May 7, 2019 AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, May 7th, 2019, 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 7th Call Stream: CLICK HERE

Regional update
Tree phenology around the state made some progress over the weekend, moving from green tip and tight cluster last week to tight/open cluster and early pink.  Remember to write down green-tip dates for early varieties to help predict and track primary ascospore maturity using local NEWA stations.  Looking forward, petal fall dates should also be recorded for plum curculio management.

Location Degree Days 4/29/2019 (Base 50°F) Degree Days 5/6/2019 (Base 50°F)
Eau Claire, WI 41 55
Gays Mills, WI 75 93
Hastings, MN 56 69
Harvard, IL 72 86
Lake City, MN 60 75
Mauston (Northwoods), WI 50 66
Mequon (Barthel), WI 33 43
Preston, MN 61 77
Rochester (Ela), WI 62 78
Trempealeau (Ecker’s), WI 51 69
Verona, WI 70 85
White Bear Lake 44 56
Woodstock, IL 82 98

Table 1. Regional DD accumulation comparing April 29 to May 6, 2019.

Rain and cooler temperatures are expected for the remainder of the week and dditional apple scab infections are expected.  Warmer temperatures return this weekend and as bloom opens up and temperatures rise above 65⁰F, this will induce prime conditions for fire blight bacteria to grow on open blossoms.  Most locations in the region have accumulated between 50-100 degree-days (DD), base 50°F and early varieties, e.g., Zestar and Duchess, along the Illinois and Wisconsin border are in full bloom and Honeycrisp are at open cluster to pink.

Historically, first generation codling moth biofix has occurred around 250-300DD from January 1st (typically May 15th-22nd) but can occur as early as 180 degree-days.  Codling moth traps should be hung over the next week to allow a period of 10 days between hanging traps and first flight.

Growers who normally monitor redbanded leafroller (RBLR) should make sure these traps are hung, as the first flight usually begins around 75 DD and has already been observed.  Overwintering larvae of obliquebanded leafroller emerge at 105 DD and are expected to appear any day.  Emerging OBLR kick off the start of the spring-leafroller complex, which also includes RBLR, spring cankerworm and green fruitworm.  These larvae can be found in tree terminals and developing fruit buds.

Scab Infections week of May 6, 2019
Depending on the location, NEWA is reporting between 40% and 50% ascospore maturity.  Unlike the infection periods the week of April 30 to May 6, where spore discharge was around 2%, the infection period for May 8 – 9 has an estimated 5% to 9% discharge.  This means, the current infection period is more significant than the last few and will continue to be more important as we near bloom and petal fall.  Orchards that received a fungicide over the weekend should have adequate protection.  Orchards with high inoculum should be re-covered if there is a break in the weather and may not want to wait until five or seven days since the last fungicide was applied.  Orchards with low inoculum or that were covered over the weekend may wait until Friday or the weekend to reapply their fungicides.

Organic growers should apply sulfur on scab susceptible varieties which will provide protection over Wednesday and Thursday.  Using Regalia (Reynoutria sachalinensis) is not recommended, as it will take time to penetrate the cuticle and act as a protectant due to the cool weather.  SDHI’s, strobilurins, e.g., Flint (trifloxystrobin), Sovran (kresoxim-methyl) and DMI’s, e.g., Rally (myclobutanil), Indar (fenbuconazole), Topguard (flutriafol) will all function ahead of the infection quite well.  Other DMI’s e.g., Inspire Super (difenoconazole) will work better than Rally (myclobutanil).  It is critical to note that during cooler weather (< 60°F), DMI’s do not perform as well.  This is not listed on the label but is important to be aware of.  These products include Scala (pyrimethanil) and Vanguard (cyprodinil).

Conventional and organic-fungicide rainfastness
Most growers have been able to apply a protectant fungicide in the last week, however many areas within the region have received over an inch of rain and this protectant has likely washed off.  With every inch of rain, 50% of the fungicide will be washed off, with 100% wash off occurring after two inches of rain.

Rain accumulation will vary across the region, with some areas receiving less than an inch to others that will receive up to three inches.  IPM growers that tank mix a single-site fungicide with a protectant fungicide will have better protection.  Even though the protectant fungicide may wash off, the single-site fungicide will offer greater rainfastness, due to the fungicide being absorbed through the cuticles of the tree’s leaves.

Ideally, pesticide applications should have at least two hours of good drying conditions prior to rainstorms.  Once captan, mancozeb or sulfur has dried, additional drying time beyond two hours is not going to change its performance.  If it begins to drizzle just as the application is completed, it should be effective as long as rain accumulations are not significant and cause wash off.  John suggests that growers note when rain starts and how much time has occurred from application to the beginning of rain.  Fungicides that are systemic or designed to be absorbed into the plant cuticle will need additional drying time beyond this two-hour minimum, and include Regalia (Reynoutria sachalinensis), Aprovia (benzovindiflupyr) and Rally (myclobutanil).

Cedar apple rust
Cedar apple rust (CAR) caused by the pathogen Gymnosporangium juniperi-virginianae, cannot spread from apple to apple or from red cedar to red cedar because the fungus experiences a two-year life cycle, alternating between hosts.  Unlike scab, rusts require an alternate host and inoculum present now 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.  John has seen a few small galls but are not easy to spot and won’t likely be more visible until we have reached high levels of relative humidity with warmer temperatures.

Photo 1. Dry cedar apple rust galls collected in Richland, Co. John Aue, 5/7/2019.
Photo 2. Cedar apple rust galls after exposure to 100% relative humidity for 30 minutes. John Aue, 5/7/2019.

Cedar apple rust infections occur between tight cluster and first cover.  Spores can be carried long distances (3-5 miles), yet most infections occur when infected eastern red cedars are within a few hundred yards of apple trees.  Spores that land on young apple tissue can germinate and if leaf tissue is wet for a long enough duration, an infection will occur.  The time required for an infection is similar to apple scab and other diseases, where temperature is a driver of minimum and maximum number of leaf-wetting hours for a successful infection.  Symptoms will usually appear one to two weeks after infection.  If you have access to red cedars, look for orange galls on the terminals and mid points of red cedar branches.

Cultivar resistance to rust is quite varied and typically a special spray for rust is only needed on one or two varieties.  Susceptibility of cultivar and proximity to an infected host will influence disease pressure.  Mancozeb applied between tight cluster and first cover should offer adequate control, however in recent years we have observed more significant CAR injury and orchards with a history of infections should consider including: Flint Extra (trifloxystrobin), Inspire Super (cyprodinil, difenoconazole), Pristine (boscalid, pyraclostrobin), Procure (triflumizole), Rally (myclobutanil), Sovran (kresoxim-methyl) or Topguard (flutriafol).  These are all listed as having “Excellent” efficacy in the 2019 – – 2020 Midwest Fruit Pest Management Guide.  More discussion on efficacy of these fungicides is on page 39 of the guide.  Cedar apple rust is not controlled by captan.  For more information on temperature and moisture requirements for CAR periods visit:

Powdery mildew
Powdery mildew (PM) germination and secondary infections favor temperatures between 50 and 70°F and when relative humidity greater than 70%.  Powdery mildew is spread by wind but will not spread during rain events 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.

Powdery mildew overwinters within buds in apple trees.  If temperatures fall below -10°F for a few days, buds infected with PM will die.  The entire region experienced temperatures that fell well below -10°F, but this does not mean that PM was eradicated.  Powdery mildew can overwinter in other locations and is possible that some colonies survived the polar vortex, but overall inoculum levels should be relatively low.  New trees coming from nurseries may have PM and may need to be treated, even though other established trees nearby do not.

Organic growers can use lower rates of sulfur to control powdery mildew.  Sulfur is a broad-spectrum protectant and conventional growers may tank mix sulfur with DMI’s and Strobilurins for added protection and resistance management.  Do not apply sulfur within 14 days of an oil application.  Do not use captan in combination with or closely following or in alternation with sulfur products.  Rally (myclobutanil) is an effective fungicide and most other single site fungicides have some activity against PM.  The aminopyrimidine class of fungicides, i.e., Scala (pyrimethanil) and Vanguard (cyprodinil) are an exception and do not provide any PM management.  Use the product label to verify use restrictions.  Fruit russeting and yield reduction can occur if sulfur is applied during hot temperatures (>80°F), especially following bloom.

It is recommended to apply fungicides for powdery mildew beginning at petal fall.  Sulfur can be used as an early spray when there is less green tissue to protect and less pressure from the fungus.  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.

Boron and zinc effects on developing blossoms
Boron and zinc are important micro nutrients used to promote flower, leaf and shoot development, and improve fruit set and quality.  Deficiencies in these nutrients can cause poor shoot and leaf growth.  Boron deficient fruit may become deformed and develop corky, dry lesions in the flesh.  For photos visit:

The first application of boron may be applied between late pink and early bloom to promote flower viability and fruit set.  This early application will not correct deficiencies identified during routine-tissue analysis.  A later application, applied between petal fall and third cover, would be used to correct deficiencies in fruit and vegetative growth.  Zinc applied at this time will stimulate early flower bud, leaf and shoot development if tissue analysis shows zinc levels to be deficient.  It is important to note that boron and zinc do not translocate from one area of the tree to another and blossoms must be open at the time of application to encourage pollen-growth tube development.

If tissue analysis from the previous season shows a boron deficiency or deficiency symptoms are observed, apply boron once or twice after petal fall.  The first application is typically timed for first cover and the second application is made at third cover.  To increase boron, apply Solubor (20% boron) as a foliar application (1 lb. per 100 gal), at this time.  Do not apply more than 0.5 lb. of boron per acre per year to avoid boron toxicity and do not tank mix Solubor with oil.  There’s a very fine line between a boron deficiency and boron toxicity, so be mindful of application rates.  Solubor is also OMRI approved.

To amend zinc, apply a foliar application of zinc chelate.  Most suppliers carry a 9% zinc chelate solution (Zn-EDTA) and recommend using one quart of Zn-EDTA per 100 gal.  Zn-EDTA is fairly compatible and if needed can be tank mixed with boron or urea.  Always verify label rate before making an application.   Applying too high of rate can result in phytotoxicity.

Note: Verify the pH of spray solutions when preparing an application.  If the pH of the spray solution is greater than seven, products like Solubor can precipitate out and are no longer effective.  It is recommended to not mix fertilizers with pesticides or oils.  Growers concerned about their spray solution pH are recommended to track and record this information on their fertilizer and pesticide records.  Add an acidifier if the pH is high.  Digital pH meters are an easy and reliable method to verify your spray water pH.  The following links highlight how water pH effects the stability of pesticides.

Winter injury and impacts on return bloom
The upper Mississippi valley has good return bloom on older varieties and newer varieties such as SweeTango and MN 55 have nice return bloom and appear to have minimal winter injury.  Return bloom and health for Honeycrisp on B9 are variable where some trees have many clusters, and the tree directly next to it will have zero clusters.  Return bloom for Honeycrisp in other parts of the state is also light, but good crops are expected on many other varieties.

Trees can abort buds and turn them into vegetative shoots when under extreme stress.  These converted buds are longer, narrower and funnel shaped where there would have been a cluster.  Light bloom on multiple varieties and rootstocks, may be more closely related to cropping history and short-term stress.  A heavy crop load followed by high winter stress may be enough to transform many fruit buds or result in a lighter return bloom.  Only 10-15% of return bloom is needed to set a crop.

If one variety or rootstock combination has significantly fewer blossoms, e.g., Honeycrisp on B9 compared to Honeycrisp on M111, this should be a red flag that those trees have a long history of accumulated stress.  The polar vortex probably is responsible for some injury, however, may not be the sole reason for trees converting buds into leafy material.  The last two years have been marked by heavy rain in the fall and could also be a contributor of tree stress.  Blossoms can be aborted as late as February or March if enough stress is induced.  Pay attention to tree stressors in the environment, e.g., too much water, not enough water, nutrient deficiency, heat stress, cold stress, etc.  Tree stress builds over time and results in extreme measures that trees must take for self-preservation, e.g., bud abortion.

High levels of stress also make trees susceptible to opportunistic pathogens like black rot, white rot or other canker fungi.  These pockets of disease will spread if the tree is extremely stressed.  The bark can be scraped off two to three-year-old wood, and if the pith or heartwood is brown, the tree is dead.  Focus on mitigating any additional stress, e.g., timely water and nutrient management, and be prepared to apply foliar nutrients during periods of cloudy and warm or cool weather once the crop is set.  Apply a phosphorous acid fungicide for potential root diseases if the region continues to experience prolonged periods of rain.

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.

Polar vortex effects on insect development
Most insects can survive a short period of extremely cold weather, including this winter’s low temperatures of -40°F.  It is likely that some insects were killed, if their overwintering locations were more exposed.  Insects that overwinter as eggs or adults are much more tolerant of cold weather compared to insects that overwinter as larvae or pupae.  Polar vortex survival also depends on whether the species is acclimated to cold temperatures, e.g., if it is native non-native or invasive.

Apple insect pests that overwinter on the tree as a larva include obliquebanded leafroller (OBLR) and codling moth (CM).  Codling moth overwinter as pre-pupae and OBLR overwinter as second or third instar larvae.  Where temperatures fell below -14°F over several nights, there could have been some impact on these overwintering larvae.  However, there is enough genetic diversity within the population that some individuals have higher cold tolerance compared to others.  Once these overwintering lepidopterans begin emerging, we will have a better idea of what populations will look like this year.

Redbanded leafroller (RBLR), green fruitworm (GFW), spotted tentiform leafminer (STLM), plum curculio (PC) and brown marmorated stink bug (BMSB) overwinter as adults and may not have been impacted by the polar vortex.  These species are more likely to experience cold weather induced mortality from late spring freezing temperatures, when they are more exposed.  European red mite (ERM) and San Jose Scale (SJS) probably survived the cold temperatures quite well.

As additional degree days accumulate more insects will be present, but at this time the spring insects are behind the crop phenology.  Insect growth regulators and Bacillus thuringiensis BT that have traditionally been applied at pink to target spring leafrollers will have little impact since populations are very low.  Growers with larger M7 trees don’t need to worry as much about lepidopteran species.  Newer plantings and high-density orchards should be scouted regularly when warm weather arrives, which will not occur within the next five days according to the predicted forecast.  Growers in the region have been quite successful at controlling early spring insects once they become active with applications of Bacillus thuringiensis (BT) or Intrepid (methoxyfenozide).

Many growers did not get the chance to apply oil last week, primarily due to cooler weather.  Oil may still be applied during early pink at a lower rate of about 1%.  Apply oil when conditions will allow for faster drying, but keep in mind that oil applications during pink could cause some flower tips to turn brown.