beginning at king bloom petal fall – likely most growers have made this initial application. This timing applies to most varieties in most years. Two more applications should be made at two-week intervals following the bloom application. Sometimes a fourth application is needed when excessive rainfall or light crops increase vegetative growth.

Rate

The rate per acre is usually calculated on a tree row volume basis and can be adjusted to two-thirds of the full-rate. The two-thirds rate is the starting rate growers should consider if they have not had experience with using Apogee. Growers with past experience will know if this two-thirds rate is too high or too low for a particular block. This suggested two-thirds rate per acre is a season-long rate. For example, if trees are at 75 percent tree row volume, then 24 ounces per acre is the seasonal rate (48 x 0.75 x 0.67).

Best results are achieved when the seasonal rate is split into three or four sprays. For example, Apogee applications at 8 + 8 + 8 ounces per acre for a total of 24 ounces per acre per season. When the fire blight risk is high, the first application of Apogee at king bloom petal fall timing should be increased to as much as 150 percent of the split rate. For example, the rate should be increased from 8 ounces per acre to 12 ounces per acre. If the first spray rate is increased, subsequent sprays, such as second and third sprays, should be reduced. The seasonal application would be 12 + 6 + 6 = 24 ounces per season instead of 8 + 8 + 8 = 24 ounces. If temperatures continue to remain high with the potential for rainfall on Sunday, June 15, 2014, and into the beginning of the week, a higher rate of Apogee is recommended.

Thinning

Apogee tends to increase fruit set, hence more aggressive thinning is often needed. If using Apogee, growers should increase thinning by 10 or 15 percent. For example, if the rate to thin was 1 pt Sevin + 8 ppm NAA, the thinning rate in blocks where Apogee has been used should increase to 1 pt Sevin + 10 ppm NAA.

Compatibility

Apogee is not compatible with calcium or boron in the tank. We also recommend that Apogee be applied after thinner application. If the two-week timing interval is also the ideal time to thin, make the thinning application first and follow with Apogee a few days later. Growers should read the Apogee label carefully. Apogee must be used with an organosilicone surfactant, and an equal weight of spray grade ammonium sulfate should be applied. Do not use Apogee on ‘Empire,’ ‘Stayman,’ or ‘Winesap’ because of the potential for fruit cracking.

To conclude, Apogee is an excellent tool to help control vegetative growth, which decreases the need for summer pruning and can suppress the spread of fire blight among shoots and within shoots. The above recommendations are the best way to maximize the use of Apogee.

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