May 19 AppleTalk Call

AppleTalk Conference Call Summary

Tuesday, May 19, 2020 8:00 – 9:00 AM
Guest Speaker: Dr. Dan Cooley, UMass
Presenter: John Aue, Threshold IPM,
Moderator: Peter Werts, IPM Institute of North America; questions or comments, or send to Josie Dillon,

May 19th Call Stream: CLICK HERE

Guest Speaker Schedule Update

  1. June 2: Dr. Brent Short, Trecé presenting on Codling Moth Lures and Mating Disruption
  2. July 14: Dr. Sara Villani, North Carolina State University presenting on Summer Fruit Rots

Regional update

Location Degree Days

(Base 43°F)

Degree Days

(Base 50°F)

Percent Scab Ascospore Maturity Percent Cumulative Scab Ascospore Discharge
Eau Claire, WI 248 84 55 48
Gays Mills, WI 307 115 82 76
Hastings, MN 331 129 66 51
Harvard, IL 274 91 88 82
La Crescent, MN 318 120 86 80
Mauston (Northwoods), WI 252 79 61 55
Mequon (Barthel), WI 168 50 39 32
Rochester (Ela), WI 221 71 62 70
Trempealeau (Ecker’s), WI 287 103 81 70
Verona, WI 276 87 76 69

Table 1. Degree-day accumulations as of May 19, 2020 using data reported by Cornell NEWA Network.

The warmer temperatures this week will bring most orchards to full bloom by the weekend and boost insect development and activity. The impact from the freeze event from May 9 to May 11 will not be fully realized until after petal fall. Growers are reporting a variable 10-90% loss, that is limited to certain varieties or locations in the orchard. Across the region there is a strong return bloom this year and many growers still expect a good crop.

Scab update
Over the past week, ascospore maturity increased significantly and locations that received rain had a 20% or more ascospore release, making it one of the largest infection periods of the season. Many locations have now discharged 50-80% of their ascospores. The warm weather through the rest of the week will result in additional spore maturity and rain forecasted over Memorial Day weekend will result in another significant infection.

Grower questions

What is the appropriate application rate for 20-20-20 foliar sprays?

  • 20-20-20 foliar fertilizer may be applied through bloom at 5 lbs. per acre. This product has 20% nitrogen, phosphorous and potassium, and at five lbs./acre, this would apply one pound of actual N-P-K. This application may also be applied twice at a half rate of 2.5 lb./acre. Boron and zinc are not as flexible and should follow the label. Most materials are designed to go into the tissue while the leaves stay wet. Once dry, depending on relative humidity and air movement, most of the material is absorbed within two to three hours. This is particularly true with boron, and calcium is likely the same. When applying at petal fall, be careful what is tank-mixed. There may be a lot of different products in the tank and will want to avoid using a calcium chloride material.

Can EBDC’s, single-site fungicides, boron, NAA, etc., be included in tank-mixes with Regulaid and Streptomycin sulfate?

  • Dan Cooley and John Aue tend to shy away from applying anything else besides Streptomycin with Regulaid. Regulaid is a strong penetrant and will enhance the uptake of pesticides and tank-mixing other products should be avoided if possible. Several years ago, growers in New York experienced crop injury from tank-mixes that had six to eight different products in the tank. The bottom line is the more products added to a tank-mix, the greater the chance of a bad reaction.
  • This weekend will be the first large fire blight infection across the region and growers should wait until late this week to apply a fungicide that could include Streptomycin and Regulaid. Captan and sulfur should not be included with this spray, but most other fungicides will be okay. As per the above comments, avoid including foliar nutrients. 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.
  • As blossoms age they become less susceptible to blossom blight. Therefore, at 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.  Please review our recent discussion with Kari Peter for additional information on fire blight management.

Insect management
Lepidopteran update
Feeding injury from red banded leafrollers, green fruit worm, spring canker worm and other lepidopterans have been observed in orchards in the upper Mississippi river valley and are likely active in other locations. To assess pressure, inspect 50-100 leaf terminals and flowers for frass, feeding or curled up first and second-instar larvae. If pressure is low, a petal fall insecticide spray will manage these larvae and if necessary, Bacillus thuringensis (Bt) products may be applied at bloom. Bt products may still be tank-mixed with Streptomycin since the products do not have active bacteria in them and will not have a negative impact on the toxic proteins in the formulated product. Conversely, tank-mixes of Streptomycin with active bacteria such as biological products used to manage fire blight, will have reduced efficacy. Most growers can wait until petal fall to manage these lepidopterans. No action should be taken if there is no evidence of their activity.

Discussion with Dr. Dan Cooley – apple scab and orchard floor sanitation

  1. Are there any new or improved methodologies for reducing overwintered scab inoculum? Can a combination of mowing, chopping and urea in early spring reliably reduce inoculum by more than fifty percent?
    • There have been no new developments since the first tests on urea and chopping were completed 20-25 years ago. These are very reliable practices and growers can expect well over 50% reduction of inoculum and upwards of 90% where urea and chopping leaf litter are used in combination. Spraying urea helps increase the breakdown of the leaves and is most effective after harvest, when applied before the leaves have dropped. Primary ascospores only travel around 100 feet and if the orchard is isolated or if there are no surrounding wild trees, the practice is very effective. The reduction in inoculum is limited by how much outside pressure may be present from wild trees or abandoned orchards adjacent to the block. Leaves that remain in the tree row will harbor inoculum and using sweepers or mowers that get under the tree is important. If these leaves are not treated or chopped, that will decrease efficacy of chopping or urea applied to other parts of the orchard floor.
    • Chopping leaf litter or applying urea, while effective, will not change timing, amount or frequency of fungicides applied against primary ascospores. Reducing inoculum also manages the likelihood of scab developing resistance to fungicides, since it is during this ascospore stage that the fungus develops genetic variability. Therefore, the more primary ascospores that can be destroyed, the lower the probability of a genetic mutation that leads to resistance.
    • For more information on chopping and urea applications, please visit:
  2. Theoretically scab should more easily be managed in high-density plantings because it is easier to have more complete spray coverage. Could this improved coverage translate to smaller amounts of fungicide being required? Have you seen any evidence suggesting scab management is easier in high-density orchards?
    • Dan does not have good evidence to support this. Updated labels require products to be applied as a rate per acre rather than a rate per 100 gallons. Reducing fungicide rates is a bit of a race towards resistance and there is a reason for minimum-labeled rates. Tree-row volume can be used to a point to reduce the amount of water applied to the orchard to the minimum amount that achieves coverage, but should not be used to reduce rates of single-site fungicides. Spray coverage requirements do not change in a high or medium-density orchard. Most growers are using 60-75 gal/acre of water and sometimes more, in high-density orchards.
  1. In a FruitNotes article from 2013,, you described an increase in fungicide use in IPM orchards over the previous decade due to changes in use patterns of DMI fungicides, e.g., Rally (myclobutanil). With new or increased fungal pressure from bitter rot and other summer diseases, do you think fungicide-use rates noted in 2013 have continued to increase?
    • Yes, fungicide use has continued to increase. Many years ago, when growers could apply mancozeb into the summer, diseases could be managed with just a few sprays. Now growers apply five to six fungicide applications to manage bitter rot, sooty blotch and flyspeck. Additionally, the weather patterns have changed and warmer, wetter and more humid summers are driving disease pressure in the northeast United States.
  1. This year the Canadian Pest Management Regulatory Agency implemented new restrictions on the use of captan in fruit orchards, Do you know how growers there are adjusting to / managing these restrictions?
    • Dan does not know how growers are adjusting to this. However, growers can use the captozeb strategy, where captan/mancozeb are applied using mancozeb at three lb. per acre and one to three lbs. of captan. This strategy can be used up to the 77-day PHI. A single-site fungicide can also be tank-mixed with captan and the restrictions on captan still allow for enough to be applied to offer resistance management, when used in combination with single-site fungicides.
    • The Canadian Captan 80 label has a max rate of 3.75kg/hectare, which would be approximately 3.33 lb./acre. Comparatively, in the United States, Captan 80 has a max rate of 5 lb./acre.
  1. Perhaps ten years ago John heard a lecture you gave at MSU’s winter IPM school about how our Venturia inequalis is not our grandparents (or great-grandparents) Venturia inequalis, i.e., how the scab fungus has evolved. There are a multitude of questions that emanated from that lecture, but could you describe the essence of that argument and how fungal evolution may continue to effect management strategies? Is there evidence of metabolic cross-resistance to multiple classes of single-site fungicides? Could the evolution of scab impact alternatives to fungicides such as plant resistance inducers or competitive colonizers of plant surfaces? Are there strategies you can envision that might allow us to utilize this genetic plasticity in inequalis to our benefit, for example, somehow selecting for less virulent strains?
    • Looking back on history of old apple varieties, the Baldwin was totally resistant to scab in the 1910-20s. Within about five years, Baldwin lost all resistance to scab and this spread across New York and the rest of the Northeast. Benlate and Dodine were some of our first single-site fungicides to be released in the 1960’s and these developed resistance within seven or eight years. Ultimately, it is always an arms race between the fungi, the plant, and the fungicide. Fungi work faster in terms of developing resistance than many other organisms and it remains a challenge to maintain efficacy of these products. Dr. George Sundin from MSU and other plant pathologists across the northeast have identified many scab strains that are resistant to several active ingredients. Mostly in George Sundin’s work, is where resistance to multiple modes-of-action has been observed. Some cross resistance has been identified at Cornell but is not widespread. In MN and WI, resistance to the DMI myclobutanil is suspected in many orchards, however, there have not been laboratory confirmations of this. Resistance activators that organic users are using are probably not going to become resistant to apple scab because the pathogen cannot get around active-host resistance. We should also note that our multi-site protectants, e.g., captan, mancozeb, sulfur, lime-sulfur are also not likely to have scab become resistant to them.
    • When comparing wild trees to managed orchards, there are vast differences in just the appearance of secondary lesions in mid to late summer. The intensity of the color of the colonies and overall damage is rarely as ugly as what can be observed in managed McIntosh blocks, where lesions are completely black. Any time a fungicide is applied, many other fungi are being killed which may offer biological control in wild trees. Growers should also consider that scab fungi are unique to each orchard and the genetic makeup and their resistance varies.
    • In order to minimize the virulence of our apple scab, the best approach is to employ multiple strategies. Following best practices for resistance management, reducing inoculum, using biological controls, and rotating modes of action will work in combination to limit evolution of scab.