June 4th AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, June 4, 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

June 4th Call Stream: CLICK HERE

Regional update
Most orchards across the region have accumulated between 250 and 350 degree days (DD), base 50°F, since January 1.  Exceptions include Eau Claire and Mequon which fall below 250 DD.  All locations are at 100% scab ascospore maturity.  A few sites have been slightly cooler with fewer degree-day accumulation and will potentially have additional spore release (Eau Claire, Mequon).  The forecast over the next week calls for a few rain showers over the weekend with highs in the 70s and low 80s, and lows in the 50s and low 60s.

Location Degree Days 6/04/2019 (Base 50°F) Degree Days by 6/07/2019 (Base 50°F)
Eau Claire, WI 247 299
Gays Mills, WI 333 389
Hastings, MN 296 352
Harvard (Royal Oak), IL 313 358
Lake City, MN 306 362
Mauston (Northwoods), WI 292 344
Mequon (Barthel), WI 205 243
Preston, MN 300 355
Rochester (Ela), WI 259 299
Trempealeau (Eckers), WI 297 353
Verona, WI 327 380
White Bear Lake, MN 268 305
Woodstock, IL 372 420

Table 1. Degree day accumulation to 6/04 and prediction by 6/07.

Degree-day calculation methods

Figure 1. Baskerville-Emin (BE) formula.

The single-sine method also referred to as the Baskerville-Emin (BE) formula, uses the 24-hour minimum and maximum temperatures to produce a sine curve and estimates degree days for that day by curve is symmetrical around the maximum temperature.

NEWA stations historically use the simple maximum-minimum formula to calculate degree days.  This formula can readily be calculated by hand and was also included in many of the Cornell Pest Management Guidelines.  The Baskerville-Emin (BE) formula uses a sine wave algorithm and results in more precise DD calculations.  This formula was implemented in NEWA in 2006.  Where NEWA apple disease and apple insect phenology models utilize DD accumulations, the BE formula is being used because of their higher precision.

Variability in degree day accumulations between different weather stations are quite common.  Maximum and minimum temperatures are collected during a ‘defined’ 24-hour period.  Another area that introduces variability in DD accumulations is how the 24-hour period is defined.  Sometimes the 24-hour day begins at midnight and for some it ends at midnight.  That is, in some systems midnight is 0:00, in some it is 24:00.  In NEWA, midnight marks the beginning of the day and is tabulated as 0:00 in the Hourly Data pages.  NEWA’s 24-hour period runs from 12:00 AM (= 0:00) until 11:59 PM.  Data is collected for NEWA’s database at the top of the hour and some NEWA weather stations may miss the true maximum and true minimum temperature for a given day, because it might have occurred at 2:16 PM.  Hence, this adds another source of variability.  Consequently, NEWA’s new weather stations with the IP100 ethernet interface, described at http://newa.cornell.edu/index.php?page=get-weather-station, will be programmed to collect the true daily maximum and minimum values.

The National Weather Service has weather observer sites reporting daily maximum and minimum temperatures that collect data, once per day, at specified times, which can affect DD value calculations. Consider the time when you look at the values from your maximum-minimum thermometer and then clear them.  If you look at these first thing in the morning and invariably at 5:00 AM, then you are collecting a true 24-hour maximum and minimum temperature for the period 6:00 AM until 5:00 AM the following day.  If you collect this data in the afternoon, the 24-hour period range would be different.  Over time, climatologists have found that “afternoon” observations typically accumulate more DD’s than “morning” ones.

Outside of the Rainwise stations used in NEWA, Spectrum Technologies is another manufacture of weather stations commonly used by orchardists and logs weather data on 15- or 30-minute intervals.  The station’s firmware has two options for calculating DD, the single-sine method and what Spectrum Technologies call the actual degree-day method”.  This method is used rather than calculating degree days using the high and low temperature data for an entire day and instead integrates the data at smaller time steps.  Degree day subtotals are calculated at 15-minute intervals to produce Degree Quarter-Hours (DQH), which are then summed over a full day.  The DQH are calculated as follows: DQH = Average Temperature – Temperature Base, where Average Temperature is average temperature over the 15-minute interval and Temperature Base is the base temperature.  If the average temperature is greater than the upper limit of the temperature range, the upper temperature limit is used instead of the average temperature when calculating DQH.  If the average temperature is less than the base.

Disease management
Primary-scab lesions from infections which occurred between green tip and bloom, are now visible in orchards.  It is essential to not apply any single-site fungicide in your orchard once scab has been found.  These include products such as Indar (fenbuconazole), Inspire Super (difenoconazole, cyprodinil), Rally (myclobutanil), Flint (trifloxystrobin), Fontelis (penthiopyrad), Luna Sensation (fluopyram, trifloxystrobin), etc.  Once scab lesions are discovered, applying a single-site fungicide will drive resistance by exposing the fungicide to millions of conidia.  The risk of resistance developing over time increases as more secondary scab are exposed to these fungicides.

Using a single site fungicide to manage powdery mildew is an exception, as captan does not have efficacy on powdery mildew.  Sulfur is a broad-spectrum protectant that may be used, but is highly susceptible to wash-off.  Rally (myclobutanil) is also a good option for powdery mildew, but should still be tank mixed with captan to reduce risk of developing resistance to apple scab.

Recent trials suggest many bio-fungicides, e.g., Regalia (Reynoutria sachalinensis), only offer low efficacy for scab management.  They are still a good option to use in rotation with sulfur or liquid lime sulfur.  In organic systems we want to minimize the number of liquid-lime sulfur applications that are made post bloom and John recommends waiting another week to determine how much scab is present, then suggests growers can look for an opportunity to apply one or two applications of liquid-lime sulfur to burn out scab.  Do not apply liquid-lime sulfur once per week over next few weeks.  Note: If applying sulfur at temperatures of 85°F or higher, fruit finish issues may occur.

Using sulfur could be maintained on McIntosh and Cortland, which are very scab susceptible, as well as new trees where powdery mildew needs management.  John suggests using a low to moderate rate of sulfur, e.g., 6-8 lb./acre.  Due to wash off, a lower rate reapplied weekly is a better option compared to applying higher rates less frequently.  Once lesions are noticeably present, John suggests using liquid lime sulfur to burn out scab.

Insect management
Plum curculio
Plum curculio (PC), like many other animals, are aware of the photoperiod and time of the season.  Due to a late start, many PC will be in a rush to complete their lifecycle with only two and a half weeks until the longest day of the year.  Historically, at 308-degree days from McIntosh petal fall, all PC have moved into the orchard from their overwintering sites and no new immigration will occur.  With very limited sightings so far, it may be possible PC have been affected by the polar vortex or have been slow to enter orchards due to cooler spring weather.  Keep monitoring for PC around the edges of the orchard and check for oviposition stings on fruitlets.  This comes with the caveat that PC is managed each year, effectively.  Organic orchards or orchards with poor PC management, can develop resident populations within the orchard.  Under these scenarios, growers need to scout all portions of the orchard, rather than limiting scouting to the perimeter.

The temperatures forecasted over the next eight to nine days are perfect conditions for PC oviposition.  It will be important to have an insecticide application for management during this time.  Neonicotinoids like Belay (clothianidin) and Assail (acetamiprid) are very susceptible to wash off and must have a two-day window without any rain to allow uptake into the leaf.  Once the insecticide is within the leaf or fruit cuticle, it cannot be washed off and will be active for a week to ten days.  Organic growers can use Venerate (heat-killed Burkholderia spp. Strain A396 and spent fermentation media), Pyganic (pyrethrins) and Kaolin Clay for PC control.  Using diatomaceous earth could have some utility but is more likely to wash off.

Refer to the May 28th AppleTalk summary for more information on plum curculio insecticide options.

Codling moth
The threshold most recommended by Extension entomologists is where five moths caught over any period of time, the date of the fifth capture becomes the biofix and degree days will be counted from that point.  Where mating disruption is used, the date of biofix occurs with one trap capture.  Some growers will not have a single massive flight with an easy to pinpoint biofix which guides timing of a larvicide application, rather they may experience a prolonged flight which will require waiting until traps reach threshold before setting the biofix.

If mating disruption is not being utilized and growers are waiting for 250 to 350 degree days for a larvicide application, John recommends using a lower rate of a diamide (Altacor, Exirel, Delegate) or spinosyn.  By applying lower rates, this allows for more frequent applications if needed.

We are currently experiencing a smaller flight, however, that doesn’t mean that will be the case for the whole generation.  Codling moth experiences overwintering mortality at -14°F, but the rate of survival depends on the developmental stage each CM was in during winter and CM maturity levels vary going into winter.  This also relates to the timing of emergence in the spring.

San Jose scale and wooly apple aphid
San Jose scale (SJS) crawler emergence are still several weeks away.  If growers are using Movento (spirotetramat) to manage SJS and wooly apple aphid (WAA), it is recommended to make the application during petal fall to late first cover.  The primary reason is Movento doesn’t move easily into leaves and needs to be accompanied by a penetrating surfactant like Regulaid or applied when the leaves are young and not hardened off.  Scaffolds, a Cornell publication, recommends a minimum of 8 oz/acre.  John recommends applying Movento in the next ten days for greater efficacy.  More insecticide options for scale and WAA will be discussed in future AppleTalk reports.

Leaf burn
Several growers have reported trees with burned leaves, which could be from one of several causes.  Make sure to identify the variety, leaf age and time the burns appeared.  This will help to identify the possible cause.  If leaf burn is present on a specific variety or two but not across the entire orchard, this could be attributed to stress from winter injury.  Other possible causes could be from the use of Aprovia, which appears as the darkening of the edge of the leaf extending into the green portion, with the entire leaf appearing dead and soft.  These symptoms can be confused with shoot blight.

Fruitless Honeycrisp management
Plant-growth regulators, e.g., ProVide (gibberellins A4A7) and Promalin (gibberellins A4A7) are potentially useful products to combat russeting and cracking in fruit and to increase fruit set after frost.  Both products are OMRI certified and contain two gibberellins (A4 and A7) that have been shown to inhibit flower production for the following year.  Many Honeycrisp growers with a lighter crop will have a heavy crop next year, which can be hard to thin down and will then lead to another off year.  By using ProVide or Promalin, this may help to stop the biennial bearing pattern, however, is not a labeled use.

Where fruit trees have no crop this year, Apogee (prohexadione calcium), should not be applied to control vegetative growth, even if concerned with vegetative growth.  Apogee will reduce shoot growth but will increase fruit bud production.  Using Apogee may induce more fruit buds for the following year.