Freeze advisory, reminders about copper, oil and protectant fungicides

April 6, 2017

Daily high temperatures across the entire region this weekend are forecast to be in the 60F+ range, with lows around 50F on two successive nights, before returning to more seasonal temps (50s/30s) for the remainder of next week. The timing of this warm weather will be earlier in the weekend for Minnesota and Iowa growers and delayed a day for growers close to Lake Michigan.

Before this warm-up, temperatures are expected to drop below freezing across the region on the morning of April 7, between 4 – 8 AM. In Eau Claire temperatures are to be as low as 27F, whereas La Crosse, Madison and SE Wisconsin will only see temps down to 30-32F. It is essential that we:

  • DO NOT APPLY copper the morning after a freeze. Wait 24-48 hours.
  • DO NOT APPLY copper if freezing temps will occur 24 hours after application.
  • Avoid oil 24 hours before or after freezing temperatures.
  • Check labels, as restrictions and precautions vary by product formulation.

The above-normal temperatures forecasted will accelerate bud development that has been stalled for the last week and by Monday evening there will be significant green showing on many if not most cultivars. There is also the possibility of a scab infection Sunday afternoon/evening with thunder storms across the region. If measurable rainfall occurs at the forecasted temperatures, as little as eight hours of leaf wetness will be enough for a successful scab infection, where green tissue is exposed.

Reminders for copper and protectant fungicide applications (See 3/31 AppleTalk call notes):
Copper at silver tip or green tip

  1. Copper primarily targets fire blight, but will manage apple scab for up to seven days. Tank mixing with a protectant fungicide is not required.
  2. Application rates should be 1-3 lbs./acre of actual metallic copper.
  3. Copper should be applied with a 1% concentration of oil to assist with spreading and adhering.
  4. Apply with sufficient water, i.e., 100 gallons-per acre (GPA), to coat bark and cankers.
  5. Copper applied for fire blight is only effective if residues remain on cankers into bloom, when the bacteria can rapidly multiply. Copper is only applied at silver tip or green tip, only because of its phytotoxicity to green tissue.
  6. If a copper application is desired for cultivars already at or just beyond green-tip, consider the following to reduce the phytotoxicity risk:
    • Reduce the rate per acre of actual copper
    • Increase the water volume, i.e., GPA
    • Apply when the spray residue will dry quickly, i.e., higher temps and with lower relative humidity.
    • Apply without oil. Note: this is not a recommended option but will reduce phytotoxicity if applying at up to TC stage

Protectant fungicides (no copper) applied between green tip and ¼” green tip

  1. Apply a multi-site protectant, e.g., mancozeb or captan. Tank mixing with a single-site fungicide is not recommended as there is insufficient leaf surface to absorb these fungicides.
  2. Mancozeb or metiram formulations can be applied with 1% oil for better coverage. If using captan DO NOT USE OIL
  3. A half-rate application of mancozeb (3lbs/A) is generally sufficient, since very little of the overwintering scab inoculum contains mature ascospores at green-tip.
  4. Also consider that rapid leaf expansion from green tip means the first application will only be effective for a limited time, usually 5-7 days.

What if I can’t spray this weekend?

  1. If pruning activity or wet soil preclude a copper application this weekend, there will likely be another opportunity. The 10-day forecast suggests cooler, more seasonal temps next week and bud development and leaf growth is likely to slow down again.
  2. Fire blight is managed with a wide array of methods, e.g., inducers of plant resistance, bacterial competition, antibiotics, plant-growth regulators, and pruning of strikes, from prior to bloom until shoot-growth elongation is completed in summer. Copper is simply the management step of choice at bud-break
  3. If you do not typically use copper for fire blight management AND if you grow only scab-immune or scab tolerant cultivars with zero recent history of scab OR if very little green tip is showing in your orchard; celebrate as you shouldn’t be spraying this weekend.

Notes for Organic Fruit Producers

  1. Using an OMRI-listed copper formulation at silver tip or green tip should be a priority only if fire blight or apple scab are active concerns. Aside from these two diseases, there are few fungal pathogens active early on the above-ground portions of our trees.
  2. I do not recommend sulfur applications as a scab preventative between green tip and tight cluster, subscribing to Michael Phillips admonition to save this material as well as lime-sulfur for the heart of the primary scab release period.
  3. I also do not recommend applying any of the biologicals until there is sufficient leaf surface to adsorb and support these products.