Enhancing return bloom with summer NAA 2006

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Some years it is desirable to try to enhance return bloom on apple varieties that tend to be biennial. This is especially important on trees that have a heavy crop load. Most years treatments of summer NAA applied at 5, 7 and 9 weeks after bloom will increase return bloom even on varieties that have heavy crop loads and tend to have poor return bloom. This timing is after any potential thinning from NAA has pasted. Fruits are often 1 inch in diameter and won’t respond to any NAA thinning action. Flower bud initiation has already begun but can be enhanced by NAA treatments during the next 30 days after the thinning period ends.

Summer Ethrel can also enhance return bloom by treatments of 200 ppm made at the same timing of 5, 7 and 9 weeks after bloom. However, summer Ethrel can some years thin 1-inch diameter fruit and summer Ethrel can advance maturity of early maturing varieties.

Summer NAA

Apply 5 ppm (2 oz/100 of Fruitone N) of NAA starting 5 weeks after bloom and apply 2 additional spray treatments at 7 and 9 weeks after bloom. The rate of NAA applied per acre should be adjusted to TRV (tree row volume) levels. The applications can be concentrated but treatments will benefit from increased water amounts. Try to not concentrate water amounts greater that 4X. These sprays can be added right to the cover sprays during that time period. Some years these treatments do not perform well especially during drought years. Varieties that have a moderate to high biennial bearing tendency should be considered for bloom enhancement sprays (see Table 1). Summer NAA treatments will not cause any adverse affects to the trees or crop. Treatments during extremely hot temperatures (maximum temperatures above 95°F) should be avoided. If surfactant or oil is included with the application, consider reducing the NAA amount by 1/3. Follow the guidelines listed in Table 2.

Table 1. Apple Variety Biennial Tendency

Variety Biennial bearing tendency
Cameo Moderate
Cortland Low
Empire Moderate
Fuji High
Gala Low
Golden Delicious High
Honeycrisp High
Jonagold High
Jonathan Low
Macoun Moderate
McIntosh Low
Mutsu High
Northern Spy Moderate
Paulared Moderate
Red Delicious High
Rome Low


Table 2. Summer NAA rate use guidelines

1. The target rate/acre is 8 oz Fruitone N (5 ppm) on full size trees (100% TRV). 8 oz/ per acre
2. Determine the target blocks TRV. Example: 75% TRV
3. Adjust the NAA rate per acre by the TRV. .75 * 8 oz = 6 oz/acre
4. Apply at 4X water concentration or less.
5. If surfactants or oil is included reduce NAA by 1/3.
6. Avoid applications during extreme hot temperatures.

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