May 12 AppleTalk Call

AppleTalk Conference Call Summary
Tuesday, May 12, 2020 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM,
Moderator: Peter Werts, IPM Institute of North America; questions or comments, or send to Josie Dillon, .

May 12th Call Stream: CLICK HERE

Guest Speaker Schedule Update

  1. May 19: Dr. Dan Cooley, University of Massachusetts presenting on orchard floor sanitation and apple scab.
  2. June 2: Dr. Brent Short, Trecé presenting on Codling Moth Lures and Mating Disruption
  3. July 14: Dr. Sara Villani, North Carolina State University presenting on Summer Fruit Rots

Regional update

Location May 8th Low Temperature °F May 9th Low Temperature °F Degree Days

(Base 43°F)

Degree Days

(Base 50°F)

Percent Scab Ascospore Maturity Percent Cumulative Scab Ascospore Discharge
Eau Claire, WI 26.5 22.5 161 42 18 7
Gays Mills, WI 34.0 26.9 213 73 42 25
Hastings, MN 32.6 28.8 235 83 23 12
Harvard, IL 28.3 27.7 184 54 56 37
La Crescent, MN 32.3 29.4 222 76 48 30
Mauston (Northwoods), WI 27.9 27.8 166 43 21 13
Mequon (Barthel), WI 27.0 25.0 107 30 12 5
Rochester (Ela), WI 26.2 24.8 141 37 33 21
Trempealeau (Ecker’s), WI 28.0 25.3 199 62 42 18
Verona, WI 32.1 29.7 190 54 35 21

Table 1. Degree-day accumulations as of May 11, 2020 using data reported by Cornell NEWA Network.

Freezing temperatures
Low temperatures on May 8th and 9th in most locations were variable and did not drop below 27°F, excluding Eau Claire (22.5), Mequon (25°F), Rochester (24.8°F) and Trempealeau (25.3). A few growers have estimated a 40-60% bud loss due to the cold temperatures. 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.

The longer the freezing temperatures stayed below 28°F, the more likely freeze damage occurred. Absolute lows may have lasted for less than an hour depending on the location. One grower reported a low of 26°F according to the NEWA station, but placed a thermometer in the lower part of the orchard and recorded 18°F. This means different parts of the orchards likely experienced different temperatures, especially lower sections where cool air will pool. Most of the browning from freeze damage that John noticed on Saturday was seen in the pistil, and on Sunday the anthers became discolored as well. There are also some varietal and rootstock differences that may lead to more damage in high density trees than in larger trees. Honeycrisp were the least damaged, and trees at late-pink and king bloom may have more significant injury. John believes most growers should have at least 10% of blossoms that are viable, which is plenty to set a crop.

Some growers applied 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. This freeze occurred at an earlier stage of phenology than when Promalin was last used in 2016. Even in the best scenario at 90% percent damage, studies show that Promalin applied within five hours won’t provide a full crop but may give 25-30% which is a significant difference. We likely won’t know the full extent of the damage until fruit set in a few weeks.

Professor Phil Schwallier, Michigan State, has previously discussed that if flesh around the ovary is damaged, Promalin will not work. Some blossoms are showing browning in the stigma and into the pistil, which 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.

KDL Label:

Promalin Label:

For more information on assessing freeze damage, visit:

Video analysis of freeze damage from Purdue:

Tree stress
Many orchards have an excellent return bloom this year and after several weeks of cold weather, trees are likely going to be stressed during bloom.  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. Soil temperatures are also low, which prevents mineralization of nitrogen and reduces root growth. Applying a foliar spray, e.g., fish emulsion, 20-20-20, urea, etc. will give the trees a boost for several weeks. Please reference the April 28, 2020 call with Dr. Amaya Atucha who discusses early season nutrient needs in more depth:

Disease management
Apple scab update
NEWA weather stations are reporting between 30% and 50% ascospore maturity. Most of the region will experience an infection event on Wednesday or Thursday this week with an estimated discharge of 10% to 25%. This infection will be more significant than the last few. Orchards that received a fungicide application around May 9th or 10th should have adequate protection for mid-week rain and those with high inoculum should be re-covered prior to the next rain. Depending on the amount of rainfall and leaf growth midweek, fungicides may need to be reapplied on Friday May 16, in anticipation of additional rain on May 16th and 17th.

Organic growers should apply sulfur on scab susceptible varieties which will provide protection over Wednesday and Thursday. Regalia (Reynoutria sachalinensis) and other biological fungicides may not perform very well this week, since the trees have minimal leaf tissue and the cool weather may slow colonization of the bacteria on the leaf surface and time required to penetrate the cuticle to act as a protectant may be extended. If there is a significant infection period, an application of lime sulfur may be applied to high-inoculum cultivars post-infection. It would be best to apply before full bloom, since lime sulfur can also have a thinning effect, when applied to open blossoms.

If growers want to add a single site fungicide to their mancozeb or captan spray, John recommends using one of the newer SDHI’s rather than a DMI or a Strobilurin. This is because SDHI’s have great efficacy against scab, powdery mildew, and rust. DMI’s do not function as well when temperatures are below 65°F. Many growers do use Inspire Super (difenoconazole, cyprodinil), which does work well in cold weather, because it has the Anilopyrimadine component in the spray. In order to best manage apple scab and reduce risk of scab developing resistance to these fungicides, all of these fungicides must be tank mixed with mancozeb or captan and should be applied prior to the rain event

Powdery mildew
Conditions for spread of powdery mildew (PM) over the next ten days will be moderate, with warm temperatures and lower relative humidity. Powdery mildew germination and secondary infections favor temperatures between 50°F and 70°F and relative humidity greater than 70%. Powdery mildew is spread by wind but will not spread when it rains because the conidia (asexual spores) cannot germinate in water. New infections can be visible within 48 hours of an infection and can begin sporulating in five days.

The most critical time to manage PM is at petal fall and applications between tight cluster and first cover may also be necessary where inoculum is high. Sulfur may be used as an early spray when there is less green tissue to protect and less pressure from the fungus. Rally remains an excellent fungicide for PM, even though its efficacy has become diminished on apple scab and has limited activity on rust diseases. Factors that influence infection risk may include warm temperatures, amount of shoot growth, unprotected tissue, and fungal activity when it is producing the most spores. 

Cedar apple rust
Between May 9 and 11 John observed his first mature galls on red cedar, which indicates this week’s rain events will release their spores and cause the first cedar apple rust (CAR) infection. 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. Spores can be carried long distances, i.e., 3-5 miles, yet most infections occur when infected eastern red cedars are within a few hundred yards. Spores that land on young apple tissue may germinate and create an infection if a film of water is present for an adequate amount of time. 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.

EBDC’s applied using the pre-bloom schedule should manage CAR infections, however, infections that happen at bloom or post bloom are likely not targeted with these higher rates. Using the extended spray schedule would be the best option to continue applications of EBDC fungicides through petal fall to manage CAR. However, the half rate of EBDC’s will not offer full protection against CAR. Not all single-site fungicides added to this half rate of an EBDC for scab and powdery mildew will manage rust. The cheapest fungicide to protect against CAR infections are the strobilurins Sovran (kresoxim-methyl) at four oz per acre or Flint (trifloxystrobin) at two oz per acre. John prefers to use SDHI’s from pink to early bloom to target the primary scab season. SDHI’s cannot be applied more than twice in a row and will therefore likely not be available for protection against CAR after early bloom. Strobilurins are effective for rusts as well as powdery mildew. The newer DMI’s are also effective on rust, e.g., Cevya (mefentrifluconazole). Captan does not manage CAR, and where Captan is applied at bloom or later, it is important to add one of these other fungicides if CAR remains a concern.

Organic growers have very few products that offer efficacy against rust. To help keep trees healthy, John recommends using an immune system activator, e.g., Regalia (extract of Reynoutria), Serenade (Strain of Bacillus subtilis) along with sulfur, which targets powdery mildew and scab, on a 10-day basis. Copper may offer some control but is likely no longer being used.

For more information on cedar apple rust, visit:

Insect management
The insects are currently trailing tree phenology and will not catch up until temperatures are regularly above 50°F. The forecasted 70°F temperatures over the weekend and into next week should trigger flights and egg hatching. By the end of the weekend early signs of lepidopteran larvae should be visible and could include green fruitworm (GFW), redbanded leafroller (RBLR), obliquebanded leafroller (OBLR), spring cankerworm (SCW). If bloom moves fast, managing spring lepidopterans could wait until petal fall, which is a practice employed by many growers. When it comes to timing an insecticide for these insects, there is no benefit in applying them before you see the earliest evidence of their feeding and should not rely on tree development to inform insecticide applications.

The insect growth regulators Esteem (pyriproxyfen) or Intrepid (methoxyfenozide) may be applied between green tip and up to bloom. Esteem cannot be applied during bloom due to potential pollinator toxicity. Agree, Deliver and Dipel (Bacillus thuringensis), Bt, products are safe to apply when bees are active in the orchard during bloom. If blossoms are still closed a Bt application will not reach larvae that have tunneled into closed blossoms. Bacillus thuringensis must be eaten by the insect to be effective and warm temperatures are needed in the 72-hour period following an application for good mortality. Bt products should be applied when it is warm (~60°F) and sunny. If an application is warranted, manage these pests in the early larval stage, e.g., first or second instar, while they are actively feeding on leaf tissue and before trees reach petal fall. After petal fall, these species will be harder to manage since many of them may be nearing the end of their life stage as a larva.

Visit this link for a list of degree-days for insect emergence, and a list of materials that may be applied during bloom: 

Codling moth
The first codling moth (CM) capture has traditionally been around petal fall or 180 DD, base 50°F, and pheromone traps should be hung as soon as possible. The Trece’ long-life lures (CM L2) are active for eight weeks, the standard-lure (1x) need to be replaced 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. The Trece Combo Meso boxes only cover half as many acres as the CM Meso box because there are two species being disrupted.

Beware of the CM look alike! Proteoteras is genus of moths belonging to the family Tortricidae. Codling moths are also in this family, but belong to the genus Cydia, and have some similarities in appearance and can be confused with these moths. Proteoteras has a similar wingspan, but is slightly narrower and lacks the bronze coloring on the wing tips of CM. For photos of CM and two look-alikes visit:

Plum curculio
The temperatures expected over the weekend and into next week, will likely be enough to trigger PC migration from overwintering sites.  Plum curculio (PC) emerge around 250 DD, base 50°F, and movement into the perimeter of orchards begins when temperatures are above 60°F. This is a gradual process and requires several days of warm weather. PC are primarily nocturnal and the greatest amount of activity will occur when temperatures at night remain warm. The entire population does not migrate into the orchard at the same time and fruitlets become susceptible to PC egg-laying when they reach 5 mm.

Black stem borer
The IPM Institute has partnered with the WI Department of Agriculture, Trade and Consumer Protection, for a third year in a row, to implement a state-wide invasive insect pest survey in apple orchards and vineyards. The project includes monitoring for black stem borer, velvet longhorn beetle, oak ambrosia beetle, and brown marmorated stink bug (BMSB) in orchards. We are working with 18 orchards to study these pests, along with 12 vineyards where we are monitoring European grape berry moth, European grape vine moth and BMSB. BMSB has been detected in 12 counties in WI and black stem borer (BSB) has been captured in seven counties. There are still many questions and variability when it comes to BSB emergence. One study shows late emergence, around 175 DD base 50°F, and when speaking with Dr. Art Agnello at Cornell University, he said they emerge around 100 DD base 50°F in combination with several days of warm weather in the 70’s °F.

Black stem borer is hard to manage because it doesn’t do any feeding on the trees. The adults bore into the tree and hollow out a gallery. Then the larvae consume a fungus that grows inside the tree, which makes the larvae difficult to kill. The only potential approach is if someone used a trunk spray of Lorsban ahead of emergence, where the adult BSB would hopefully encounter some of the residue on the trunk. This is not an orchard-wide pest but is targeting trees that are already stressed and in decline. BSB is attracted to ethanol released by stressed trees. Most of these infestations and tree decline occurs on three to four year old trees and are not worried about larger and older trees. To some extent, BSB is considered part of the Sudden Apple Decline and Rapid Apple Decline complex.