April 20th AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, April 20th, 2021 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM,
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

April 20th Call Stream: CLICK HERE


Green Tip Date

Ascospore Maturity (%) Ascospore Discharge (%) to date Degree Days
(Base 50°F)
Rainfall since green tip (in)

Min Air Temp (°F)


Eau Claire, WI


10% 5% 69 1.54”


Galesville, WI


23% 10% 98 0.84”


Gays Mills, WI


7% 3% 87 0.70”


Mauston (Northwoods), WI


5% 3% 46 0.90”


Mequon (Barthel), WI

4/4 9% 5% 64 1.73”


Rochester (Ela), WI


13% 8% 78 1.04”


Verona, WI


10% 5% 73 1.91”


La Crescent, MN

3/23 15% 6% 107 0.75”


Hastings, MN


7% 3% 44 1.20”


Harvard, IL


5% 3% 80 0.54”


Table 1. Degree days and ascospore maturity downloaded on 4/19/21 from Cornell NEWA system.
Find your local station today: http://newa.cornell.edu. *Temperatures recorded between 12:00 AM and 8:00 AM on 4/21/21.

Apple scab
The upper Midwest typically receives at least three inches of rain in April and some snow. With this early, cool and dry spring weather, most of the orchards are 1.25” – 2.25” behind the average precipitation for April. While there are many chances of rain over the next ten days, there is not a large rain event forecasted which will likely lead to a apple scab infection between now and the end of the month. The result of delays in spring rains will lead to a buildup of primary apple scab ascospores in the orchard and when a significant rain fall does occur, expect a significant scab infection to also occur. In addition to leaf growth creating unprotected plant tissue, fungicides will also degrade from UV exposure. Typically, fungicides during primary scab season are reapplied every five to seven days. This year, it is probably fine to extend those intervals, but growers should be cautious and ready to respond if there is a change in the forecast. Additionally, the cool temperatures and dry weather is not conducive to disease pressure from rust and powdery mildew. Therefore, if in the past you have included single site fungicides such as strobilurins, sterol inhibitors and SDHIs, in your early sprays at tight cluster or pink, you may be able to delay adding these to your tank mixes until bloom. Large orchards that cannot respond quickly to changing weather may still want to consider single site fungicides in upcoming sprays.

Pre-bloom insects
Insect phenology likely will not sync with crop phenology in the next ten days. Ultimately the trees will push through to bloom even with a lack of degree day accumulation to drive emergence of rosy apple aphids, tarnished plant bug, red banded leafroller, green fruitworm and plum curculio. We can still plan to hang traps at our normal time, e.g., May 1st for codling moth. Normally we expect the first redbanded leafroller flight to occur between 111-176 DD base 50F. If cool weather persists, we can expect plum curculio to be delayed.

WI Bee App
The University of Wisconsin has developed a new smartphone app that has been largely motivated by apple growers asking the question, “Do I have enough wild bees to pollinate my orchard?” Over the last ten years the Gratton lab has been working with orchards to learn about wild bees in Wisconsin and their role in providing pollination services. This is only the second year of the app and the goal is for the WI Bee app to serve as a management tool for growers. The app has been modeled after a similar mobile app that was developed by the Northeast Pollinator Partnership, where research in New York indicates that an average of 0.8 bee visits per minute indicate a healthy population that could provide pollination services to orchards. In 2020, nine apple orchards in Wisconsin participated and generated a total of 162 five-minute surveys during bloom. A lot of data from around the state is required to derive more solid conclusions. The hope is that more orchards can participate this year.

The WI Bee app is available in the Apple and Android app stores and may be accessed from the project page. There are over 400 native bees in Wisconsin and WI Bee focuses on six categories of bees that are most important for Wisconsin orchards. The app has tutorials and a quiz you can take to test your knowledge before completing the pollinator observations. The pollinator assessments take five minutes and growers should complete three surveys within a day to improve quality of the data. Since bees are very sensitive to the weather, and their activity may change because of this, more than two days of observation data should be used before deciding to transition away from managed pollinators. The best time to complete the observations during bloom would be on a warm sunny day when wind is low and you can see your shadow. Generally, if it is nice for you to be outside, it is nice for the bees. Growers own the data they collect and will be able to see their data throughout the season on the WI Bee Dashboard. Last year’s report is available HERE, and if you would like to contact the WI Bee team, Hannah Gaines Day or Colleen Satyshu, send an email to .

Managing risk of freeze injury
Radiation freezes are the most common freezes we experience and often come after a period of warm and sunny weather, where the night is clear, and the air is dry. The current conditions are more indicative of an advective freeze, as opposed to a radiation freeze. The advective freeze is characterized by horizontal movement of large air masses over a large geographic area, e.g., the Polar Vortex. Under these circumstances, there is often no inversion and therefore a lack of layers of warm and cool air that would be mixed by winds, frost fans or helicopters.

Critical temperatures for fruit damage to trees at tight cluster are 27°F for a 10% bud kill and 21°F for a 90% bud kill. Orchards with bud development that has reached pink, 28°F will result in a 10% kill and 24°F will result in a 90% kill. If temperatures drop below freezing, there is the option to turn on frost fans. If it is windy, the wind will have the same effect. The longer the freezing temperatures stay below 28°F, the more likely freeze damage will occur.

Forecasts on smartphone apps that use national weather service data are predicting different low temperatures over the next few nights and hourly data suggests temperatures to get down to 25 – 28F. On the night of Monday, April 19th, 25F was reported at one orchard in western Wisconsin. On Tuesday, April 20th early morning temperatures reached 29F in western Richland (about 75 miles west of Madison) and there was no significant change in temperatures between ridge tops and valleys. Further to the north temperatures reached 27F and 28F in Hastings and Chippewa Falls, early Tuesday morning. The duration of these cool temperatures is also significant, and more injury can be expected if temps fall below freezing for upwards of four hours.

John is optimistic that the cool temperatures and minimal solar gain over the last week have allowed the trees to be somewhat acclimated to the cooler weather. The relative humidity is also considerably higher and may help. What damages flowers is the difference between dew point and actual temperature and when relative humidity is low, more damage can occur. Comparatively, the higher the relative humidity the less damage we will receive. Most varieties that are between tight cluster and early pink should have escaped injury Monday night and will need to see if this holds for Tuesday night.

To assess damage, take 20-30 bud samples and cut them in half north to south along the stem to blossom end. Note any browning or blackening in the pistil area, especially the ovary, which indicates it was killed by the freezing temperatures. Make sure to survey different areas in the orchard as well as tree canopy sections, e.g., top, middle, and bottom.

Some growers apply KDL (potassium) or Promalin (N-phenylmethyl-1H-purine 6-amine, Gibberellin sp.). Promalin supplies a plant hormone called gibberellins, which is the same chemical that seeds produce while developing. During the time between blossoms opening up and fruit setting, trees are waiting for the release of these gibberellins, which signals fruit development and will hopefully reduce the chance of bud rejection from the tree. The forecasted-freeze temperatures will occur at an earlier stage of phenology than when Promalin is recommended to be used. Even in the best scenario at 90% percent damage, studies show that Promalin applied within five hours will not provide a full crop but may give 25-30% which is a significant difference. We likely will not know the full extent of the damage until bloom or even fruit set.

 Professor Phil Schwallier, Michigan State, has previously discussed that if flesh around the ovary is damaged, Promalin will not work. If blooms show browning in the stigma and into the pistil, this may eventually recover. There has been other research showing that even if the ovary is damaged, applying Promalin can help to save the bud and will eventually set fruit, however it will likely be seedless and not store well.

For more information on critical temperatures for bud and blossom development, visit:

 Tree stress
The cool temperatures are inducing additional stress on varieties between tight cluster and pink. Regardless of nutrient reserves in the roots, trees cannot move nutrients up into the canopy when cool weather significantly slows evapotranspiration. Producing blooms is a huge drain of energy for the tree and it uses a lot of carbohydrate reserves. Trees with reddening on leaf margins or pale green leaves are examples of nutritional deficiencies that can result from this stress. Soil temperatures are also low, which prevents mineralization of nitrogen and reduces root growth.

Foliar fertilizers will help manage these nutrient deficiencies, but it is advised to hold off on Boron and zinc applications until the phenology is more advanced, e.g., late pink to early bloom, and stick to a foliar nitrogen sources such as fish emulsion, urea (3 lb./100 gallons of water) or 20-20-20 fertilizer (5 lb./100 gallons of water). Note: this assumes applications are made at 100 gallons of water per acre. More water is recommended for full absorption of nutrients into the leaves. Lastly, once IPM growers switch to captan, risk of injury from additional products in tank mixtures may occur. Organic growers should also consider that any foliar fertilizer that includes an oil as a carrier may result in phytotoxicity if mixed with sulfur. To mitigate these risks, now is a good time to apply these products.

Oil and freezing temperatures
Dormant oils are normally not applied once the crop phenology reaches pink and bloom. Additionally, horticultural oils are not good for honeybees and native pollinators. The conundrum is that there has not been good weather since April 5th for applying oil to control mites and scale. During a hard freeze, microscopic cracks can form in one-year old wood and green tissue. When oil is applied after a freeze, it may penetrate these cracks, causing injury to the tree. However, when the forecasted low is only around 29-32°F, damage is much less likely. Orchards whose phenology is behind and still have cultivars with buds at silver tip or green tip are okay. If the cultivars were beyond green tip, damage is likely but should be manageable at ~10% loss depending on the length of time temperatures stay below 23°F. Mite populations can be assessed by examining spurs for overwintering mite eggs with a10x hand lens. Look at the base of the buds where the bark is wrinkled. If there are 30+ mite eggs that are red and not clear, then there is a potential population.