Apogee application time in apples

When applying Apogee to control vegetative growth in apple trees, growers should consider timing, rate and compatibility.

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. 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 plant pathologist George Sundin’s work has shown that Apogee greatly reduces the potential for shoot blight. When applying Apogee to apples, growers should consider timing, rate per acre and compatibility with other chemistries in the tank.

Apogee 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. In recent years, we are recommending growers apply slightly earlier than petal fall as most growers miss that king bloom petal fall timing, and if there is more than 3 inches of growth, Apogee will not work as well. Growers should try and time these applications for less than 3 inches of shoot growth, which in many years coincides with king bloom petal fall. 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.

Apogee 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. 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 2/3). Best results are achieved when the seasonal rate is split into three or four sprays. For example, Apogee applications should be applied 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 or prior to 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 (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 for many days this coming week, a higher rate of Apogee is recommended as the epiphytic infection potential (EIP) for fire blight is high at all sites across northwest Michigan.

Apogee compatibility

Apogee is not compatible with calcium or boron in the tank. We also recommend Apogee be applied after the 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.

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