April 13 AppleTalk

AppleTalk Conference Call Summary
Tuesday, April 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

 April 13th Call Stream: CLICK HERE


Location Green Tip Date Ascospore Maturity (%) Degree Days

(Base 50°F)

Eau Claire, WI 4/3 6% 69
Gays Mills, WI 4/3 3% 86
Hastings, MN 4/5 4% 44
Harvard, IL 3/30 3% 83
La Crescent, MN 3/23 9% 107
Mauston (Northwoods), WI 3/30 3% 44
Mequon (Barthel), WI 4/4 5% 64
Rochester (Ela), WI 3/30 7% 81
Galesville, WI 3/21 14% 98
Verona, WI 4/3 7% 74

Table 1. Degree days and ascospore maturity downloaded on 4/13/21 from Cornell NEWA system. Find your local station today: http://newa.cornell.edu.

Welcome and reminders

Thank you all for joining us for another season of AppleTalk.  It has been quite a year and the conversations that John and I have had with many of you suggest the 2020 harvest was a great success and that we are all looking forward to a productive 2021 season. It is still not too late to join and referrals from other growers is the most effective way we build and expand our community of apple growers focused on sustainable and organically focused crop management.

Please keep in mind the following items as the season gets underway:

  • When you join the call please mute your line. If your phone does not have a mute button, select *6 to mute/unmute your line.
  • Submit your questions at least 24 hours prior to the call so we can include them in our agenda.
  • Let us know if there is a guest speaker you would like us to have on an upcoming call.
  • Write down your green tip date to track apple scab ascospore maturity. This date is important to predicting the percent of mature ascospores in the orchard.  If you have a NEWA station at your orchard, you can enter this in.  If you are looking at a nearby station, you can enter in your local green-tip date.
  • Make a note to write down your 95% petal fall date for McIntosh or another mid-season blooming variety. This date will be needed to track degree days for plum curculio movement into the orchards post bloom.

Understanding degree days
Degree-days are a weather-based indicator used to estimate development of crops, insects, and diseases over the course of the growing season. Degree days are calculated by subtracting the lower-development temperature threshold for a plant, insect, or disease from the average daily temperature within a 24-hour period. The default lower temperature thresholds for insects use a base of 50°F, and a disease such as apple scab uses a base temperature of 33°F.

There are several different ways the NEWA stations can calculate these degree days and for the purposes of AppleTalk the Base 50F option is what will be reported. This will most easily allow you to compare NEWA degree days to any of your own calculations you make.  However, you may observe some differences between these degree days and what NEWA uses for their insect models.  All of the insect models in NEWA use the BE degree-day model and is an option you can select in the degree-day calculator.  This is a more accurate predictor of degree-day accumulations as it relates to insect development, however, uses a more complicated singe-wave based algorithm and cannot be easily calculated by hand, which is why in AppleTalk the simple method of degree days is what will be reported in the tables.

Weather impacts on crop management
La Nina conditions are persisting into spring which has generated a strong west to east jet stream. The flow of this jet stream is split with one flow moving across the top of the northern states and one across the southern states. The split in this flow was responsible for keeping cold artic air, i.e., polar vortex, form dipping into the upper Midwest this year. This pattern is to continue through April and if it holds should help mitigate against the risk of a frost or freeze event over the next ten to fourteen days. The normal-average temperatures at the beginning of April are traditionally in the 50s and lows around 30 and by end of month have a high of 60 and low of 38. Currently the average highs in the 60s and average lows in the 30s. Temps will remain slightly below normal through the week and no moisture is forecasted. Rain fall has been below normal for most of Minnesota and Wisconsin and the National Weather Services has these states in a drought. Even though many areas received an inch or more of rain over the last week, the soils are deficient of moisture right now.

Tree phenology
It is not uncommon to see a widespread in the tree phenology across an orchard or a region.  Currently orchards are between green tip and tight cluster, with more varieties at an early to mid-tight cluster. It is also common for terminal buds to open first and normally shoots are not always fruit buds and these may not best represent average development for your orchard. This variability may be related to the lack of heavy rain to washout frost that is remaining in the soil. The frost was not deep this winter and the lack of rain caused frost to leave the ground unevenly.  For example, differences in slope or shading may result in more early development on south and west facing slopes.

In 2020 trees broke bud in mid-April and temperatures remained cool, which allowed for more even growth and development up to tight cluster.  The subsequent heat wave caused bloom to occur at the same time for most varieties.  Currently, there is both a lot of green tip and tight cluster in the orchards and this should allow time for more pre-bloom activities as it relates to disease management, including inoculum reduction efforts. It should also be expected that bloom and petal fall will be more staggered across different varieties.

Fire blight
Copper is traditionally applied between silver tip and green tip to target fire blight bacteria that overwintered in cankers on the tree. A small amount of scab protection can also be achieved from a copper application. At this time, if a copper was not applied, it is likely too late to make this application, unless the orchard is still solidly at green tip with no tight clusters showing. At the current growth stage and with cool temperatures, bacterial colonies in fire blight cankers are not actively growing. Fire blight cankers become much more active at pink tip and the goal of this early application is to ensure enough copper is applied to persist until early pink through bloom to inhibit bacterial growth. If there is still enough copper residue remaining at petal fall, this can be redistributed by rainfall on to the developing fruitlets and cause russeting. For the standard copper products on the market, approximately three to four inches of rain between the application and fruit-set will mitigate this risk. Therefore if a copper application has not yet been applied, it is advised to not make it.

If fire blight was a problem in an orchard last year, much more vigilance will be required in making sure streptomycin is timed properly and will be discussed during bloom.  Other copper products such as Cueva are not a good replacement for a fixed copper which targets overwintering fire blight.  However, may have some utility in preventing shoot blight post bloom. Expect much more discussion on fire blight as we get closer to bloom.

Apple Scab
The first infection period for apple scab occurred this past weekend between Friday and Sunday, however, many orchards have single digit ascospore maturity and where inoculum is low, the copper spray should have provided some protection against scab. If the orchard did not receive a fungicide prior to this infection period, it is too late to apply a fungicide for “kick back”. Any application made will provide protection against the next infection period. Do not be complacent over the next month, as this infection period will likely cause problems for high inoculum blocks and since it takes a minimum of 21 days for scab lesions from green tip infections to become visible, it could easily be bloom or petal fall before these lesions become visible.

Between green tip and bloom the EBDC class of fungicides, e.g., Roper Rainshield, Manzate, etc. are the protectant fungicides that should be used to manage scab. Avoid Captan applications right now, as they can cause crop injury if applied too close to or combined with dormant oil sprays. Traditionally, it is recommended that by tight cluster, growers should be preparing for major scab and powdery mildew infections and should be applying SDHI, strobilurin or sterol inhibitor classes of single-site fungicides. However, temperatures will remain cool over the next two weeks and ascospore maturity lags behind the crop phenology. Therefore, the best option is to remain on a protectant spray program or include fungicides from the Anilopyrimadine class, i.e., Scala (pyrimethanil), Vangard (cyprodinil) or possibly Inspire (difenoconazole, cyprodinil). These fungicides will offer better cool weather performance over the other fungicide classes and since powdery mildew is not a concern right now, scab is our only target for these fungicide sprays. Another option is the fungicide Syllit (dodine) and is a product that very easily builds resistance in scab and applications should especially be limited and may only be used up until pink. Now is the only time when it can be used and could be an option for very high inoculum orchard.  The label indicates it may have an impact on some of the spores that are on the ground.  John is doubtful but also hopeful an application like this might help lower inoculum.

Apple scab is well tuned to the development of apples. The amount of ascospore development in terms of pre-bud break is typically small. During spring rains, mature spores could be released (pre-bud break), but would require green tissue to be present for an infection to occur. The bulk of ascospore release will occur between tight cluster and petal fall. Biological fungicides should be avoided until there is significant leaf surface present and temperatures are warmer. If you are looking for an opportunity to apply an EBDC in five or six days from now, depending on current temperatures, we may not see a lot of growth over the next week.

Additional notes on mancozeb and disease management:

  • The full rate of mancozeb, referred to as the “Pre-bloom schedule” is 6 lb./acre and the half rate or “Extended-spray schedule” is 3 lb./acre and interpreting these two schedules can be confusing.  The full rate allows for four six-pound applications where the half rate is seven three-pound applications up to the 77-day pre-harvest Interval (PHI).  If the goal is to apply mancozeb past bloom, then it is important to only use 21 pounds maximum for the season and not apply past the 77-day PHI.
  • If there is not a lot of rain, there will be a buildup of ascospores waiting to release during a major rain event. Many minor rain events will release spores more frequently which is ideal.

Organic scab management at green tip
Reducing the inoculum on the orchard floor within the leaf litter is one of the most effective strategies organic and low-input orchards can do to improve scab management. Chopping, mulching, etc., is very important and has been proven to reduce overwintering scab inoculum by 50%. Biological fungicides can be used by organic and non-organic growers for apple scab and other diseases later in the season. However, many of these products are live cultures and require a minimum amount of leaf surface and warmer temperatures to thrive, they may not be very effective at green tip and are best saved until there is much more leaf tissue present. If fungicides need to be applied in organic systems, sulfur and lime sulfur are the primary pre-bloom options. Lime sulfur is best applied post-infection as it offers some eradication and too frequent of applications as a protectant can reduce yields, tree, and soil health. Lime sulfur must be applied when it is not too hot and is not compatible with oil. Sulfur alone does not redistribute or adhere very well to tissue and has a short length of efficacy and could be applied as a protectant prior to rain events.

Under our current conditions, organic growers can consider the following:

  • Minimize applications of lime sulfur due to phytotoxicity and save for infection periods that anticipate a large ascospore release. Lime sulfur also kills lesions and is another reason to save these applications to later in bloom or post bloom.
  • If there is a block with severe scab the last few years, then sulfur may be a good option to apply ahead of the next rain event. But ideally, wait until further along into primary scab season before beginning sulfur applications, at a minimum wait until a very consistent tight cluster through the orchard.

Oil applications for early season mite and San Jose scale management |
Currently insect development is behind the tree phenology and while we are normally recommending to apply oil this time of year, as described below, it is advised to not include above a 1% oil if you are planning a spray during the next week when temperatures will consistently be below 60F. Oil will not be very effective when applied under these cool conditions. If applying oil with EBDC, use only as a low rate where it will act as a a spreader/sticker when applied at <1% and will not yield much benefit as an insecticide. If it is not possible to apply a 2% oil before pink, this may lead to higher pressure from mites and scale.

Oil applied between silver tip and bloom is important to suppress mites and San Jose scale. Dormant oils are easiest to work with when temperatures are at least 60 °F or higher. As temperatures increase, overwintering mite and San Jose scale respiration rates increase, and the oil application will do a better job at suffocating mite eggs and San Jose scale. There is a very wide range of application rates growers like to use. Most growers apply a 1 – 2% oil, but 3 – 5% oil has also been observed. These higher rates may be necessary where scale and mite pressure is most severe. We will discuss mites and scale in greater detail later this spring, but the dormant oil spray is one of the most important opportunities to manage these two pests.

Esteem (pyriproxyfen) may be applied pre-bloom to improve management of San Jose scale and early leafrollers. San Jose scale is found on the bark and significant green tissue is not needed for efficacy of an Esteem application. In the past John has recommended ½ inch to tight cluster at a minimum for applying Esteem. Most of the developed tight cluster buds are apical, or at the ends of last year’s growth or fruit spurs. These are important buds to protect from scale, however they do not represent a high percentage of all the buds on the tree. Wait until most buds are at tight cluster. There is no rush to apply Esteem this week due to cooler temperatures.

Final thoughts
Many growers have still been finishing up pruning and every year there are some reports of growers doing pruning into tight cluster. The silver leaf disease does spread this time of the year and it seems that it is likely spread by late pruning. Once transmitted into the tree, it is there for the rest of its life, but this may not lead to a significant role in tree death. Even though the leaves show symptoms, the disease is in the woody tissues of the branches and leader. Late pruning needs some warm and dry weather to get the cuts to heal over and the current conditions are less than ideal for pruning.