May 4 AppleTalk

AppleTalk Conference Call Summary
Tuesday, May 4th, 2021 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM,
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 4th Call Stream: CLICK HERE


Location Green Tip Date Ascospore Maturity (%) Ascospore Discharge (%) to date Degree Days

(Base 50°F)

Fire blight infection 5/2 – 5/4

(assumes rain accumulation + FB last year)

Eau Claire, WI 4/3 54% 21% 113 Yes
Galesville, WI 3/21 77% 70% 151 Yes
Gays Mills, WI 4/3 41% 35% 166 Yes
Mauston (Northwoods), WI 3/30 68% 63% 160 Yes
Mequon (Barthel), WI 4/4 39% 34% 118 Yes
Rochester (Ela), WI 3/30 73% 72% 156 Yes
Verona, WI 4/3 42% 36% 158 Yes
La Crescent, MN 3/23 73% 58% 168 Yes
Hastings, MN 4/5 68% 20% 112 Yes
Harvard, IL 3/30 53% 45% 166 Yes

Table 1. Degree days and ascospore maturity downloaded on 5/03/21 from Cornell NEWA system. Find your local station today:

Assessing soil moisture

Despite the recent rains for most growers, soil moisture is becoming critically dry if significant rains do not materialize. This concern applies to all trees, just not newly planted. Even with the 0.75 to 1.5” of rain that we received; growers should check soil moisture at a depth of six inches. One method of doing this is the “ball test”. Soil should form a nice ball when squeezed and if you toss in the air and catch it on your palms, it should stay intact on the first toss. Depending on how many times you toss it into the air before it breaks apart should serve as an indicator as to how dry or wet the soil is. While crude, it is a helpful and low-tech method that can be effective. Dig small holes to check moisture in the row middle to get an indication of moisture levels across the whole field. Even though we had a nice rain, need to still address water deficits through irrigation and watering. Most locations received about 1.3 inches of rain for the month of April and the normal is at least three inches of rain plus some snow.

Disease management

Spring disease complex (Apple scab, powdery mildew, and cedar apple rust)

The everchanging weather patterns continue to pose challenges for apple scab management. Orchards that received rain between Saturday and Monday night should expect this to be the most significant infection period of the season. The ten-day forecast continues to lack significant rainfall and extending spray intervals may be justified, however, apple scab inoculum remains very high for many orchards. In this most recent rain, orchards may have had 40 – 50% of their spores for the season released. We should consider that 50% in a high vs. low inoculum orchard is a significant difference.

Bloom is a good time to apply an SDHI fungicide, (i.e., Aprovia, Fontelis, Miravis, Sercadis), along with a protectant fungicide to manage powdery mildew and rust, in addition to apple scab. If you are concerned about bitter rot, it is advised to not apply a strobilurin, (i.e., Flint, Sovran, Luna Sensation, Merivon), at this time and save these for later in the season. If powdery mildew has been a problem in your orchard, this past weekend offered good conditions to encourage its spread. The next five days, the risk of powdery mildew infections decreases with the falling temperatures.

The next rains may only have a 10% -20% spore release if you had a high release during the rain between May second and third. Use the NEWA stations to compare seasonal spore maturity vs. what has been released. The next ten days to two weeks we will likely reach 100% ascospore maturity for many and is a very critical phase to manage scab infections.

Organic strategies for scab management

The trees have developed enough vegetative-leaf growth that make application of biological fungicides an option for organic growers. The efficacy of products such as Regalia and Serenade on apple scab is fair at best, however, most scab trials occur in trial blocks with very high scab inoculum, and it is possible performance may fair better in low inoculum orchards. Considering the high risk of scab infections, applications of sulfur, liquid-lime sulfur or potassium bicarbonate have a better track record of managing apple scab. Liquid lime sulfur may be applied post infection, whereas sulfur and potassium bicarbonate should be applied before the infection period

Fire blight

The 80-degree temperatures on Saturday May first and Sunday May second, followed by rain on the evening of May second and into May third resulted in the first fire blight infection of the season for many orchards. If your orchard did not receive any rain, then there was no fire blight infection. Bacterial growth exploded at a very rapid rate and potentially a very dangerous thing if an orchard received rain before it cooled off. For streptomycin to be effective, it must be applied within 24 hours of the infection. The forecast for the rest of the week is calling for cool temperatures with highs in the low 60s and lows back into the upper 30s, with minimal chances of rain until Saturday May 8th. Under these conditions, there will be very little to no fire blight infections.  Additionally, since we have had very little rain to date, fire blight cankers should have been suppressed by rain redistributing copper.

Trees are only susceptible to fire blight infections if blossoms are open, and moisture is present. These basic parameters need to be met for an infection to occur: 1) Inoculum or signs of fire blight in neighborhood last year, 2) open blossoms, 3) moisture, and 4) warm temperatures of greater than 65°F. Be observant for cankers and bacterial ooze that could spread infections, even as risk decreases. At the current stage of tree growth, transmission down shoots where new tissue can be exposed, is a primary source of fire blight infections. Rapid or easily transmitted fire blight will be a possibility until shoots stop growing.

Streptomycin applied 24 hours before or after an infection period or rain event will provide control for up to 48 hours of bacterial growth inhibition after the infection, though some say it can provide up to 72 hours of protection when applied preventatively. The fire blight Epiphytic Infection Potential (EIP), which references bacterial growth, should be tracked starting when blossoms open and restarted when streptomycin is applied after an infection period. If including Regulaid with Streptomycin, no other fungicides or foliar nutrients should be included with this tank mixture. Regulaid is a strong penetrant and will enhance the uptake of pesticides and tank-mixing other products should be avoided if possible. Research from Dr. George Sundin at Michigan State shows that Regulaid is not required a second time if applied with Streptomycin already. It changes the permeability of the flower so a second application of Regulaid is not needed. Additionally, Captan and sulfur should not be included with this spray, but most other fungicides will be okay.

All other bactericides such as copper or Kasumin (kasugamycin) need to be applied before an infection occurs. Copper products like Cueva (copper octanoate) should be applied as the blossoms open. Copper applications may be beneficial on non-bearing trees or trees where fruit russeting is not a concern. This is an excellent time to scout for oozing fire blight cankers that could be painted with copper. Cankers that are not oozing by now most likely are not active fire blight cankers.

Apogee (prohexadione calcium) may also be applied to help minimize fire blight risk and is generally applied at pink, or as soon as possible. Instead of maximizing shoot growth reduction by applying three to four applications, applying once at a higher rate will help maximize fire blight control. Apogee is often applied at 6oz per acre at pink and again at 6-9oz per acre at petal fall to help with fire blight, according to Allen Teach of Sunrise Orchards, Gays Mills, WI. If the rate is too high, it may make the fruit “stick” to the trees and will be very difficult to thin. If not applied at pink, apply Apogee at 6oz per acre or slightly less. When applied at king bloom petal fall, Apogee can increase fruit set. Apogee rates based on tree size and tree row volume are discussed in the Michigan and Midwest spray guides

As blossoms age they become less susceptible to blossom blight.  Therefore, once the trees reach petal fall the risk of a fire blight infection to the blossoms is minimal and Streptomycin should not be applied. However, shoot blight remains a risk on vigorous non-bearing trees.  These may be treated with a range of biological products.

Insect updates

Plum curculio

Remember to write down your McIntosh petal fall date to denote the start of tracking degree days which predict the end of plum curculio emigration from overwintering sites into orchards. During bloom lookout for wild plums, cherries and apples that serve as external sources for plum curculio. Eliminating these external sources along fence lines and hedge rows can improve PC management in organic orchards or where softer chemistries are used.


Green fruitworm and leafrollers

The spring lepidopteran complex is finally starting to emerge because of the weekend of heat.  Obliquebanded leafroller (OBLR) are coming out of diapause and first-instar larvae for Redbanded leafroller (RBLR) and green fruitworm (GFW) have been observed. OBLR are a bit more obvious since they are feeding on leaves, whereas RBLR and GFW are more likely to be found feeding on blossoms. Locations near Chippewa Falls or the Twin Cities may be a few days behind. Everyone one else who is beginning to observe these insects have the option to apply Bacillus thuringensis products, e.g., Agree, Dipel, Javelin, etc. or the insect growth regulator (IGR) Intrepid. Do not apply the IGR Esteem during bloom.  Ideally, these insecticides should be applied when temperatures are above 60°F.


1st instar European red mite nymphs can be observed on the small leaves around fruiting spurs.  Early in the season they will not be observed on the vegetative shoots. This is the largest population of ERM that John has observed in a long time, likely due to the poor conditions for applying oil. Orchards with high populations of predator mites, may still get good suppression of ERM populations if no oil was applied this spring. If the weather remains dry, two spotted spider mites will move up into trees, from broadleaf weeds and grasses that begin to dry down from lack of water.


Normally during bloom is when rosy apple aphids begin to appear on fruiting spurs and growing shoots.  John has been seeing some leaf curling from aphids, but not RAA. This would be the most problematic on non-bearing trees and can often be managed by petal fall sprays. At this time, note where aphids are observed, but avoid insecticide sprays that may be toxic to pollinators.

Bloom thinning

Developing fruit are at many different growth stages due to the variable weather conditions in April and while we are still in bloom, it is important to begin thinking about thinning sprays. Varieties impacted by the freeze events may still require thinning. Bloom thinning can be a very safe time to apply mild thinners and achieve a small amount of thinning to make these decisions easier at petal and especially as fruit reach 8 – 10 mm. Where blossoms have been damaged, the carbohydrate model may not be as effective, and thinning will be more challenging as fruit size up. Therefore, bloom and early petal fall, are a good opportunity to apply mild thinners, with minimal risk of over thinning. A good example of this is where king bloom has been damaged.

Climate impacts on tree physiology

Most orchards experienced blossom injury from three or four nights of freeze where the temperature dropped below 28F in mid-April. At first it appeared many orchards had minimal injury and as we have assessed the crop, the injury is becoming more obvious. At first it appeared late varieties like Honeycrisp were minimally affected by the freeze since they were at green tip or early tight cluster, as opposed to pink. Conversely, varieties which include Zestar, which opened early, damage is more severe. As Zestar blooms have aged and are at petal fall, there is a lot more damage than what was observed ten days ago. As Honeycrisp phenology has moved along, it has become apparent for some there is a significant lack of return bloom in orchards that had a balanced cropload and employed use of plant-growth regulators, such as NAA to encourage return bloom. John sees this impact as an indicator of something else going on and is different than locations with poor return bloom due to overcropping. Now John is observing many orchards in southern Wisconsin do not have a consistent return bloom across many varieties, apart from Gala.

John’s new hypothesis relates to how temperatures during dormancy impact fruit bud formation. Typically, we think of fruit bud formation begging when shoot growth stops, but new research shows that on Honeycrisp, next year’s fruiting buds begin developing as early as petal fall. Most other varieties begin forming fruit buds in June at terminal set and continues into the fall. New research shows that even during dormancy fruit buds continue to develop and this development is regulated by the ambient temperatures, therefore our fluctuating winter temperatures and specifically mild winters may create additional challenges outside of early blooms.

John believes the poor return bloom may be tied to our mild January and March. In February and March, the fruit buds in apples are forming proteins. These proteins form at temperatures between 0 – 10°C (32 – 50°F) and accumulate in the fruit bud. As temperatures drop below freezing, protein production will stop, and accumulated protein dissipates from the fruit. If temperatures remain above freezing for too long, proteins, may keep accumulating to a level which can cause developing fruit buds to abort and turn into a vegetative bud instead of a fruit bud.

This winter had a significant number of days with temperatures between 32 – 50°F, which would have been ideal for protein accumulation. The average temperature in March was 40°F for southern WI, which was above normal and there was not a single day in March where the temperatures were significantly below freezing. Could have the above average temperatures in January and March resulted in enough protein accumulation to abord buds?

So, what has this injury looked like? In Honeycrisp, John has seen the vegetative leaves remaining and instead of a blossom emerging, vegetative growth emerges making the leaf emergence look funnel shaped. Another phenomenon has been buds that only pushed out two or three blossoms instead of five blossoms. Damage to the structures inside the ovules, flesh of apple and what becomes the seeds that push out pollen tubes, have also been damaged and appear as brown tissues in the center of the blossom.

This injury varies from orchard to orchard and within orchard, and by rootstock and variety. If you have a light bloom, examine flowers as most will not be healthy. Even on Honeycrisp that were protected by freeze because of their later bloom appear to be unhealthy.

If you have a poor return bloom, evaluate if you over cropped in 2019 or 2020. For example, over cropped Honeycrisp in 2019 should have resulted in a light bloom in 2020 and have a heavy return bloom in 2021. Yet, there are increasing reports where this occurred and return bloom has been poor this year. This has been observed across the region. In this instance, we can see where the fruit buds were. It looks like a fruit bud and then has a strange funnel shape growth pattern. John believes after another week of growth; we may not see the difference between the aborted fruit buds and vegetative buds.