July 13 AppleTalk

AppleTalk Conference Call Summary
Tuesday, July 13, 2021, 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM, jgaue@mwt.net
Moderator: Peter Werts, IPM Institute of North America; questions or comments, pwerts@ipminstitute.org

July 13th Call Stream: CLICK HERE

Location Green Tip Date

(Estimated)

Mac Petal Fall

(Estimated)

Degree Days

(Base 50°F)

Jan 1 – Present

CM May 13 Biofix

(86/50)

CM May 19 Biofix

(86/50)

LWH from Petal Fall Relative Humidity

Hours from Petal Fall

Eau Claire, WI 4/3 5/16 1205 1096 1033 205 461
Galesville, WI 3/21 5/7 1273 1135 1074 275 314
Gays Mills, WI 4/3 5/12 1288 1127 1070 167 372
Mauston (Northwoods), WI 3/30 5/10 1296 1133 1073 195 374
Mequon (Barthel), WI 4/4 5/18 1095 999 947 163 363
Rochester (Ela), WI 3/30 5/12 1182 1063 1010 169 503
Verona, WI 4/3 5/12 1295 1145 1085 178 365
La Crescent, MN 3/23 5/9 1363 1179 1115 147 768
Lake City, MN 4/4 5/16 1335 1193 1128 143 412
Hastings, MN 4/5 5/17 1228 1107 1039 94 254
Harvard, IL 3/30 5/11 1265 1105 1047 198 397

Table 1. Degree days and ascospore maturity downloaded on 7/12/21 from Cornell NEWA system. Relative Humidity hours added on 7/14/21. Find your local station today: http://newa.cornell.edu. Note: Leaf wetting hours for sooty blotch and flyspeck use an estimated petal fall date, unless entered by the station operator. NEWA model allows you to add your last systemic fungicide or petal fall date.

Regional roundup
After a period of significant rain fall for most growers, we now enter a week of dry and sunny weather with highs in the mid-80s and lows in the upper 60s. There are no forecasted highs in the 90s over the next ten days.  After an excessively hot June and early July, the next ten days look pretty good for working in the orchards.

This week’s notes bring a special focus on codling moth, apple maggot and summer disease management.  Please see our June 29th notes to review monitoring and management recommendations for Japanese beetle, woolly apple aphid and other secondary pests.

Disease management
Sooty blotch and flyspeck
Acknowledgements
An important objective of AppleTalk is to keep us all informed when new advances in the science require us to recalibrate our decision-making processes. Management of sooty blotch and flyspeck (SBFS) is one of these situations.  There have been several important studies over the last 25 years and in the last ten, more work has been done to validate a SBFS model that uses accumulation of relative humidity hours vs. leaf wetting periods. The following summary outlines the evolution of this model and includes information I obtained through recently published papers and conversations with Dr. Dan Cooley, UMass, who developed the model used by NEWA and Dr. Mark Gleason, who developed our preferred model using relative humidity and emails correspondence with Juliet Carol and Dan Olmstead, Cornell, who manage the NEWA network and explain the nuances of how NEWA is calculating wetting events, etc. The conclusions summarized at the end explain how to use relative humidity hours to time SBFS sprays and will be the preferred recommendation in AppleTalk going forward.

The application of the sooty blotch and flyspeck model is a classic example of how we can elevate our use of predictive models in an IPM system to minimize preventative fungicide sprays, when compared to calendar-based programs. However, confusion emerges as we examine the various ways this model has been validated and implemented across the eastern United States. The biggest point of contention being how wetting periods are tracked; the technology used to monitor them; and the thresholds used to time the spray. The model used in NEWA has a threshold of 175 leaf-wetting hours (LWH) and counts all hours. However, this is contradictory to other Extension publications which use the same threshold, but discount all wetting periods less than four hours.  Additionally, I have read that all hours can be counted, but a threshold of 200 – 270 LWH should be employed. After inquiring with the pathologists who helped write the NEWA model, it has been clarified this model is a modification of the original Brown and Sutton model, which only counted leaf-wetting periods > 4 hours, however, had a threshold of 200 – 250 beginning with the first rain 10-days after petal fall. But Brown & Sutton used a DeWit LW meter to develop their model, and it used a string that would tighten or loosen as it dried or became wet; even the authors acknowledged it might not work with electronic sensors.

Hartman and Smigell tested the model using electronic sensors and found the DeWit required more wet hours than the electronic sensor. Further testing was completed in Kentucky, where the treatment threshold was adjusted to work with an electronic leaf-wetness sensor. They adapted a technique first used to study SBFS, wherein fruit are covered by bags for varying periods during the season to determine when fruit were first infected by SBFS. At regular time intervals of approximately 1 week, fruit were arbitrarily bagged. Fruit bagged before 175 LWH measured from 10-days after petal fall did not develop SBFS. Based on this, the researchers recommended a 175-LWH treatment threshold counting all leaf-wetness hours. Under this revised model, two to four fungicide applications were saved relative to calendar- based prophylactic sprays.”

However, none of these approaches are a good fit for our climate conditions in the upper Midwest where high levels of relative humidity remain the primary driver for sooty blotch and flyspeck infections. The 2021 growing season is providing us the perfect opportunity to examine why relative humidity is a better predictor of SBFS over LWH.  Ten or so years ago Mark Gleason and Patty McManus validated a model where 192 hours of RH above 97% was a better predictor than 175 hours of leaf wetness for our region. However, NEWA stations record hours of relative humidity equal to or greater than 90%, not 97% RH.  It has been mentioned by pathologists these instruments may not reliably track very high RH levels and is why the NEWA stations are set to measure 90% or greater RH.  A new study published by Gleason et al. in 2017 offers us the best approach moving forward for determining when to apply fungicides for SBFS. This model uses all relative humidity hours from first cover, which is approximately ten days after petal fall and has a threshold of 385 RH hours equal to or greater than 90%. These data may be accessed by going to the ‘Daily Summary’ page under the ‘Weather Data’ tab on the NEWA website.  You will have to manually tally the RH hours from your first cover spray to accurately determine if you are above the 385 RH threshold. If you examine the tables at the top of this week’s notes you can see the incredible differences between RH and LWHs. At this point, nearly all locations will have accumulated enough RH hours to trigger a SBFS spray.

Unlike other disease models which are preventing infections from happening, the SBFS models simply predicts symptom appearance. A fungicide such as Topsin M (thiophanate methyl), that can eradicate the pathogen must be applied, otherwise symptoms will still appear. NEWA stations located at airports do not often have a leaf-wetness sensor but can still give us RH data for our calculations. It is also note worthy that NEWA stations will calculate one minute of leaf-wetting as one hour, which may be helpful next year when we revisit apple scab management and LWH used to calculate scab infections.

References:

  • Hafizi Rosli, Derrick A. Mayfield, Jean C. Batzer, Philip M. Dixon, Wendong Zhang, and Mark L. Gleason. 2017. Evaluating the Performance of a Relative Humidity-Based Warning System for Sooty Blotch and Flyspeck in Iowa, 2017. Plant Disease 101:10, 1721-1728
  • Hartman, J. R. 1995. Evaluation of fungicide timing for sooty blotch and flyspeck control, 1994. Fungic. Nematicide Tests 50:10.
  • Smigell, C. G., and Hartman, J. R. 1997. Evaluation of fungicide timing for sooty blotch and flyspeck control, 1996. Fungic. Nematicide Tests 52:31.
  • Smigell, C. G., and Hartman, J. R. 1998. Evaluation of multi-layer fruit bags for cork spot, sooty blotch and flyspeck control, 1997. Biol. Cult. Tests 13:39.

When do we start protecting against fruit rots?
Initially John thought due to our dry spring, we would have less issues with fruit rots, but now finding that historical data is showing the hot and dry conditions can be a driver of sun induced bitter rot infections and lenticel infections. Zestar always seems to be a good canary in the coal mine for looking for fruit with dark spots/potential fruit rots. John is seeing some fruit that are already showing some signs of sunburn.  The most damaged area may be quite small and nice and round and not necessarily where the apple has shown color because the apple was aligned in a different position when infrared rays were doing damage.  Captan maintained on fruit would help protect against fruit rot.  Considering how light the crop is, growers should hold off on more expensive bitter rot fungicides until later in August. Bitter rot management will be discussed during the next call.

Insect management
Second generation codling moth
The first flight of the second codling generation was documented late last week and last week John thought the flights immediately after the 4th of July seemed early for a second gen flight but are in alignment with DD accumulations.  At this time, all pheromone lures and trap liners should be replaced. Rather than continuing to count degree days from our first biofix, it would be easier to set a new biofix and count degree days the same way as first generation. Orchards with a strong biofix should not delay the first larvacide to 350 DD, as we often do during first generation.  During this second generation larvicides should be timed at 250 DD and then reapplied after wash-off or the lifespan of the insecticide.  The preferred larvicides for second generation management would include Altacor, Exirel, Delegate.  Combined apple maggot management can be achieved by using Assail. Organic growers may use the codling moth virus, Bt products or Entrust, in combination with mating disruption.

Codling moth and potential for a 3rd generation?
It is possible that our early accumulation of degree days could be enough to trigger a 3rd generation that is unable to complete its lifecycle. Larry gut, MSU codling moth and mating disruption researcher, determined if there is enough DD accumulation in May and June, CM larvae in the fruit may be exposed to long photo periods around June 21st and the subsequent eggs they lay will not pause in their development and move immediately into a third generation. This is a not well understood phenomenon and we are only speculating if this will happen.  Essentially, John suspects the early CM activity could have resulted in 5th instar larvae being exposed to long photoperiods around the solstice.  Normally these late instar larvae are present after the solstice.  Fifth instar larva would be the biggest worm in the apple and is constantly pushing out frass or when it leaves fruit to pupate. The question we can speculate on is that during the solstice did we have any 5th instar worms?

If by harvest you are still capturing codling moth it is critical to evaluate the degree day totals and determine if those moths are still second generation or if there is a potential for them to be third.  If based on degree-day accumulations, they are still second-generation moths, then you may continue using the same larvicides as you did during second generation.  If you suspect a third generation then using a different mode of action is essential.  The codling moth virus sold as Madex HP or Virosoft has a zero-day pre-harvest interval and may be a good option for some. Note: The label only allows two Altacor applications per year at the 4.5 oz. rate; Delegate allows four applications at the 7 oz. rate and Assail allows four applications at the 8 oz. rate.

Codling moth resistance management
Earlier this year John had recommended using neonicotinoids, i.e., Belay (clothianidin) and Assail (acetamiprid) to manage 1st gen CM, in orchards not using mating disruption. This would also offer simultaneous management of secondary pests such as potato leafhopper and green apple aphids. Additionally, late plum curculio would also be suppressed by these neonicotinoid applications.

Orchards that followed this strategy should also evaluate second generation codling moth trap data for signs that may indicate neonicotinoid resistance. Early development of resistance in the codling moth population may not result in enough damage at harvest to suspect resistance. However, codling moth trap captures during second generation that are significantly higher than first generation may indicate the development of resistance, or at the very least, a spray was missed or poorly timed during first generation.

Apple maggot
Growers continue to report early, and high apple maggot captures across the region, with one location reporting more than twenty in one trap.  Consider hanging at least three traps per ten acres and more in blocks with a history of apple maggot injury or where there is pressure from wild apple trees outside the orchard. Organic growers interested in trap out should hang as many traps as they can stand! Officially, Michigan state recommends upwards of 400 traps per acre.  These do not need to be hung all at once but should be fully deployed before the peak apple maggot activity in your orchard.

Neonicotinoids are the primary organophosphate alternative for AM management. Assail (acetamiprid) is often a popular choice because it may be used for both AM and second-generation CM. Recent conversations with Peter Jentsch, research entomologist at the Hudson Valley Fruit lab in New York, suggest that Assail (acetamiprid) is the best apple maggot insecticide option, even over a product like Imidan (phosmet).  His trial data suggests that imidacloprid products perform poorly, though we have not observed that in the Midwest. Field trials with Assail show performance that is equal to what may be achieved with organophosphates.  This performance hinges on the curative properties of the neonicotinoids as a larvacide or ovicide. The neonicotinoids have also been found to repel the adult flies and prevent egg laying. The downside to using Assail has been poor management of the summer generations of leafrollers. Subsequently the use of generic imidacloprid products, e.g., Admire Pro, Wrangler, Montana, in a tank mix with a spinosad or diamide insecticide, has been quite popular. This tank mix results in a reduced risk, yet broad spectrum spray which can manage AM, CM, and summer leafrollers, e.g., obliquebanded and redbanded leafrollers.

Even though we have been successfully managing AM this way, imidacloprid and most of the neonicotinoids do not offer extended-contact knockdown of the female AM fly. Assail does have more mortality on the adult fly and wears off quickly. The main control from Assail and imidacloprid is in the egg laying as an ovicide and survivability of the eggs, and as a repellant. Overall, we have found this strategy to perform well and are yet to hear about AM failures from using the neonicotinoids. However, this does make it hard to spot spray for AM.

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