May 25th AppleTalk

AppleTalk Conference Call Summary
Tuesday, May 24th, 2021 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM,
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 25th Call Stream: CLICK HERE


Location Green Tip Date Degree Days

(Base 50°F)

Eau Claire, WI 4/3 280
Galesville, WI 3/21 319
Gays Mills, WI 4/3 340
Mauston (Northwoods), WI 3/30 333
Mequon (Barthel), WI 4/4 264
Rochester (Ela), WI 3/30 309
Verona, WI 4/3 329
La Crescent, MN 3/23 359
Hastings, MN 4/5 306
Harvard, IL 3/30 338

Table 1. Degree days and ascospore maturity downloaded on 5/24/21 from Cornell NEWA system. Find your local station today:

Regional summary
Warming temperatures and increasing likelihood of rain across the region will bring increased risk of secondary scab infections; the first signs of plum curculio activity and the start of the codling moth flight. Fire blight risk is minimal for varieties past petal fall, but non-bearing trees with bloom are at risk.

Apple scab
The NEWA models show all locations in Table 1 are at or past 95% primary ascospore maturity and remaining spores will be released when at least 0.1” of rain is received. Infections will occur if temperatures are above 50°F during the rain and leaf wetness is sustained. Orchards north of Eau Claire, WI and along Lake Michigan, such as Mequon, WI are only at 70% primary ascospore maturity. The total number of scab infection periods varied between four to eight, and most occurred in the last ten days. It was considerably warmer during these recent rains and scab lesions appear within 14 days. Scab lesions have already been found in southern Wisconsin and rain will spread secondary infections. Orchards should be covered for scab ahead of the upcoming rain.

When scouting, look for lesions on the backside the of the leaf, this is where primary scab infections tend to show up first. Scab lesions found on the top of leaves tend to come from secondary infections, but this is not universally true and primary lesions on the topsides of leaves can appear. To better determine when the infection occurred, look at the age of the leaf and location along the length of the growing shoot. This is indicative of when the infection occurred. A lesion on a newer leaf reflects a scab infection that happened more recently than a lesion on an older leaf at the beginning of this year’s growth.

Powdery mildew
Even though the weather patterns have been oscillating between hot and dry and cool and wet, the periods of hot weather have resulted in significant powdery mildew infections for some orchards. Powdery mildew (PM) has been observed on Cortland, McIntosh, Gingergold and Honeycrisp. Remember that captan and EBDCs (mancozeb) do not protect against powdery mildew. Sulfur is the only “broad-spectrum protectant” active on powdery mildew. When temperatures are not solidly in the 80s°F, sulfur can more safely be applied. When temperatures are in the 80s during the application and drying, the risk of crop injury increases.

The strobilurin, sterol inhibitor and SDHI fungicides are all options but have varying efficacy between “Fair”, “Good” and “Excellent”. If you are seeing powdery mildew or have a history of powdery mildew a sterol inhibitor such as Indar, Procure, Rally and Topguard are all rated as “Excellent” and would be the best option. If you have powdery mildew or scab resistance to the sterol inhibitor fungicides, then a strobilurin or SDHI would be the next best option. Fungicides with a good rating include: Flint, Luna Sensation, Fontelis, Merivon and fungicides with a “Fair” rating include Aprovia and Inspire Super. Depending on what other diseases need to be managed, a “Good” rated fungicide might be just fine.  Unless there is a significant amount of scab pressure and minimal powdery mildew pressure, avoid a “Fair” rated fungicide.  This table is on page 53 – 54 of the 2021 – 2022 Midwest Fruit Pest Management Guide.

Cedar apple rust
Across southern Wisconsin, John has observed cedar galls that have already matured and released their spores. Locations to the north or northeast may still have active cedar apple rust galls. The CAR galls resemble orange golf balls which can grow more than two inches in diameter. When they are mature, they swell and produce telial horns during rainy weather which release spores. Once the spores are released, the telial horns collapse and eventually the gall will fall off. 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 CAR is between tight cluster and first cover. Most infections occur when infected eastern red cedars are within a few hundred yards and spores land on young apple tissue. Symptoms appear one to two weeks after the infection. EBDCs applied from tight cluster to first cover provide good control when applied as a protectant but offer no post-infection activity. However, we have still observed CAR infections in orchards using the extended spray schedule of EBDCs through petal fall. Susceptibility of cultivar and proximity to an infected host will influence disease pressure. Unlike scab, rusts require an alternate host and inoculum is not reflective of how much rust was in the orchard last year. The strobilurins, SDHIs and sterol inhibitor fungicides offer varying efficacy on rust, check your label if you need additional rust control. However, none of the fungicides would stop the transmissions of spores from leaves of apple trees back to red cedar.

A note on EBDC fungicides
The EBDC fungicides mancozeb sold as Roper Rainshield, Manzate Pro-Stick, Fortuna, etc. have a 77-day pre-harvest interval, but may only be applied up to this 77-day PHI at the 3 lb. rate. The pre-bloom schedule allows for four applications up to a total of 24 lbs. per acre per season. The extended-spray schedule allows for up to seven applications at 3 lb./acre for a total of 21 lbs. of mancozeb for the season and allows you to apply up to the 77-day PHI.

A common comment I have heard is growers applying 4 or 5 lbs. at some point during the season, if this is done, you can no longer follow the extended spray schedule and you are bumped into the “pre-bloom schedule”.  Based on the label instructions, at no point should 4 or 5 lbs. of mancozeb be applied, either apply 3 lbs. or 6 lbs. per acre. While it is not illegal to apply 4 or 5 lbs. it is somewhat a waste, since it bumps you out of the extended spray schedule and the only benefit is that you saved a few dollars per acre.

Fire blight
Newly planted trees continue to push out blossoms and some growers are still reporting the occasional bloom appearing. There is a serious fire blight risk for these newly planted trees and these blossoms are susceptible to infection from rapid bacterial growth that will occur during this hot weather. It is important to remove the blossoms or treat these young trees with low doses of copper or streptomycin if there is a fire blight infection.

Insect management
Codling moth
Low numbers of codling moth have been reported over the last ten days. However, a strong flight establishing our biofix in southern Wisconsin occurred on May 19 and 20. Growers to the north should expect a flight with this warm weather, if it has not occurred already.

For those who may be new to codling moth management, we define a “biofix” as marking the first and significant-sustained flight, where moths are captured multiple days in a row or exceed a threshold of five moths per trap per week. Once a biofix date is established, begin tracking degree-days (base 50°F) and monitor traps weekly. If you are calculating degree-days by hand, be aware that codling moth has a top developmental threshold of 86°F, which means all temperatures above 86°F should be counted as 86°F, e.g., a high of 95°F would be changed to 86°F degrees, if calculating by hand. Degree days are also available from your nearest NEWA station here,

Codling moth fly between 6pm and 11pm when wind speeds are between three and five miles per hour and when temperatures are above 62°F, without rain. Assign the biofix date for the warmest, calmest night. When checking traps, fluttering CM had likely flown within the last 48 hours. Most female CM can live for seven to 14 days yet will mate and deposit most eggs on the evening they emerge. Every day that a sexually viable female emerges and is not able to fly, egg fecundity is decreased by 20%.

Growers using mating disruption do not need to shut down 100% of mating for it to be effective. When we vastly reduce the number of moths exposed to our insecticides, this helps improve the efficacy of the insecticides that are applied and reduces selection pressure which can mitigate resistance risk.

Resistance management considerations
Codling moth is a significant global pest of apples. While not present in every apple production region of the world, all locations around the world with codling moth have reported resistance to one or more insecticide mode-of-action. In the United States, resistance to the organophosphate classes of insecticides is the most common example of resistance. Growers in Michigan have reported codling moth resistance to insect growth regulators and Avaunt (a common insecticide we use for plum curculio). Codling moth resistance to the neonicotinoids has been reported in Europe. The point is codling moth is a highly adaptable species and considering that three – four insecticide applications are sometimes required for the first generation and two or three applications for the second generation, resistance management is paramount! There are a lot of insecticide options for codling moth, but some of our other pests do not. This positions neonicotinoids as the primary organophosphate and pyrethroid replacement for plum curculio, aphids, plant bugs, Japanese Beetle. If we add in these pests plus codling moth, it could be quite easy to accumulate six neonicotinoid applications during a season. Even though these are not all targeting codling moth, we need to consider non-target exposure when applying insecticides for these other pests.

Neonicotinoid resistance management for codling moth:

  • Option One: Neonicotinoids used for first generation codling moth, plum curculio, rosy apple aphid, green apple aphid and Japanese beetle. For the second generation of codling moth, diamides (Altacor or Exirel) or spinosad (Delegate) should be used.  Apple maggot may be controlled with Exirel and neonicotinoids would need to be used sparingly and only target apple maggot and not codling moth. In this scenario Wrangler (imidacloprid) would be a preferred apple maggot insecticide over Assail (acetamiprid).  Assail and Wrangler are neonicotinoids and Wrangler has no efficacy against codling moth. Mating disruption is also an option!
  • Option Two: Altacor, Exirel or Delegate are used for first generation codling moth. Plum curculio would be managed with Avaunt and the neonicotinoids Assail may be used to manage second generation codling moth and apple maggot. The other neonicotinoids may also be used to manage late activity from Japanese beetle and apple maggot. Where rosy or wooly apple aphids are a problem during first generation codling moth, Beleaf 50 SG may be applied. Beleaf 50 SG has a supplemental label for use on first generation codling in Wisconsin. Unfortunately, this label has not been extended to growers in Minnesota, Illinois, or Iowa. So, make sure aphids and plant bugs are the target, but it would certainly be active on any codling moth present. Mating disruption is also an option.
  • Organic Management: Organic codling moth management should absolutely be based in use of mating disruption. However, insecticide applications may still need to supplement MD in organic or IPM Orchards.  Organic growers should consider timing applications of the codling moth virus (Cydia pomonella granulosis virus) sold under the trade names Virosoft and Cyd-X. Entrust, PyGanic, Grandevo and Venerate are all OMRI approved insecticides labeled for codling moth and several other important pests of apples. The same rotational approach should be considered when using these on codling moth or for other insects.

As resistance builds, the effects are not immediately visible for a few years.  The number of successful resistant worms that survive are so low that problems do not appear during scouting or on the pack line. Evaluate the trap captures between first and second gen CM; the first sign would be larger populations of adults during second generation vs. second generation.

Plum curculio
Plum curculio movement has begun, and injury has been found in NE Wisconsin and several locations in southern Wisconsin.  It should be expected that more injury from this past weekend to become visible. Thursday and Friday are expected to be very cool and could slow PC movement until nighttime temperatures are back in the 60s. The last several nights have been ideal for oviposition injury. In the past we have discussed scouting early sizing varieties first, but as more fruit reaches 10 mm in size, any fruit at or above 10mm will be susceptible to PC oviposition. At this time, scouting should not be limited to only the perimeter and the interior of smaller blocks should be scouted and larger blocks should be scouted ten to fifteen rows in from the edge too.

Please make note of the 95% petal fall date and report back to us. Tracking degree days from petal fall may be used to predict when their emergence from overwintering sites ends, which occurs after 308 DD base 50°F have accumulated. These degree days from petal fall will be included in the next summary table if you submit them to us. Without your actual petal fall date, NEWA estimates petal fall based on degree days accumulated from January first, which is likely to be inaccurate.

Rosy apple aphid
We are seeing a strong resurgence of rosy apple aphid this spring. If there are no longer any blooms on the trees, an insecticide such as Belay should be used over Avaunt for dual activity on PC and RAA. By early summer RAA will move to alternate hosts such as plantain and other broadleaf weeds.

John’s interview with Phil Schwallier
Many growers in western Wisconsin and eastern Minnesota have had some rain in the last week, while southeastern Wisconsin remains very dry. This rain has been helpful but need to consider this is not enough to replenish sub-soil moisture that has been depleted.  Therefore, if your orchard received 0.5” to 2” of rain in the last week, you are not out of the woods. The evapotranspiration rates of the trees are reaching 1.4” of water per week and growers that received rain benefited but have no reserve. A few rare locations received three or four inches of rain in May, but most growers have only accumulated one inch of rain in May.  Michigan is much like this, having only received one inch of rain this month.

Dr. Phil Schwallier of shared his opinion on these drought conditions with John:

  • The dry sub soils should not impact return bloom initiation unless we see flagging or symptoms related to severe water stress in the trees. If you look at the trees in the afternoon and leaves are limp, this is an indication of water stress. If this continues through fruit bud formation, this will impact the number of buds that develop for next year.
  • More importantly, an even bigger stress is how many fruit the trees set. Even with variable return bloom and freezes that damaged flowers, many growers do have a better fruit set than anticipated. More frozen blossoms set fruit than expected, which is good. If there is not enough rain to satisfy tree needs and trees are slightly over cropped, this will negatively affect fruit bud formation for 2021.

As fruit begin to size, the influence of carbohydrate deficits and surpluses become even more significant. It is important to use the carbohydrate model and adjust rates of Fruitone (NAA) or MaxCel (6Ba) accordingly.  The model takes some of the guess work out of thinning by using the current weather forecast to determine if thinner rates should be increased, decreased, or applied at the standard rate. The model calculates the general carbohydrate balance, which has been found to correlate well with natural drop and decreasing tree sensitivity including sensitivity to chemical thinners. Cool, sunny periods of good carbohydrate supply leads to a natural drop and less response to thinners (increase thinner rate).

Cloudy, hot periods result in carbohydrate deficits and lead to stronger natural drop and stronger response to thinners (decrease thinner rate). The carbohydrate model needs to be consulted at the time of thinning to help determine rates. The grower needs to use their experience with thinning and adjust rates based on what they have applied in the past to the target variety and crop stage. This model does not tell you what product to use or what rate to apply but is particularly helpful when using MaxCel (6-BA) or Fruitone (NAA), that are dose dependent. Conversely, Sevin (carbaryl) is not dependent and will generally provide the same level of thinning regardless of rate. You can access the carbohydrate model from your local NEWA station.

When thinning with carbaryl: Remember that an application of carbaryl can be reactivated by light rain showers if rain is received within 48 – 72 hours of the carbaryl application. Carbaryl is not dose dependent and one pint per 100 gallons of water will work the same as one quart per 100 gallons of water. Carbaryl has never been a strong thinner and is a good at singluating or removing side blooms that are 2 – 3 mm smaller than king fruit, e.g., 10 mm lateral and 12- 14 mm king; carbaryl will remove some laterals.

If using NAA or 6BA avoid an adjuvant or acidifier or penetrant. Regardless of your carbaryl rate, the carbaryl should not impact of NAA.