April 12 – Management at Green Tip and First AppleTalk Conference Call

The first conference call of the season is still scheduled to begin on Tuesday, April 12, 2016 at 8:00 AM.  We will send all registered users the conference call number.  If you have not yet registered for AppleTalk and would like to, please contact, Peter Werts, pwerts@ipminstitute.org or call (608) 232-1410.  We would like to have your registration complete prior to the first call and will continue to accept registrations during the season.

Green tip has arrived to southern Wisconsin and northern Illinois
Over the past week many orchards across southern Wisconsin experienced bud-break in early breaking varieties, e.g., Idared and Zestar.  By Wednesday, March, 30 some had reached the McIntosh green-tip stage of development.  Orchards further north are not far behind; orchards on the Chippewa ridge are currently at or beyond silver-tip on Paula Red.

In response to bud break, growers are contemplating their first sprays of the season.  However with colder and wet weather in the near-term forecast, patience is probably a virtue.  With the current temperature forecast, trees are unlikely to push much new growth for the next 5-7 days, so there will be little change in growth stage.  Therefore, it seems prudent to hold off any copper applications until at least mid-week next week.

Apple scab and spore maturity
Using estimated green tip dates between March 26 and 31, 2016, ascospore maturity from the models provided by Cornell NEWA stations in Gays Mills WI, Woodstock IL, Lake City and La Crescent MN indicate 2-4% maturity.  This very small amount of mature spores can result in scab infections, if green tissue is present.  Assessing scab inoculum is essential during these early infections.  If you have a low-inoculum orchard, your risk of a mature spore landing on the very small amount of tissue present is fairly low.  Scab susceptible varieties or high-inoculum blocks, may be at risk of an infection.  Comparatively, when we reach ¾ inch green tip or tight cluster and have 20-50% spore maturity, there are more spores released during a rain event and there is more green tissue to catch those spores, resulting in a significant infection period.  Remember, if you do not have any green tissue, you are not at risk of a scab infection.

Forecasted temperatures during predicted rain storms in the next five days are to remain in the upper 40s to low 50s F.  At these temperatures, models require between 9 and 21 hours of continuous wetting for an apple scab infection to occur, according to the revised Mills Table, http://www.fruit.cornell.edu/tfabp/revmills.htm.  Wetting events should be combined when two or more successive wetting periods have less than 24 hours of drying between them.

You can access the Cornell NEWA stations for Illinois, Minnesota and Wisconsin at, http://newa.cornell.edu/index.php?page=apple-diseases.  Stations located at orchards in our region include:

Illinois

  • Woodstock
  • Poplar Grove

Minnesota

  • Delano
  • Elgin
  • Hastings
  • La Crescent
  • Lake City
  • Webster
  • White Bear Lake

Wisconsin

  • Gays Mills

Moderate to high-scab inoculum, 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 and Koverall, 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, 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 will not supply sufficient water to ensure good coverage.  In these scenarios large volumes of water are necessary to allow the pesticide to penetrate into the tree bark.

Using copper at bud break
Pesticide applications at apple bud-break are directed against fire blight (Erwinia amylovora) and/or apple scab (Venturia inaequalis).  At this early growth stage and with the cool temperatures, bacterial colonies in fire blight cankers are not rapidly growing. The copper-containing fungicides that inhibit bacterial growth should be applied early and can be phytotoxic if significant green tissue is present at the time of application.

There are many copper-containing products that can be used for fire blight at bud-break.  Formulations may contain copper hydroxide, copper sulfate, 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 russetting 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 russetting.  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 russetting increases.

Suggestions to mitigate risk of phytotoxicity and russetting Injury

  1. Apply when drying conditions are good (low humidity).
  2. Do not apply within 24 hours of a freeze event.
  3. Eliminate the oil from the application.
  4. Reduce the rate per acre (but not the total gallons per acre).

Early season insect management
A 1-2% delayed-dormant oil spray may be applied to target over wintering insect eggs including San Jose scale (SJS) and European red mites.  Efficacy against high populations of SJS may be improved by using the insect growth regulators Esteem or Centaur.  These additional insecticides should only be necessary where SJS pressure was high last year.  Oil sprays should be applied when temperatures are above 40°F and when temperatures will remain above freezing for next 24-48 hours following the application.  Performance of oil sprays will be best when the relative humidity is less than 65% and temperatures are warmer than 60°F.

The GPA application rates of water are also very important.  It is unlikely to get adequate coverage in a semi dwarf orchard, e.g., M.7 trees, when applied at volumes of less than 100 GPA.  Applying at volumes >100 GPA are necessary to ensure adequate coverage.  On high density plantings, full dilute applications, appropriate to their tree size should be applied.

You can read more about maximizing the benefits of your oil sprays:

Most of the traps for monitoring insects are hung at Pink, however, pheromone traps for green fruitworm (GFW) and redbanded leafroller (RBLR) should be hung at green tip.  Growers with young plantings or high-density trees have grown increasingly concerned about GFW injury.  We have also seen GFW injury showing up in our pre-harvest damage assessments.  We can use the pheromone traps to identify the end of the flight and begin scouting for hatched larva 100 degree days (DD) after peak flight, e.g., tight cluster to bloom.