April 23, 2019 AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, April 23, 2019, 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments, pwerts@ipminstitute.org

April 23 Call Stream: CLICK HERE

Orchard floor management reminder
Hastening the decomposition of fallen leaves on the orchard floor is an important step toward reducing scab inoculum.  Flail chopping, raking under trees manually, machine-raking or sweeping, are all viable options that will destroy leaf litter.  At this time, an application of urea to the orchard floor might not deliver as much value and would be better saved for the fall.

Regional degree day (DD) accumulation and crop phenology update

Table 1. 2019 Degree Day Accumulation to 04/23/2019

April temperatures have been relatively normal in the past two weeks, minus the snow accumulation April 10th – 13th.  The forecast predicts high temperatures to be in the mid to upper 50’s and low to mid 60’s for the next ten days.  Rain accumulation between half and one inch is expected and will help warm soil temperatures.

The temperatures forecasted through the end of April will allow trees to move into ¾ inch green tip to tight cluster within the next week, if not already.  Areas north of La Crosse, WI have experienced cooler temperatures and will likely be a week to ten days behind and growers in northern Illinois are already reporting tight cluster on some varieties.

Tree-stress and soil moisture
The snow storm that delivered four to thirteen inches of snow across southern Minnesota and central Wisconsin on April 10-13 likely impacted soil moisture and temperature.  Most of the melting snow should have been absorbed by the soil.  Every 10 – 12 inches of snowfall equals about an inch of water, which equates to about 110 tons of water being added per acre.  Considering this large volume of low-temperature water (32-33°F), after-effects on soil temperatures can be reasonably expected.  Areas that received substantial snowfall will likely require more solar radiation and take longer for soil temperatures to reach normal spring temperatures.

Data from soil probes indicates soil temperatures are relatively variable across the region.  In Northern Wisconsin and Northern Minnesota, soil temperatures remain a bit cooler than the majority of the region, falling between 33°F to 39°F.  In Central Minnesota and Wisconsin, soil temperatures fall between 45°F and 50°F.  Soils in the southern half of Minnesota and the southern half of Wisconsin fall between 48°F to 53°F.  In Northern Illinois, soil temperatures range from 45°F to 58°F.  To investigate further, check out this page on the NOAA website where these soil temperature probe data are displayed.

New tree planting recommendations
Soil temperatures are of minimal concern for growers preparing to plant trees.  Low-soil temperature may have played a role in slowing bud development, but air temperature and photoperiod have greatest influence on plant development.

Many growers have planted trees in the past couple weeks and several growers are planning on planting within the next week.  Monitor soil moisture levels to ensure that feeder roots are not suffocated.  Prior to yesterday’s rain, soils were relatively well drained after the warm weekend weather.  From now until the end of bloom, trees will have low energy and are entering a highly stressful period.  If trees went into dormancy relatively healthy with plenty of stored nutrients, they will be able to handle this period of stress.  Keep in mind that trees do need adequate moisture to dissolve nutrients stored form last fall over the next three to four weeks.

Soil saturation poses a greater risk during planting and it is recommended to delay planting until the soils dry out.  Overly saturated soils at planting time creates an opportunity for Phytophthora to invade newly planted root systems.  Phytophthora requires free water in soil to transport the zoospores and waiting until the ground dries and irrigating young trees as soon as they’re planted or start breaking are advised rather than planting in wet soils.  When choosing to delay planting, storage precautions must be followed to limit negative effects:

  1. Upon arrival, open boxes of new trees and the plastic liner to prevent fungal development.
  2. Inspect the quality and condition of the trees right away. Contact the nursery immediately if the trees do not look good.
  3. Roots need to be moist and covered with sawdust, hay or mulch. Do not let the roots dry out.
  4. Store the trees at 34°F in a cooler or cold room with good ventilation. Do not store the trees in a room that is also storing apples or that had apples stored in it immediately before the trees arrived. Ethylene produced by the ripe apples will stress the trees. If apples were stored in the room, it should be thoroughly aired out before storing the trees. The goal is to prevent trees from breaking dormancy while keeping temperatures below 40°F.
  5. Do not soak the tree roots in water prior to planting. Tree roots need oxygen too and soaking in water will suffocate the fibrous feeder roots.

Zinc and Boron application rates and timing
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: https://www.canr.msu.edu/uploads/files/Applenutrition-EricHanson.pdf.

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.

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.

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 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 to 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.

Copper at green tip
Functionally, copper sprays are only able to kill fire the blight bacteria (Erwinia amylovora) when a copper ion that has been dissolved in water comes in contact with an active fire blight canker.  A small amount of water is necessary for copper activation (air humidity can often be enough), however too much water, such as periods of heavy rainfall, will completely rinse the copper away.  Even a small amount of copper residue remaining at petal fall will cause fruit russeting.

It is not too late to apply copper if within ¼ to ½ inch green tip.  Once beyond ½ inch green tip, applying copper is not advised as it may cause phytotoxicity.  With rain forecasted within the next week, growers can choose to apply a lower-soluble product.  Solubility can be decreased with the addition of hydrated lime to soluble coppers as well, which reduces phytotoxicity by acting as a buffering agent.  The least soluble forms of copper include copper sulfate, pentahydrate and cuprous oxide.  Refer to last weeks notes for more information on formulation, rates and adjuvants: http://www.ecofruit.wisc.edu/appletalk/april-9-appletalk/

To reduce the risk of phytotoxicity if applying during or near ½ inch green, there are several options:

  1. Eliminate the oil or reduce the concentration to 0.5% or 0.25%.
  2. Reduce the amount of copper applied per acre from two to one pound of actual copper:
    • Badge SC: 4 pints/acre (1.14 lb. actual Cu)
    • C-O-C-S: 2 lbs/acre (1.025 lb. actual Cu)
    • Kocide 3000: 3 lb/acre (0.90 lb. actual Cu)
  3. Apply copper when it will dry quickly. The faster it dries, the more likely it will be nontoxic to green tissue.

Note:  If there are trees showing tight cluster or flower buds, do not apply copper.

Note: Ultra low oil rates may minimize oils effectiveness as an insecticide, however, the oil will still act as a spreader/sticker when tank mixed with copper products or an EBDC fungicide.

Several growers feel their ability to manage fire blight management at bloom has improved because of access to the disease modeling available in the NEWA stations.  However, a lot of growers have the mindset that it is best to stick with what is currently working.  John believes that under certain circumstances, copper may not be necessary or needed.  On established trees, if growers have not seen fire blight and management practices during bloom have been successful, it may be okay to not apply copper.  On new plantings and younger trees, John would not recommend skipping copper.  It is impossible to know if new trees coming from the nursery have fire blight cankers, therefore skipping copper is not worth the risk during the crucial time for growth and development in young trees.  John also suggests applying copper to susceptible varieties.

Follow these links for information on Preparing Tank-mix Bordeaux Mixture and 5 Tips For Working with Copper, or check out the general guide from last week’s blog, Demystifying Copper for Disease Management.

Oil concentration and timing
With a shorter pre-bloom period expected, making multiple oil applications at lower rates to target San Jose scale (SJS) and European red mites may not be possible.  Instead, consider making one application of oil at a higher rate (2-3% rate dilute) on a warm day.  Oil performs best when relative humidity is high (65%) and temperatures are warmer than 60°F.  This is because oil and water emulsion takes longer to dry and therefore is working for a longer period of time to dissolve insect and mite cuticles.  With later oils during tight cluster and pre-pink, it is best to apply oil during periods of lower relative humidity.  The most effective time to target the overwintered, immature San Jose scale is during the delayed-dormant period, from silver tip to half-inch green.  Developing foliage will increase spray shadowing as the season progresses, reducing application effectiveness.

Note: Do not apply oil if freezing temperatures are forecasted 24 hours before or after the target application date.  For more information on managing SJS, visit: San Jose Management: Scaling it Up as Buds Break.

Historically, apple growers in the Midwest have been using two to three applications of oil up until early pink.  In New York, SJS has been a growing problem and has some correlation to the reduction in use of oils.  The downside of using oil is eliminating the possibility of using Captan in the early season.  It is also important to note that oil can cause phytotoxicity.  Oil dissolves parts of the cuticle on the insect or mite and does the same when in contact with plant cuticles.  The later that oil is applied, the higher possibility for oil to cause phytotoxicity.  Research that occurred during the to mid-1900’s found that oil can suppress development in trees and cause plant injury if applied at bud break.  In the past, oils were less refined and had lower unsulfonated residues with higher molecular weights, meaning there were more impurities found within the oil itself.  Oils that we use today are generally more refined and lighter by removing any sulfur, nitrogen and oxygen compounds, leading to fewer plant injury issues.  John recommends that growers reduce the oil rate and split into two applications, which will help to manage multiple arthropods, e.g., SJS, mites and aphids.  For more information on the components of horticultural oils, visit: https://extension.colostate.edu/docs/pubs/insect/05569.pdf.

Oystershell scale, on the other hand, overwinters beneath the dead-mother scale as eggs and cannot be effectively targeted pre-bloom with oil.  Oystershell scale (OSS) will begin to hatch at 364-449-degree days, base 50°F.  On a normal year this may occur between May 15 and June 1.  We will revisit OSS management strategies in the coming weeks.

Pre-bloom insect management
There are several insects to keep an eye out for during the pre-bloom period.  More information regarding pre-bloom insect management will be discussed in the next call.

  1. Leafrollers: The main concern with leafrollers occurs in non-bearing orchards.  When plum curculio sprays are applied at petal fall, leafrollers are often not cleaned up very well by these sprays.  Make sure to scout non-bearing orchards for obliquebanded and red banded leafrollers.  Some products to keep in mind for managing these pests:
    • BT (Bacillus thuringenis) Products: Dipel and Deliver
    • Insect Growth Regulators: Esteem (pyriproxyfen) and Intrepid (methoxyfenozide).
  2. Tarnished plant bug (TPB): TPB is a pest that growers have historically been concerned about. However, we generally do not see a lot of economic injury from this pest at harvest and growers are encouraged to avoid sprays or support those applications with careful scouting.

Mating disruption can be set up at any time now.  It is recommended that mating disruption is distributed about two weeks before the first codling moth flight occurs.  However, there is no reason to delay mating disruption at this time.

Pre-bloom scab management
With the thunderstorm and rain event we had yesterday in much of the region, scab infection events occurred in several locations.  All of these locations experienced a scab infection event within the past two days (Location: inches of rain):

  1. Hastings, MN: 0.54 in.
  2. Rochester, WI: 0.27 in.
  3. Verona, WI: 0.96 in.
  4. Harvard, IL: 0.08 in.
  5. Woodstock, IL: 0.07 in.

The level of concern for an apple scab infection depends on how much inoculum a grower has in the orchard and how much green tissue is present.  Cortland, McIntosh, Zestar and Gala varieties or any others that had significant scab last year, are at a higher risk of there being a successful infection during the recent rain.  Blocks with little to no scab last year or varieties such as Honeycrisp with some resistance to scab, are at a much lower risk from the rain resulting in a successful infection.  There is a greater likelihood for scab to occur in a susceptible variety as the crop phenology progresses.  In summary, the level of concern for scab infection is variable depending on conditions in each orchard.

There are few materials that work well against scab in cool weather.  Two examples of single-site fungicides that work well at green tip are Scala (pyrimethanil) and Vangard (cyprodinil).  The best time to use these products are when temperatures are relatively cool, below 65°F.  Cornell and Michigan State University have recommended using Syllit (dodine) at early green tip but it is important to note that it cannot be applied after pink.

As crop phenology progresses, DMI fungicides, e.g., Rally (myclobutanil), Indar (fenbuconazole), Inspire Super (cyprodinil, difenoconazole), strobilurin fungicides, e.g., Flint (trifloxystrobin), Sovran (kresoxim-methyl) and SDHI fungicides, e.g., Aprovia, (benzovindiflupyr), Fontelis (penthiopyrad), Luna Sensation (fluopyram, trifloxystrobin), Sercadis (fluxapyroxad) and Merivon (fluxapyroxad, pyraclostrobin), may be applied as tank mixtures with captan or EBDC fungicides, e.g., Roper Rainshield (mancozeb).  If growers have been using strobilurins twice during primary scab over the last few years, the risk of developing resistance is quite high.  This includes any product with a strobilurin component including the pre-mix SDHI products like Luna sensation, Merivon and Pristine.  When resistance occurs, there is no way to fight it and growers run the risk of losing the chemical class altogether.  Consider using Aprovia or any of the newer SDHI products that do not have a pre-mix, e.g., Fontelis and Sercadis.  Make sure to rotate classes to reduce the chance of resistance.

Note: DMI’s have reduced efficacy when it is below 65°F.

Minnesota Apple Growers Association is doing squash mounts at three sites.  According to many plant pathologists, including Dr. Patty McManus (UW-Madison) and Dr. George Sundin (Michigan State), it is very difficult to accurately do squash mounts.  Having accurate counts is very dependent on who is completing them and if they are consistent.  Squash mount data from Minnesota has dramatically varied over the last few years.  If growers are going to hold off fungicide use based on squash mount data, John believes that is quite risky.  It is better to use tools like NEWA stations and other disease modeling programs that use DD, which scab development is known to be based on.  While scab is the primary concern through bloom, there are other trunk diseases that need to be managed simultaneously, e.g., rust and powdery mildew.