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John Aue Notes on Bud-Break Sprays
April 14, 2015
Pesticide applications at apple bud-break are directed against fire blight (Erwinia amylovora) and/or apple scab (Venturia inequalis). At this early growth stage the bacterial colonies in fire blight cankers are not rapidly growing. However, the copper-containing fungicides that inhibit bacterial growth can be phytotoxic, and therefore, are applied early, prior to significant growth of green tissue. Very small amounts of apple scab inoculum can be mature at bud-break, and a single early scab infection period can result in primary scab lesions, which make later disease management a headache, especially in high-inoculum blocks or scab-sensitive cultivars.
Moderate to high-scab inoculumn, highly susceptible cultivars and fire blight is NOT a concern
Application of a half-rate of protectant (captan or an EBDC) is recommended at green tip.  Do not apply if green tissue is not present or immediately eminent. The effective life of any fungicide applied at this growth stage is probably no more than a week.  If an application is made before green-tip, there will be no redistribution during a rain event. Any of the EBDC formulations, e.g., Penncozeb, Manzate, Dithane, Polyram, Roper Rainshield, are typically used for this application. A half-rate of these fungicides are ~3 lbs./acre. These fungicides may be tank-mixed with oil. Do not tank mix captan with oil, as this is a highly phytotoxic mix. A one percent concentration of oil (one gal./100 gal. water) aids distribution of the fungicide and also functions as an insecticide for some pests that overwinter on the trees, e.g., San Jose scale and aphids.
Concentrate sprays
During pre-bloom sprays, when there is little green tissue available to capture fungicides, many growers choose to concentrate these sprays by applying at 40-50 gallons per acre (GPA). The goal is to increase the amount of fungicide that gets deposited on the leaves and flowers buds. If these concentrate sprays are applied, good weather conditions with low wind are critical to ensure adequate pesticide coverage. If pesticides containing oil or Esteem are applied to target San Jose scale or European Red Mite, concentrate sprays may not supply sufficient water to ensure good coverage. In these scenarios large volumes of water is necessary to allow the pesticide to penetrate into the tree bark.
Copper application for fire blight
There are many copper-containing products that can be used for fire blight at bud-break. Formulations may contain copper hydroxide, copper sulphate, copper oxychloride, etc.  All of these formulations function the same way. They supply copper ions, i.e., metallic copper, and it is these copper ions that inhibit bacterial or fungal growth. The hydroxyl or sulphate portion of the molecule, does not.  When choosing a copper product for fire blight, it is important to compare the amount of copper ions or copper metal contained in the different products.  This is often represented on the package either as a percentage of dry product by weight, e.g., 50% equals one pound copper to two pounds formulated product, or as pounds per liquid volume, e.g., 2 lbs./gal. If copper is applied to apples at bud-break, we want to apply 1-2 lbs. of actual copper per acre, according to Dr. Patty McManus, UW-Madison fruit pathologist.  As Dr. Rosenberger, from Cornell has noted in his Scaffolds articles, the other important factor in copper is the efficacy. The longevity of the copper will be dependent on the particle size of the copper salts in the formulated products.  The smaller the size, the less likely it is to be dislodged by rain, and theoretically the better the copper will be distributed on fire blight cankers.  Getting information on particle size for particular copper products can be difficult, however your distributor would likely be able to assist.
Application rates (GPA) and adjuvants
Applying copper with higher volumes of water, e.g., 75-125 GPA, depending on tree size, along with a one percent concentration of oil, i.e., 1 gal./100 gal. water, may improve the distribution of the copper within cankers.
Understanding risks of copper applications
Most copper applications are made early not because fire blight bacteria are rapidly growing at the time of application, but because of the phytotoxicity they cause when applied to green tissue. Note: Once copper products are dry, they are no longer phytotoxic. Fruit russeting is a second risk factor with copper sprays on apples. If there is still sufficient copper residue remaining at petal-fall, this can be redistributed by rainfall on to the developing fruitlets and cause russeting. For the standard copper products on the market, it is thought ~3-4 inches of rain between application and fruit-set mitigates this risk.
When using early copper fungicides, we need to apply enough product, that it is active when bacterial growth is rapid. However, if we apply closer to pink, when the fire blight bacteria, Erwinia amylovora, is more active, we increase the risk of causing phytotoxic injury.  If too much copper is applied and we do not receive enough rainfall between the application and petal fall, the risk of fruit russeting increases.
Suggestions to mitigate risk of phytotoxicity and russeting Injury.
- Apply when drying conditions are good (low humidity).
- Do not apply within 24 hours of a freeze event.
- Eliminate the oil from the application.
- Reduce the rate per acre (but not the total gallons per acre).
