May 28th AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, May 28, 2019, 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 28th Call Stream: CLICK HERE

Regional update
Across the region ascospore maturity is now at or near 100%.  There have been enough rain events that most of these spores should be released.  There are likely infections that are still incubating and conditions for new infections will persist for the next two weeks.  Degree-day accumulation is between 180-230 for most of the region, excluding Eau Claire, Mequon and White Bear Lake which fall below 180 DD.  Woodstock Illinois has accumulated 270 DD, the highest in the region.  The forecast over the next week calls for spotty rain showers, with highs in the 60s and 70s and lows in the 50s.

Location Degree Days 5/28/2019 (Base 50°F) Degree Days by 5/31/2019 (Base 50°F) Ascospore Maturity 5/28/2019
Eau Claire, WI 146 187 95%
Gays Mills, WI 231 266 100%
Hastings, MN 182 218 100%
Harvard (Royal Oak), IL 224 250 100%
Lake City, MN 194 230 100%
Mauston (Northwoods), WI 188 220 100%
Mequon (Barthel), WI 134 156 95%
Preston, MN 197 232 100%
Rochester (Ela), WI 196 228 100%
Trempealeau (Eckers), WI 196 230 100%
Verona, WI 226 257 100%
White Bear Lake, MN 156 195 96%
Woodstock, IL 270 297 100%

Table 1. Degree day accumulation and ascospore maturity to 5/28, and degree day prediction by 5/31.

Disease updates
Primary vs. secondary scab management
Slow growth has kept leaves in an immature state longer than we would normally expect by this time in May.  These leaves remain very susceptible to scab infections and as leaves mature or harden off, their cuticle becomes more resistant to scab.  Orchard floors with knee-high grass and unkept plant debris present a hospitable habitat for fallen inoculum to survive, but ascospores in plant material that are left exposed on bare or mowed ground will disintegrate.  Not all lesions from primary-scab infections have shown up and orchards could easily see an explosion of primary scab over the next couple of weeks.  Due to the high pressure this year, a single-site fungicide may be tank mixed with captan through second cover.  Mancozeb may still be used for growers on the extended-spray schedule and if they have not reached their maximum of 21 lb. for the season.

Secondary scab will likely appear on varieties where scab has been a problem the previous year.  Once secondary scab appears, avoid treating scab with single-site fungicides, e.g., Aprovia (benzovindiflupyr), Flint (trifloxystrobin), Rally (myclobutanil) and Sovran (kresoxim-methyl) that were used during primary-scab season.  Captan should be the only fungicide used to reduce the spread of scab, once secondary infections are found.  After first or second cover, if no scab has been found, intervals between sprays may be stretched and rates can be adjusted.  However, consider that protecting tissue from black rot and white rot would still warrant applications of Captan at five pounds per acre.

Image 1. Primary-scab lesion on smaller leaf with healthy leaf for comparison.  Quarter is shown for size.

Image 2. Closeup of scab lesion from leaf shown in Image 1.

The scab lesion in Image 1 and Image 2 is slightly different than what we normally expect to see.  The edges of the primary-scab lesion are not well defined and not perfectly round.  It is just beginning to produce conidia and will become darker and rounder as it matures.  This is not atypical of primary-scab lesions.  Once the lesion develops conidia, the conidia will be on the upper and lower side of the leaf.  Primary-scab lesions almost always appear on the bottom side of the leaves, since spores are generally released from the orchard floor.

Organic growers past bloom or very close to petal fall should not rely too heavily on sulfur for scab and powdery mildew (PM).  Sulfur is an excellent protectant for PM, however conditions have been poor for powdery-mildew infections.  Powdery mildew inoculum will die at -15°F and it is expected that much of the inoculum was destroyed by the polar vortex.  Sulfur is not resistant to wash off and after primary scab, John is not fully supportive about continuing sulfur applications.  Applying a biofungicide, e.g., Regalia (Reynoutria sachalinensis), makes more sense at this time of year.  Liquid-lime sulfur continues to be an option, especially as temperatures remain cool.

Rust and powdery mildew
Most locations have not had much risk for rust or PM.  The amount of rain should delay PM growth and spread; however, we have had enough rainfall for rust.  New trees should still be managed for PM in case it was brought in from the nursery.

Winter injury and canker diseases
The images below (Image 3, 4 and 5) are of Jonagold shoots where trees suffered winter injury.  These are of one-year old wood that pushed out vegetative buds last summer, but later died due to the winter temperatures.

Image 3. Winter injury in Jonagold shoot.

Image 4. Winter injury in Jonagold at node.

Image 5. Winter injury in Jonagold at internode.

Image 3 shows the node between two and one-year old wood.  The node is cracking in the tissue around the terminal bud where one-year old wood emerged last year.  This is common on virtually all the shoots that have experienced die-back.

Image 4 shows peeled-back bark on the one and two-year-old wood.  The cambium is green on both sides, even though the one-year old wood died back.  Image 5 is the same piece of wood as Image 4 but shows the internode or the junction between one and two-year old wood.  There is virtually no live cambium in the region, and it was entirely decayed.  This is where the winter injury occurred, not in the one-year old wood, but in the junction.

Jonagold is not very winter hardy and may reflect the lack of tolerance to cold temperatures.  This type of winter injury can likely be observed on other cultivars.  The one-year old wood with winter injury is a prime host for fungi that produce black and white rot cankers.  John suggests clipping these out, but the best route is to maintain a cover of Captan and use more frequent sprays of Regalia (Reynoutria sachalinensis).  As leaves mature and are producing more carbohydrates, Regalia will keep the tree in a defensive mode and the tree will be able to better withstand infections from slow growth canker fungi.

Fire blight
Risk of fire blight is not as high as it was during bloom, but bacteria still has the potential to replicate tenfold every couple of hours.  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.

Insect updates
Codling moth
We typically discuss delaying the first codling moth treatments when gaps between flights occur, but these gaps only happen if cool weather prevents a codling moth flight.  Since we are expecting cool weather this week, it is likely that growers will not catch moths regularly and treatments could be delayed.  One flight was reported at the end of last week between May 23 and May 26, with biofix being set on May 25.  Codling moth traps should be checked nearly daily for CM flights.  A continuous flight means treatments may be required as early as the 250-degree-day codling moth treatment threshold.  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,

The total population for each generation emerges in a shape of a bell curve over a 600 to 900 degree-day period, where we typically have 1000 DD between each generation of a biofix.  This suggests that after 250 DD from biofix, only a small portion of the total population will have emerged – about a three percent hatch.  Orchards with high pressure or a large first flight, e.g., more than ten moths per week, often apply the first larvicide at this time.  If the initial flight is light or inconsistent due to cooler temperatures or rain a stronger flight can occur after the first biofix, e.g., two-weeks later.  If this occurs the first larvicide can be applied at 350 DD.  At 350 DD from biofix, 15% of larvae have hatched and at 450-550 DD up to 50% larvae hatch.

Note: For a full discussion on using the degree-day model to manage codling moth, please review the May 21, 2018 AppleTalk notes.

Plum curculio
Plum curculio (PC) continues to be delayed relative to tree phenology.  The past few years have been defined by hot weather in the 80s and 90s during bloom, which allowed PC migration into the orchards before petal fall.  This year movement is slow and cover sprays at petal fall are not necessary.  King fruits in southern Wisconsin and northern Illinois are in the 4-5mm range, some varieties are even at 10-12mm.  Once king fruit reach 5mm or larger, they become a target for PC oviposition, i.e., egg laying.  When plum curculio first move into the orchard, they must feed and mate before they can cause typical plant scarring, which often takes up to a week to occur.  It is essential to scout the perimeter of the orchard and focus on historical hot spots and locations that have the largest fruits.

Many growers have been moving away from the organophosphate insecticides over the last decade and until recently, Avaunt (indoxacarb) has been the most commonly applied OP alternative.  Many of the newer insecticides use a combination of contact mortality, systemic activity as an anti-feedant or ingestion, to manage PC.  The neonicotinoids Actara (thiamethoxam), Assail (acetamiprid) and Belay (clothianidin) have contact mortality for only a few days, after several days the insecticide penetrates the fruit and is more of an anti-feedant and keep the female from laying an egg.  Likewise, they can kill the egg if successful oviposition does occur.  Exirel (cyantraniliprole), a diamide insecticide related to Altacor (chlorantraniliprole), may be used to manage PC and CM.  It has similar activity to Avaunt, which requires ingestion.

Note: Actara does not work on codling moth, however Belay may be used on first generation codling moth and Assail is labeled for both first and second-generation codling moth.  These neonicotinoids all have different wash off potential, costs and manage other pests including aphids.  John Wise has developed most of the wash off research based on management of codling moth larvae survival.  A PC weevil is going to eat much more tissue and consume more insecticide residue that may otherwise not control CM, so even with some rain, these insecticides may still be working on PC.

Venerate (heat-killed Burkholderia spp. Strain A396 and spent fermentation media) is a relatively new product and is approved for organic PC management.  As described in the active ingredients name, Venerate is a bacterial by-product.  According to the label, “Venerate controls insect targets by enzymatic degradation of exoskeletal structures and interference with the molting process leading to mortality through contact and/or ingestion.”  John Wise, Michigan State University entomologist, suggests Venerate is showing good control of PC in organic orchards.  Since Venerate needs to be ingested, it should be applied as a complete cover, perhaps with kaolin clay, and it may be best to apply after initial PC activity has been detected.

If growers are thinking of applying a PC application at petal fall, John suggests being cautious as it is likely too early for an insecticide application until PC are spotted while scouting.  We are also in a crucial period with scab management where applications cannot be spread out too far.  If using Avaunt or Venerate, e.g., need to be ingested and should be applied 50-70 DD earlier than a neonicotinoid.  Neonicotinoids like Belay and Assail can be applied once some feeding has been observed.  Avaunt or Venerate should be applied when evening temperatures are in the 70s or 80s and fruit have reached a size susceptible to PC injury.  Several locations still have some bloom and should avoid applying an insecticide as they are toxic to bees until bloom is finished.

Stink bug and tarnished plant bug
A few stink bug adults have been identified in orchards.  Stink bugs and tarnished plant bugs are more common when blooming plants and broadleaf weeds are located in the alleyways.  John is not worried at this time but wanted growers to be aware.  It is likely that stink bugs will lay eggs in the next month.

Spring-lepidopteran complex
In orchards that received an application of Bacillus Thuringensis or an insect-growth regulator, in the last week, remaining lepidopterans have been suppressed.  There have been a few sightings of early instar redbanded leafrollers (RBLR) hatching.  Monitor terminals for these newly emerging RBLR, even if an insecticide was applied in the last week.

Oystershell scale, San Jose scale and dogwood borer
Orchards are not at great risk of finding San Jose scale (SJS) crawlers until several weeks after bloom.  Oystershell scale (OSS) has been identified in a few orchards.  If OSS has been present in your orchard within the past few years, monitor for crawlers.  Oystershell scale are much less tolerant to cold winters than SJS.  Temperatures below -25°F will have an impact on OSS overwintering.  In comparison, SJS can survive in temperatures as low as -45°F.  During the polar vortex, temperatures fell well below -25°F and likely caused some mortality in OSS.  John recommends not applying an insecticide for OSS yet, as many insects have been slow to develop due to cooler spring temperatures.

Growers should hang dogwood borer traps in blocks that are two to seven years old.  This will provide an accurate idea of dogwood borer populations within your orchard.

Flower thrips were originally limited to western states but have become increasingly present in the Midwest.  Thrips have asymmetrical mouth parts sometimes referred to as rasping/sucking mouthparts.  These mouthparts allow thrips to cause the distinct injury on the midrib of developing terminals on trees.  Their oviposition injury causes a blemish called pansy spot.  This was observed several years ago in 2010 or 2011, when a large population of thrips blew into the region during apple bloom and has not been observed since.  There are no established thresholds for terminal infestations on trees and generally there is no reduction in crop quantity or quality from thrips injury.

If thrips infestations are found on newly planted or non-bearing trees, treatments may be made before growth is stunted.  While the injury may be unsightly, thrips should not require an insecticide application on mature trees.  If thrips are observed while scouting, a beating tray can be used to determine the extent of the infestation.  A beating tray is piece of white cloth on a frame of any size that can be held under a shaken branch to sample insect populations that may not be detected by visual scouting.  This scouting technique is highly recommended as a yearly practice, especially for spotting western flower thrips.  Both Delegate (spinetoram) and Entrust (spinosad) are labeled for thrips.  A non-ionic surfactant should be added to help spread the insecticide into the tightly closed growing points.

One grower is noticing a large difference in degree-day accumulation at NEWA sites within their general region, why is this?

  • NEWA sites can often reflect microclimates and can have differences depending on where they are located, i.e. sun vs. shade, slope vs. flat terrain. This will likely not be an issue for thinning and insect development but is good to be aware of.

NOTE: The discussion on thinning will be posted separately, along with a thinning guide at a later time.