May 11 AppleTalk

AppleTalk Conference Call Summary
Tuesday, May 11th, 2021 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM,
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 11th Call Stream: CLICK HERE


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

(Base 50°F)

Eau Claire, WI 4/3 74% 25% 115
Galesville, WI 3/21 90% 71% 153
Gays Mills, WI 4/3 67% 35% 165
Mauston (Northwoods), WI 3/30 86% 72% 160
Mequon (Barthel), WI 4/4 61% 38% 117
Rochester (Ela), WI 3/30 85% 63% 158
Verona, WI 4/3 68% 40% 157
La Crescent, MN 3/23 89% 60% 168
Hastings, MN 4/5 86% 35% 112
Harvard, IL 3/30 74% 46% 164

Table 1. Degree days and ascospore maturity downloaded on 5/11/21 from Cornell NEWA system. Find your local station today:

Regional summary
Another heavy frost was experienced on Saturday morning of May 8th and again the morning of May 11 throughout portions of Minnesota and Wisconsin. The week of cool weather has continued to slow crop and pest phenology. However, insect activity has not ground to a halt and obliquebanded leafrollers, green fruitworm, variegated and others continue to emerge after the weekend of 80-degree weather of May first and second. Orchards on the IL/WI state line are past full bloom with many varieties at petal fall and early cultivars such as Zestar already sizing to 3 – 5mm. To the north SweeTango and First Kiss are at full bloom and Honeycrisp was nearing full bloom at the end of last week, yet some locations near the Twin Cities are still have Honeycrisp at pink.

We continue to see extended bloom with varieties having a few blossoms still at pink while most are at petal fall.  Additionally, blooms with only one, two or three flowers continue to be observed and are at varied stages of pink to bloom. If you still have bloom hanging on, keep the bees in the orchard to encourage pollination. Looking ahead, temperatures conducive to accelerating insect active will arrive early next week. There is no rain in the forecast until the weekend of May 23 and going forward there will be several more chances of rain the week of May 23.

Drought impacts on soil moisture
The National Weather Service periodically measures soil moisture in top 30” of the soil. The latest analysis from the eastern Dakotas and upper Midwest states consisting of 430 sampling sites are showing all sites with less than 50% of the average soil moisture typically expected by May 1st. Some were below and none were above 50% of the average soil moisture we should have in the top 30” of soil come May 1st. If we do not get more rain soon, it will be hard to play catch up with irrigating/watering trees. In August high temperatures will increase evapotranspiration rates and is also when fruit is sizing. If temps stay cool or if we get more rain, this may not be an issue. But we need to begin formulating a contingency plan to water trees and this is not limited to new plantings and dwarf trees, even larger trees will be in trouble.

Fire blight
As the orchards transition from bloom to petal fall, combined with the cool temperatures and lack of rain, applications of streptomycin are largely unnecessary for many. Growers in locations where trees are just now coming into full bloom, such as those located near the Twin Cities, may still have some fire blight risk, but only if all conditions for an infection are met. Orchards at or past petal fall have a very minimal risk of a fire blight infection, under our current conditions. Orchards that still have bloom hanging on should consider that as the blossom ages, the susceptibility to an infection decreases, on top of the reality that we just have not had any rain to spread the infections!

Remember, temperatures above 65°F will lead to rapid bacterial growth and since our temperatures have been in the low 60s to 40s, there just is not a lot of bacterial growth occurring at this time. Furthermore, under cool temperatures, fire blight bacteria will die if it is not able to reproduce, therefore inoculum does not build up the way it does for a disease such as apple scab. 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 last year, 2) open blossoms, 3) moisture, and 4) warm temperatures of greater than 65°F. Extended bloom are often caused by cooler temperatures, which lower fire blight risk. For the few orchards that still have cultivars in full bloom, keep these parameters in mind over the next week.

Spring disease complex (apple scab, powdery mildew, cedar apple rust)
As dry weather persists, many orchards are stretching fungicide intervals to ten – 14 days. In a normal year, if that still exists, this practice would be ill advised during primary scab season. However, if there is no rain, there are no scab infections, and the practice can be reasonably justified. Warmer temperatures at the end of this week will increase the risk of powdery mildew infections and if your orchard has a history of powdery mildew, this would be one reason not to stretch spray intervals. Pressure form cedar apple rust is also low, due to the lack of rain. However, be cautious as apple scab inoculum continues to build and if or when we do get rain, plan on a significant apple scab ascospore release. Regarding the buildup of scab inoculum, mature-scab spores do have a life span and are tied to temperatures. High temperatures in the 80s or 90s are required for unreleased ascospores to expire and under the cool temperatures we are experiencing, scab spores will continue to be viable.

There are many fungicide options for petal fall sprays, however, orchards with low scab inoculum and minimal issues with powdery mildew and cedar apple rust may consider only applying a protectant fungicide until a more imminent threat of rain presents itself. Conventional growers may consider including a sterol inhibitor such as Rally (myclobutanil), Indar (fenbuconazole) or Ceyva (mefentrifluconazole) or Top Guard/Rhyme (flutriafol). Except for Cevya, most sterol inhibitors are very effective on powdery mildew and apple scab. Applying an SDHI fungicide is also an option, but consider avoiding premix products that include a strobilurin, if you want to save strobilurins for bitter rot and summer fruit rot management.  This would limit SDHI options to Fontelis (penthiopyrad), Aprovia (benzovindiflupyr) or Sercadis (fluxapyroxad). Merivon (fluxapyroxad + pyraclostrobin) and Luna Sensation (fluopyram + trifloxystrobin) have the addition of a strobilurin.

Organic management of the spring disease complex
Organic growers have a range of fungicide options that can be considered, beyond elemental fungicides, now that there is more leaf expansion to absorb some these biologicals. In trials completed several years ago at the University of Wisconsin, products such as Regalia, Serenade and Oxidate had a fair to poor performance rating on powdery mildew and cedar apple rust. However, these trials were completed under worst case scenarios and growers who better manage inoculum, could experience better results.

Sulfur continues to be one of the best broad-spectrum protectants against powdery mildew for organic and conventional growers. If you have a history of powdery mildew, consider an application of sulfur at petal fall, and this will also protect you against apple scab. Liquid lime sulfur continues to be a good option as a protectant and for post-infection control of scab.  Since our temperatures are cooler, the risk of phytotoxicity from sulfur or liquid lime sulfur is lower.

The last organic fungicide option that is worth consideration is potassium bicarbonate sold as Kali-green or Milstop, among other brand names.  Last spring Kari Peter presented some information on this and had some ongoing trials.  Unfortunately, she has not provided an update on the 2020 trials at this time. Dr. Kari Peter recommends applying potassium bicarbonate prior to the infection at a 3 lb. per acre rate. Rates can go as high as 5 lb. per acre, but the relative efficacy of this higher rate is unknown (this is what her ongoing research was looking at). In her high inoculum control blocks there was upwards of 80% scab infection on leaves and fruit, where she had applied the potassium bicarbonate, these 40 – 50% control of scab on leaves and fruit was achieved. Again, growers who are able to manage inoculum can reasonably expect better performance from potassium bicarbonate.

Insect management at petal fall
Codling moth
Codling moth traps should be up now, but if you have not completed this, not to worry the cool nighttime temperatures will not result in any codling moth flight any time soon, additionally, our degree days continue to lag and we expect the first codling moth flight 175 to 250 degree days from January first.  can be hung at any time.

The emergence of the spring lepidopteran complex of obliquebanded leafroller, redbanded leafroller and green fruitworm is in full swing. Obliquebanded leafroller overwinter as second or third-instar larvae and emerge around tight cluster where they feed on floral parts and developing fruit. Larvae of GFW, OBLR, RBLR, spring canker worm and variegated leafroller will feed on growing terminals and blossom clusters as temperatures increase.

At petal fall, an insecticide timed for plum curculio will often mange the remaining populations of these lepidopterans. However, the cool weather will delay plum curculio emergence and if you feel the need to manage these insects, following our bloom protocol for lepidopteran management is advised. This means rather than applying an application of Avaunt (indoxacarb), Belay (clothianidin) or even Imidan (phosmet), instead we defer to the insect growth regulator Intrepid (methoxyfenozide) or the Bacillus thuringensis products (Agree, Deliver and Dipel).  These are our only pollinator safe options at the time to manage this pest complex.

Regarding timing of Bt products, 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.

Even where return bloom is good, it may be a good idea to hold off on petal fall thinning. The below description of the carbohydrate model is dependent on healthy trees and blooms.  It is not to say the carbohydrate model does not work under our conditions, we just don’t know how predictable the response from thinners will be, where blossoms have been injured.

Attached to these notes is a separate document which summarizes several years of interviews we conducted with Dr. Phill Schwallier, now retired Michigan State University horticulturalist.

Carbohydrate model for petal fall thinning
The carbohydrate model is a tool that can be used to help determine application rates of chemical thinners and achieve more reliable thinning. 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), and are dose dependent. Conversely, Sevin (carbaryl) is not dependent and will generally provide the same level of thinning regardless of rate.

Beginning at petal fall and before fruit begin to put on size, is a relatively safe time to thin, since thinners are much less responsive at this time. For example, to achieve a 50% fruitload thinning when applying Sevin (carbaryl) at 10-15 mm fruit, the same application will generate only 25% fruitload thinning at petal fall. Application timing is dependent on variety and level of desired thinning. Since petal fall occurs at different times, Sevin (carbaryl) should only be applied as a spot application. Applications to the entire orchard at petal fall are not advised.

The carbohydrate model is available on NEWA under the crop management section, .

Check out this Michigan State University Extension publication for more information on predicting apple fruit set model:

Development of a Fruitlet Growth Model to Predict Thinner Response on Apples:

Recommended rates of chemical thinners based on the target variety are published in the Midwest and Michigan commercial tree fruit spray guides. Please consult these resources for specific rates. Note: These rates are based on a healthy and undamaged fruit set. The Midwest Fruit Pest Management Guide is available online for free and thinning recommendations may be found on pages 36 – 38: