May 3 AppleTalk – Grower Panel Thinning Discussion

AppleTalk Conference Call Summary
Tuesday, May 3, 2016, 8:00 – 9:00 AM
Thinning Panel Discusion with Sunrise Orchards, Ferguson Orchards and the Apple Barn Orchard and Winery
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 3rd Call Download: Click Here

Wisconsin Pollinator Protection Plan
Sara Ecker, Ecker’s Apple Farm, WAGA board president

Wisconsin is home to over 400 species of wild bees.  There are many things apple growers can do to help ensure we have bountiful populations of both wild and managed bees to pollinate our orchards each spring.  The Wisconsin Pollinator Protection Plan addresses pollinator protection across urban, rural, agricultural and non-agricultural areas.  The complete plan is available online at

Bee nutrition and diseases
Orchardists who maintain their own hives can learn more about hive health in the “Beekeeping to Maximize Pollinator Health” section of the Wisconsin Pollinator Protection Plan.  If commercial honey bees or bumble bees are brought in, growers should communicate with their beekeepers to agree upon a date for hive arrival and removal, and discuss any concerns relating to pesticide use before and during bloom.  Colony density and distribution in the orchard also needs to be discussed.  Too many hives placed in one location can lead to inadequate forage resources and an increased likelihood of disease and parasite spread.

Pollinator habitat
Orchardists have good opportunities to enhance foraging habitat and nesting sites for pollinators.  Diverse and abundant pollinator communities can be maintained long-term if adequate nesting habitat is located near forage.  On the farm we can protect existing nesting habitat by leaving dead wood, brush piles and bare patches of soil in surrounding forests and fields.  To improve foraging habitat, we can add wildflower strips or flowering hedgerows in areas not likely to be subject to pesticide drift.

The USDA Natural Resources Conservation Service (NRCS) offers financial and technical assistance to farmers and landowners through a variety of conservation programs.  Through the Environmental Quality Incentives Program (EQIP), orchardists can install pollinator habitat on the edge of fields through the 327 Conservation Cover and 386 Field Border Conservation Practice Standards.  Additional options include establishing or enhancing larger pollinator foraging and nesting sites in areas not in agricultural production.  For more information, contact your local NRCS office using the USDA service-center locator,

Pesticide exposure
Pollinators can be exposed to pesticides through direct contact with residues on treated plants or adjacent areas contaminated by drift.  Pollinators can also be exposed directly, when they are active in areas, while pesticide applications are being made.  In addition, pesticide residues have accumulated in colonies of native and managed bees when pollen and nectar contaminated with pesticides is brought back to the hive.

Pesticides vary greatly in their toxicity to pollinators, and some combinations, including adjuvants, can be more toxic than the individual pesticides.  Exposure to pollinator-toxic pesticides can result in mortality and changes in behavior, navigation, colony weight and/or reproductive rates.  While there is still much to learn about pesticide impacts on pollinators and considerable new research is being reported or underway, there are a number practices orchardists can follow now to protect pollinators from pesticide exposure:

  • Use the EPA bee-advisory box to identify pesticides toxic to pollinators and look for statements on labels that read, “highly toxic to bees”, “toxic to bees” or “extended residual toxicity.” Avoid using these products when bees are present, and always follow label instructions.
  • Do not apply insecticides highly toxic to pollinators until after apple bloom. Historically it was common for apple growers to apply an insecticide at the pink stage of blossom development for tarnished plant bug, spotted tentiform leafminer, aphids and leafrollers.  Research suggests that these pests rarely cause economic injury, and delaying the first insecticide application until petal fall can reduce exposure to pollinators.  Some fungicides can also be toxic to pollinators.  During bloom, apply these when bees are not actively foraging, whenever possible.
  • Apply pesticides toxic to pollinators when bees are less active, e.g., evening, nighttime or early morning.
  • Manage orchard groundcover to reduce blooming weeds and the likelihood that pollinators will be actively foraging on weeds during pesticide applications or within the period of residual activity for the products applied. This can be accomplished by mowing, planting competitive orchard grasses during orchard establishment and/or through selective herbicide applications.
  • Identify and protect nesting and foraging sites from pesticide drift by making sure your sprayer is properly calibrated, and adjust spray schedules and application procedures when wind speed and wind direction could result in pesticide drift to these sites.

Thinning discussion
Grower presenters Allen Teach, Sunrise Orchards; Tom Ferguson, Ferguson’s Orchards; and Steve Jacobson, Apple Barn Orchard and Winery

Across the region most growers are now in full bloom, with select varieties still at pink and early breaking cultivars nearing petal fall.  Now is the time when we can begin to assess bloom and crop potential to determine our thinning approaches.  Here are three different techniques to thinning from several experienced growers.

Additional comments on thinning were created during a conversation with Duane Greene, professor of pomology, University of Massachusetts-Amherst.

Assessing blossom health during bloom
While weather for the most part has been favorable this spring, areas across Minnesota, Wisconsin, Illinois and Iowa did experience freezing temperatures in mid-April.  The following can help assess blossom health and potential impacts from freeze injury:

  • Inspect the pistil length when king bloom opens. Flowers with short pistils may be damaged.
  • Are any flowers misshapen?
  • Are spur leaves twisted, misshapen or damaged? Are they brown or yellow?
  • If you are finding damaged pistils and flowers, thinning should be more conservative.
  • If the king blossom is lost and a couple of side blossoms look good then applying a petal fall thinner could be ok. In this scenario one of the side blossoms will take over; if no thinning is completed there will likely be too many fruit and thinning will need to be completed at a later time.
  • Use the Predicting Fruitset Model to measure fruitlet growth to predict what fruit will drop during fruitset and following a thinning spray. For more information visit:

 Cornell apple carbohydrate thinning

  • The carbohydrate model is based on carbohydrate demand of growing fruit. Actively growing fruit demand carbohydrates and can create a deficit.  The supply of carbohydrates is delivered by spur leaves.  On days with lots of solar radiation, e.g., sunny days and cooler weather, there is a good supply of carbohydrates, i.e., no deficit and no tree stress.  When it is warm and cloudy there is a higher demand for carbohydrates, but the spur leaves are not producing them, due to low solar radiation.  This creates a deficit and tree stress.
  • It is recommended to take note and use caution when thinning during really warm temperatures when the carbohydrate model may go below 20 grams. This suggests the tree is already under a lot of stress and fruit may over thin if thinners are applied at too high of rates.
  • Please note: This model has little to no utility before fruitset. The carbohydrate model was not developed to account for damaged cluster leaves, which can result from cold injury.  Damaged cluster leaves are not as efficient as healthy leaves since photosynthesis is impaired by the damaged tissue.  There has not been much experience with using the model when cluster leaves, blossoms or young fruit may be damaged.
  • For more information visit,

Conservative approach to thinning and key thinning windows

  • Bloom/Petal fall: Apply thinners that will be less influenced by weather conditions.
  • 6 mm fruit: Begin assessments of fruit growth. Fruit will begin to grow rapidly after this stage.
  • 7-9 mm fruit: We should get a good idea of fruitset since we can compare reduction in fruitlet growth.
  • 14 mm fruit: Possible thinning window. MaxCel (6-benzyladenine) works well for later thinning.
  • 18 mm fruit: This is the cut off for thinning. Ethrel (ethephon) can work as a rescue thinner in some situation.

Tank-mixing considerations
It is understandable that any opportunity to be efficient with applications of sprays should be taken advantage of, however, we should approach tank mixing with some caution at petal fall.  Please consider the following:

  • It is common practice for growers to apply additional adjuvants with every spray, Li-700 is a common example. If any adjuvant acts as a penetrant, it will open up the leaf cuticle and increase uptake of any chemicals applied, including thinners.  This could result in thinners performing better than anticipated, e.g., you may take off more fruit than expected.
  • Thinners are often applied with much more water than your regular fungicide/insecticide tank mix.
  • Very little research exists to identify how a thinner + insecticide + fungicide interact when applied as tank mix. A good example of this is the concern regarding mixing Fontelis with other chemicals which contain oil.  These tank mixes may or may not result in fruit or plant-tissue damage and it is recommended to apply thinners separately from the fungicide/insecticide petal-fall spray.

Panel discussion
Allen Teach, Sunrise Orchards
The first thinning applications for Honeycrisp are applied as soon as bees are removed from the orchard and as close to petal fall as possible.  Allen applies 10 ppm NAA + 2 qt. of carbaryl per acre and applied water at 250 gallons per acre (GPA).  The full dilute application is essential to getting good coverage. Allen has always thinned at petal fall, because early thinning sprays are essential for establishing return bloom for the next season.  Ten days after the application Allen reassess the crop to determine if additional thinners are required.  Gala has been a more challenging apple to thin, but Allen typically begins at petal fall with a 10 ppm application of NAA at 250 GPA of water.  He has found that one year this works, and other years it has not been successful.

Steve Jacobsen, Apple Barn Orchard and Winery
Steve does not apply thinners at petal fall and rather waits for king fruit to size up to 6-7 mm before beginning his thinning program.  Honeycrisp are treated at 6-7 mm with 150 ppm of MaxCel + 1 qt of carbaryl at 100 GPA of water.  Any additional thinning is completed via hand thinning.  Gala’s also receive the same combination of 150 ppm of MaxCel + 1 qt. of carbaryl applied at 100 GPA, however a follow up application of 15 ppm of NAA + 1 qt of carbaryl is applied ten days later.

Tom Ferguson, Ferguson’s Orchards
Honeycrisp is currently at 75% king bloom open in Galesville while Eau Claire and Chippewa Falls still have lots of pink.  Unlike waiting until petal fall, Ferguson’s Orchards begins thinning during bloom.  Applying thinners during bloom is a very safe approach.  We are very unlikely to over thin as these applications only accomplish 10 to 15% of all thinning that needs to be completed.  Early thinning also helps with return bloom.  For bloom thinning they apply 2 gal of liquid-lime sulfur (calcium polysulfide) + 2 gal (dormant) oil / 100 gallon of water per acre.  Once at petal fall or as soon as bees are gone, 10 ppm NAA + 1 qt. carbaryl are applied to Honeycrisp.  At petal fall Gala receive 100 ppm MaxCel + 1 qt. of carbaryl and have not had problems over thinning.  This year they are planning on applying more water per acre.  Note:  Bloom thinning with liquid-lime sulfur plus JMS stylet oil or fish oil are good options for organic producers.  It is important to remember that if oil is applied, the next fungicide application cannot include captan.

Why bloom and petal fall thinning?
Thinning at bloom and petal fall is recommended because chemical-thinning agents are far less weather dependent and therefore are less likely to have negative impacts on thinning.  This can be challenging for many growers when the potential crop load is unknown.  Consequently, these stages of fruit development are safe times to thin because we know there will be a thinning effect and the risk of over thinning is very low!

Fruit grow very slow from petal fall to 5 mm and initially have low demand for carbohydrates.  Therefore, variations in temperature and weather have less impact on the fruit, since it has not begun to grow rapidly, keeping its energy demands low.  Applying a growth-regulating thinner, e.g., Amid-Thin (NAD) or NAA, at this time will allow for easier thinning since the chemical will have greater influence on the carbohydrate level compared to weather conditions.  Any temperature effect is buffered, however, thinners should only be applied if the temperature is below 85 °F.  Applying during cooler temperatures is okay.

Using Amid-Thin for petal fall and bloom thinning
The goal of applying Amid-Thin (NAD) or liquid-lime sulfur + oil is not to get 100% of the thinning completed.  By completing part of the thinning at bloom or petal fall, it will give us a better idea of what else needs to come off beginning at 7 mm.  During petal fall, we know the weather, bloom quality and if the bees had good conditions.  Considering this information when thinning at petal fall could make thinning decisions easier once fruit are larger.

Amid-Thin is not temperature dependent and does not exhibit a dose response, making it a good option for petal fall.  If NAA was applied at petal fall, we would still have the concerns about a temperature response to thinning.  Trials completed by researchers at the University of Massachusetts used a flat rate of 8oz./100 gal. at petal fall.  At this rate no over thinning has been observed and heavy clumping of fruit was also reduced.  Amid-Thin would only be applied at petal fall, thinning thereafter would still rely on NAA or MaxCel between 7-10 mm.

After petal fall thinning it is still recommended to quantify the fruit growth using the fruit-growth model developed at Cornell.  The carbohydrate model provides guidelines and while not precise, because it is based on the weather conditions, it is the best resource we have to help guide timing and rate of thinning, especially with NAA and MaxCel.

Gala and Honeycrisp are good varieties to try petal fall thinning with Amid-Thin.  Getting return bloom on Honeycrisp is often a challenge and research suggests the earlier the crop load can be reduced, the better return bloom we can expect.  Amid-Thin is not recommended for bloom or petal fall thinning on Fuji, Empire or Red Delicious.  NAA would be the preferred option on these varieties for petal-fall thinning.

Resources for precision thinning
Precision Crop Load Management, New York Fruit Quarterly, 2013,

Thinning apples with more confidence, Good Fruit Grower, 2014,

Insect and disease updates
Scab and rust infection
The infection we had Last weekend was likely a large and released a significant amount of mature ascospores.  The severity of the infection depends on inoculum from last year and amount of time from last infection period.  Lesions from an infection period won’t become visible for another 10-14 days.  John has found occasional lesions on cluster leaves.  These lesions are small and difficult to see and would have come from infections closer to green tip.  Note: Liquid-lime sulfur is an option for organic grower interested in burning out visible lesions.  Conventional producers have used a variety of fungicides to burn out scab, however, this practice is not advised due the resistance concerns for DMI, SDHI and QoI fungicides.

To predict scab development growers can use either degree day models or squash mounts.  Growers in Minnesota send leaf samples to the Minnesota Apple Growers Association where ascospore maturity is assessed by examining the developing spores on the leaves.  John’s opinion is the degree day model is more accurate than squash mounts.  The potential for error is large when reading squash mount to determine maturity.  Maturity of spores are released between tight cluster and petal fall, a lot of susceptible tissue is present at this time.

Cedar apple rust cannot spread from apple to apple or from red cedar to red cedar – the fungus must go through the two-year life cycle, alternating between hosts. The infection period for cedar apple rust (CAR) is between tight cluster and first cover.  Spores can be carried long distances (3-5 miles), yet the majority of infections occur when infected eastern red cedars are within a few hundred yards.  Spores that land on young apple tissue may germinate and infect if a film of water is present for an adequate amount of time. Symptoms appear one to two weeks after infection.  EBDCs applied from tight cluster to first cover should offer adequate control.  Susceptibility of cultivar and proximity to infected host will influence disease pressure.  Unlike scab, rusts require and alternate host and inoculum is not reflective of how much rust was in your orchard last year. “Heavy” rust infections are produced by long-wetting events with little rain.

For more information see: Efficacy of Selected Fungicides Against Apple Diseases, 2016 Midwest Fruit Management Guide, page 31.

Our spring lepidopteran complex is in full-feeding mode.  Growers can expect to find larvae of obliquebanded leafroller, redbanded leafroller, spring canker worm, variegated leafroller and green fruitworm all feeding on growing terminals and blossom clusters.  These pests can be controlled with petal fall sprays for plum curculio or with applications of Bacillus thuringiensis (Bt) when it is warm and blossoms are open.  If blossoms are still closed a Bt application will not target larvae that have tunneled into closed blossoms.

The cooler nights will make it unlikely that plum curculio will migrate into the orchard very far and movement will likely be slow.  Perimeter application on varieties that have reached petal fall is all that may be needed at this time due to the cool night time temps, lower activity.

Early season lepidoptera
Insecticides applied for early season lepidoptera, e.g., green fruitworm, obliquebanded leafroller, need to be consumed by active larvae.  Consider waiting to apply an insecticide for these pests until temperatures warm.

Green fruitworm
Feeding damage to fruitlets and foliage, i.e., large smooth holes and chewing along leaf margins of terminal shoots, is typical of green fruitworm (GFW).  Because GFW develop quickly and have one generation, populations are usually not large enough to justify control.  As GFW increase in size, nearing pupation, they will consume less leaf tissue.  For greatest efficacy apply an insecticide while GFW is small and actively growing.

Spring cankerworm
Spring cankerworm are common and are a small, dark grey to brown larvae, whose movements resemble an inchworm.  Damage can be seen as foliar feeding to all sections of the leaf excluding the midrib and fruitlets.  In two to three weeks the larvae will be pupating.  Old feeding can be determined by observing the amount of new growth beyond the damaged tissue.

Forest tent caterpillar
Forest tent caterpillar look similar to the eastern tent caterpillar but do not form tents.  Forest tent caterpillar is not typically an economic pest.

Obliquebanded and redbanded leafrollers
The larvae of obliquebanded (OBLR) and redbanded (RBLR) leafrollers can be distinguished by inspecting their head and thoracic shield; OBLR is tan to brown or blackish and RBLR is yellow or green.  Now is the time to set OBLR traps.  Scouting for foliar feeding now may help predict future management decisions later in the season.  For bearing trees planted on vigorous rootstocks, feeding damage on 5% or more of terminals or 3% of fruit clusters indicates treatable populations.  Thresholds for high density planting are lower.  There are no thresholds established for non-bearing trees.

OBLR and RBLR have multiple generations per year.  OBLR fruit-feeding injury can result in more economic damage later in the season after codling moth management ceases.  Monitoring populations at this time can help reduce the incidence of injury near harvest.

Codling moth
Codling moth (CM) overwinter as a full grown larva on trees; temperatures below -15⁰F will provide some mortality.  The first CM capture has traditionally been around petal fall and pheromone traps should be hung as soon as possible.  The long-life lures (CM L2) are active for eight weeks: if traps are deployed May 1 replace lures by July 1.  If the standard-lure (1x) is used, wait until McIntosh bloom to hang traps; replace lures after three to four weeks.  Hang CM pheromone traps in the upper third of canopy.

If mating disruption is used, hang at least one CM combo lure (CMDA) per block and at least two oriental fruit moth (OFM) traps per orchard.  The CMDA lure will attract female and male CM moths.  Damage caused by OFM is very similar to CM damage.  OFM lures will also attract lesser appleworm (LAW); OFM has three flights per season, first flight can begin as early as pink.  LAW flights correspond with CM.

Additional resources
A new issue of Scaffolds for the week of May 2 has been posted and is available at:

A version compatible for mobile devices is available at: