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Shoot Growth Suppression
Apogee®, a PGR for vigor control in apples, will reduce the length of shoot growth. Apogee is also labeled for the control or reduction of fire blight in apples. Shoot growth suppression by Apogee® is very consistent when the first application is properly timed and where a sufficient dosage is applied during the active growth season. Apogee® acts to retard shoot growth by blocking the production of gibberellic acid (GA). By decreasing the level of GA in the plant, Apogee® will inhibit the shoots’ ability to elongate, thereby resulting in shorter shoots. Since there is some residual GA in the plant, it usually takes about 10 days for shoot extension growth to slow. One application of Apogee® will last for about 2 to 4 weeks, so repeated applications are required when factors are favorable for further shoot growth.
It is critical that the first application be applied when the longest shoots on the trees are between 1 and 3 inches long. In most situations, this will be between late bloom and petal fall. Careful monitoring is essential to time this application.
The interval between sprays can range from 1 to 4 weeks, depending on tree vigor and the dosage of Apogee® that had been previously applied. Subsequent applications should be made when the first few shoots show signs of regrowth. Failure to reapply Apogee® when conditions are still favorable for growth can result in loss of growth control.
The rate of Apogee® recommended for shoot growth control varies from 3 to 12 ounces per 100 gallons of dilute spray. The rate of Apogee® to be applied per acre should be calculated by determining the tree row volume of a block and multiplying the dilute rate by this figure.
The total dosage and timing of sprays will depend on the inherent vigor in an orchard. The vigor in an orchard is dependent on many factors, including fruit load, rainfall, variety, rootstock, soil type, nutritional status, and pruning severity. Pruning is a major factor in increasing the vigor and shoot growth of trees. These factors should all be considered when selecting an application regime for a specific orchard block.
Apply Apogee® with sufficient water to obtain thorough coverage and use a nonionic surfactant. The sprayer must be able to deliver the spray to the parts of the tree that have excessive vigor. Conversely, if shoot growth suppression is desired in only a portion of the tree, it is possible to spray only that portion of the tree.
If calcium is present in the spray water, the water is said to be “hard” and it deactivates Apogee®. Therefore, when the source of spray water is hard, a proprietary water conditioner or high-grade, sprayable, ammonium sulfate should be used. The amount of conditioner to use depends on the degree of hardness. Test strips for water hardness can be obtained through swimming pool and spa suppliers.
Likewise, calcium fertilizer materials applied in the tank with Apogee® deactivates it. Therefore, it is recommended that an Apogee® application be delayed until ½ inch of rain has fallen after a spray of calcium has been applied. It is permissible to apply calcium sprays a few days after an Apogee® spray without a reduction in effectiveness.
Apogee® is labeled to decrease June drop in apples when applied at between 10 and 12 ounces per 100 gallons. This would be 30 to 36 ounces per acre for an orchard with a tree row volume of 300 gallons. However, at lower rates typically used for shoot growth control, Apogee® also may increase the set of apples. Avoid the use of excessive rates of Apogee® during the thinning window to minimize this effect. When Apogee® is used for shoot growth control, the aggressiveness of the thinning program may need to be increased. This may include increasing the strength of the thinner or making an additional application of thinner, depending on the circumstances.
The quantity of Apogee® needed per application and the number of applications needed to reduce shoot growth in a specific apple block is dependent on the inherent vigor in a block. Listed below are some suggested rates and timings that may prove to be useful.
In an orchard with a tree row volume of about 300 gallons, the following treatment regimes seemed to work well: 16, 12, and 8 ounces per acre in a pruned orchard, and 12, 10, and 6 ounces per acre in an unpruned orchard.
Management of fire blight using Apogee®
Apple trees are less susceptible to fire blight when sprayed with Apogee® for shoot growth reduction. Results obtained in Pennsylvania tests indicate that the rate of 12 ounces per 100 gallons applied in a single application when new shoot growth is 3 inches long is more than 95 percent effective in preventing shoot infection in mature York Imperial trees following wound inoculation with the fire blight pathogen. Making 2 applications at the 6 ounces per 100 gallons rate at 7-day intervals was 99 percent effective in preventing infection, while making 3 applications of 4 ounces per 100 gallons was 88 percent effective in preventing fire blight shoot infection. Apogee® is not directly active against the fire blight bacterium (Erwinia amylovora) but induces resistance in the treated trees. The time required for resistance to become active in the tree following treatment is 7 to 10 days. The resistance remains active in newly developing shoots for about 21 days. On high-vigor trees that are highly susceptible to fire blight shoot infection, repeated applications are necessary until 3 weeks before the end of the terminal shoot growth period. Follow the growth reduction timings and rates to control fire blight using Apogee.
Return bloom of biennial apple varieties can be increased with post-thinning applications of ethephon or NAA (Fruitone N or Fruitone L). Apply 3 to 5 weekly sprays, beginning 5 weeks after full bloom or when developing fruits are larger than 25 millimeters and are no longer responsive to chemical thinners. Ethephon should be used at 8 fluid ounces per 100 gallons, or NAA at 2 ounces per 100 gallons. Avoid the use of ethephon on summer-ripening varieties (McIntosh, or Macoun) due to advanced ripening. On nonbearing trees, ethephon may be used at 1.0 to 1.5 pints per 100 gallons to promote flower bud development.