April 9 AppleTalk: Welcome back to another season

AppleTalk Conference Call Summary
Tuesday, April 9, 2019, 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments, pwerts@ipminstitute.org

April 9th Call Stream: CLICK HERE

AppleTalk call and blog logistics
Thank you for registering for AppleTalk!  Your participation makes this program possible.  After each AppleTalk, you can expect the call recording and agenda to be posted to the blog following the call.  Written summaries will be posted when complete, within 24 to 48 hours, and distributed via email in PDF format.  John Aue, Peter Werts and Josie Dillon will begin scouting orchards in the coming weeks and are available to answer questions as the season begins.  If you still need to register for AppleTalk visit: https://ipminstitute.org/projects/specialty-crop-grower-services/appletalk/register/.

The regular season of AppleTalk will begin weekly on Tuesday April 23.

Early season orchard management
Hastening the decomposition of fallen leaves on the orchard floor is an important step toward reducing scab inoculum and is beneficial to both IPM and organic producers.  Flail mowing, machine-raking or sweeping are all excellent methods to disrupt overwintering inoculum.  It is not too late to distribute 5% urea solution or lime, which will speed up decomposition of leaf litter.  Compost or mulch can be applied to cover up leaf litter.

NEWA weather stations
The Network for Environment and Weather Applications (NEWA) weather stations provide forecasts and historical weather data for around 20 weather station across IL, MN and WI.  Visit the links below to see if there is a station near you.  The NEWA site offers a friendly interface to monitor insect and disease development and access crop management tools.  Access to the site is free of charge and you don’t need to have a station to use this resource.  This is a resource that we will be using and referencing season long.  If you have any questions with accessing a station please contact Peter, Josie or John.  Access your local station here:

Regional Extension publications and newsletters
Weekly Extension publications and newsletters distributed by Michigan State University (MSU) and Cornell University are excellent resources.  These resources deliver a broad range of regional topics and we recommend subscribing if you have yet to do so.  See links below.

Regional degree day (DD) accumulation and crop phenology


Degree Days (Base 50)

Verona, WI


Trempealeau (Eckers), WI


Mequon, WI


Malone (Lake Winnebago), WI


Rochester (Ela), WI


Lake City, MN


Harvard, IL


Woodstock, IL


Table 1. 2019 Degree Day Accumulation to 04/09/2019

April temperatures are slightly above normal for the first 9 days of the month.  The forecast for the next five days is showing below average temperatures with the possibility for snow and rain from April 10 – 12 in Minnesota, Wisconsin and Northern Illinois.  Orchards which receive snow will likely take longer for soil temperatures to increase.  Spring rains will also help to warm soil temperatures.  Normal average high and low temperatures are figures we refer to when determining DD accumulation and normality in temperature patterns.  If we are within five or six degrees of normal temperatures, we won’t see a noticeable difference in tree or insect growth and development.  Through the rest of the month, excluding April 10 – 12, temperatures are forecasted to be within the normal range and no warm spells are expected.  The coldest temperature is forecasted to be around 20°F on Saturday night.  If temperatures remain cool, we will warm slowly and have no rush for planting trees and applying fertilizer.

Green tip on McIntosh often occurs between ~38-63 DD base 50°F, based on historical observations in the northeast United States.  As of today, only a few areas are getting close to green tip.  With temperatures forecasted to be cooler over the next few days, green tip will likely not occur for at least another week.  Areas north of La Crosse, WI could potentially have a later green tip date.

Winter injury
December temperatures which fluctuated between the low 30s°F and mid-40s°F were ideal for meeting the trees chilling requirements.  This means trees were likely at their maximum winter hardiness going into winter.  Mid-winter warmups, like the one that occurred in late December (50°F), do cause a loss of chilling hours and could make it harder to regain winter dormancy.  There was no January thaw and temperatures remained normal, meaning trees were likely at full dormancy during the polar vortex that occurred the last few days of January and first week in February.

Apple buds are generally cold hardy at -25°F to -30°F.  Temperatures that occurred during the polar vortex ranged from -30°F to -55°F.  It is unknown at this point how severe winter injury will be in 2019.  Some factors to consider:

  • In newer varieties and root stocks, cold hardiness is unknown.
  • Winter injury to flower buds could have occurred.
  • Trees pruned five days before the deep freeze would have been at a greater risk of winter injury.
  • Trees with excessive vigor, e.g., too much nitrogen or trees with low vigor due to some other stress were at a greater risk for winter injury.
  • Trees with a very heavy crop the season before would have been stressed and more susceptible to winter injury.  This could also include trees that were not harvested.
  • Snow did provide some good insulation to mitigate against root injury.
  • Varieties like Jonagold, where WI is their northern limit, were at a greater risk, however, we lost most of those in the 2013 deep freeze.

One grower reported that during pruning, brown rings were observed on the inside of the cambium in Jonagold.  The shoots were about ½ inch in diameter and the trees were three years old.  John speculates this could have been from water stress during its first year.  It is important to look for damage to the cambium layers of the tree, when assessing for winter injury.

John has concerns going forward when considering climate change.  Climate change will add more stressors to our orchards but may not show up as symptoms on the tree for two to three years, depending on the level of stress.  This will position trees to be more susceptible to other stressors, e.g., pest stress, water or heat stress and nutrient deficiency.  These stressors are all interrelated.

Winter injury resources

Preparing for copper and oil at green tip
Copper applied at silver tip or bud-break target fire blight and apple scab.  At this early growth stage and with cool temperatures, bacterial colonies in fire blight cankers are not actively growing.  Fire blight cankers become much more active at pink and enough copper needs to be applied at this time to persist until early pink through bloom to inhibit bacterial growth.  The downside of applying too high of a rate of copper, is that if excess copper is present at petal fall it can cause fruit damage and russeting.

Note: Once copper products are dry, they are no longer phytotoxic.  If there is still enough 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, approximately three to four inches of rain between the application and fruit-set will mitigate this risk.

Tips to mitigate risk of phytotoxicity and russeting:

  • 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 of copper per acre (but not the total gallons of water per acre).

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 formulations function the same way by supplying 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 metallic copper contained in the different products.  This is often represented on the package either as a percentage of dry metallic copper by weight or as pounds per liquid volume.

Another important factor in choosing copper is its longevity, which is 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 throughout fire blight cankers.  Getting information on particle size for particular copper products can be difficult, however your distributor would likely be able to assist.  Some options include:

  • Kocide is 30% metallic copper
  • COCS is 50% metallic copper
  • Badge contains 2.18 lb. copper per gallon. Apply 2 – 4 qt per gallon.

For more information visit:

  • Demystifying Copper for Disease Management, Brian Lehman and Kari Peter, Penn State, Click Here.

Copper application rates and adjuvants
At bud break, use 1 – 2 lb. of metallic copper per acre and apply with a high volume of water, e.g., 75 – 125 GPA, depending on tree size.  A 1% concentration of oil can be used to improve the distribution of the copper within bark and cankers.  This helps to distribute the copper to break surface tension.  To reduce risk of bud damage, do not apply oil when temperatures have been, or are forecasted to be, below 32°F within 48 hours of the applications.  Copper is not the greatest scab material out there but will give about 5-7 days of scab protection.

Track rainfall from the application date, if three plus inches of rain has accumulated by bloom, it’s likely the copper will be gone, and you will need to be more aggressive in controlling bacteria compared to a dryer spring.

Oil applications for early season mite and San Jose scale management
Oil applied between silver tip and bloom is important to suppressing mites and San Jose scale.  Dormant oils are easiest to work with when temperatures are at least 60 °F or higher.  As temperatures increase, overwintering mite and San Jose scale respiration rates increase and the oil application will do a better job at suffocating mite eggs and San Jose scale.  There is a very wide range of application rates growers like to use.  Most growers apply a 1 – 2% oil, but 3 – 5% oil has also been observed.  These higher rates may be necessary where scale and mite pressure is most severe.  We will discuss mites and scale in greater detail later this spring, but the dormant oil spray is one of the most important opportunities to manage these two pests.  The post from the Jentsch Lab on San Jose scale management offers some important insights to consider: http://blogs.cornell.edu/jentsch/2019/04/08/weighing-in-on-san-jose-scale-management/

Green tip fungicides
The forecasted temperatures for April will be conducive to scab development and John does not recommend delaying first scab spray.  If you had scab last year, it is recommended to start management in high inoculum blocks earlier, rather than later, since there may be significant ascospore maturity before tight cluster.

The amount of ascospore development in terms of pre-bud break is typically small.  During spring rains, mature spores could be released (pre-bud break), but would require green tissue to be present for an infection to occur.  The bulk of ascospore release will occur between tight cluster and petal fall stage.  John does not recommend using any of the biological fungicide until there is significant leaf surface present and temperatures are warmer.  Additionally, copper applications may negate the impact of these biological products.  For orchards which receive snow on April 10 – 12, snow cover will slow scab development.

If there is very minimal bud maturation five to seven days after an initial copper application, a mancozeb application may not be needed.  Need to look at leaf growth and degree day accumulations (NEWA station) to have a better understanding of the maturation and reproductive rate of the fungi.

  • Don’t mix copper and mancozeb. There is no advantage for this.
  • The full rate of mancozeb, referred to as the “Pre-bloom schedule” is 6 lb./acre and the half rate or “Extended-spray schedule” is 3 lb./acre and interpreting these two schedules can be confusing. The full rate is four six-pound applications where the half rate is seven three-pound applications up to the 77-day pre-harvest Interval (PHI).  If the goal is to apply mancozeb past bloom, then it is important to only use 21 pounds maximum for the season and not apply past the 77-day PHI.

Biocontrol resources
The National Organic Program does not allow the use of antibiotics, which include streptomycin applied to treat fire blight.  Cornell is working on a three-part series focused on biocontrol of diseases titled, “Biocontrol Bytes”.  All three parts are now available, see the link: https://blogs.cornell.edu/biocontrolbytes/