May 21 AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, May 21, 2019, 8:00 – 9:00 AM
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 21st Call Stream: CLICK HERE

Regional update
Most areas have accumulated between 100-165-degree days, since January 1st.  The forecast for the next week predicts highs in the 70s and lows in the 50s, which will ramp up insect development and push trees from full bloom to petal fall, if not already.  Ascospore maturity across the region is at 70-100%, with most areas falling in the 80% to low 90% range.  Eau Claire and White Bear Lake are slightly behind at 73% and 75%.  The rain expected over the next week to ten days will push scab ascospores to full maturity and conclude the primary scab season.

Growers are encouraged to routinely check weather data supplied by their local Network for Environment and Weather Applications (NEWA) station for ideal spray application times.  To access your local NEWA station, please see the following link:

Location Degree Days 5/20/2019 (Base 50°F) Degree Days by 5/24/2019 (Base 50°F) Ascospore Maturity 5/20/2019
Eau Claire, WI 99 135 73%
Gays Mills, WI 145 192 90%
Hastings, MN 126 159 82%
Harvard (Royal Oak), IL 137 188 91%
Lake City, MN 134 174 85%
Mauston (Northwoods), WI 112 154 84%
Mequon (Barthel), WI 80 124 73%
Preston, MN 126 166 87%
Rochester (Ela), WI 116 147 90%
Trempealeau (Eckers), WI 126 171 88%
Verona, WI 135 187 92%
White Bear Lake, MN 109 141 75%
Woodstock, IL 165 217 100%

Table 1. Degree days, degree day predictions and ascospore maturity as of May 20, 2019.

Disease updates
Apple scab
Some locations have experienced the largest ascospore release of the season over the past week including Hastings (32%), Lake City (30%) Mauston (30%), Mequon (36%) and White Bear Lake (35%); however, the severity of this infection period is dependent on the overall inoculum present in the orchard.  Past weather data from IA, MN, WI and IL suggest three to four inches of rainfall is typical by mid-May and in these first three weeks of May some locations received an additional one to seven inches of rain.  Areas that remain an inch or so below the average precipitation may not have experienced the same threat level due to relatively low wash-off rates.  Areas with the highest rainfall include Hastings (7.04) and Lake City (6.45), while Gays Mills has only received 1.41 inches.  All other locations have had three to five inches and fall within the normal range.  When determining the need to reapply protectant fungicides, e.g., captan or an EBDC, remember that that one inch of rain removes half of the fungicide and two inches of rain is enough to remove all fungicide protection.  Leaf expansion will also leave unprotected tissue susceptible to scab infections.  Single-site fungicides may not be completely washed off during heavy rains and their weathering is influenced more by leaf expansion.  Full rates of fungicide should be used during periods of rapid leaf growth and when precipitation forecasted.

Most orchards are close to maximum maturity of ascospores, and according to Dr. Patty McManus (UW-Madison), up to 5% of these ascospores will never fully develop.  The recent rain showers should have released all the available mature ascospores.  Usually, peak-ascospore development occurs at petal fall.  This year it will likely occur at bloom, because apple scab has a lower development threshold of 33°F and fruit trees develop at 42°F.

Orchards that have received the 6 lb. rate of EBDC fungicides will need to switch over to captan beginning at petal fall, because a maximum of 24 lb. per year is allowed for this pre-bloom schedule and applications are not allowed post bloom.  If the 3 lb. rate has been used, the maximum amount of product is 21 lb. per year and may be applied up to the 77-day pre-harvest interval, which is often around second cover.  Due to this year’s wet weather, growers may apply another single-site fungicide with their protectant fungicide, even if two applications of a single-site fungicide have been applied.

Growers that are getting close to using all of their EBDC applications at 3 lb. per acre and have concerns about captan causing fruit damage or phytotoxicity at petal fall, could consider applying captan now.  Use captan only when applying another fungicide and switch back to the 3 lb. EBDC for sprays at petal fall that may include thinners, foliar nutrients and insecticides.  Most of the issues with Captan causing phytotoxicity are associated with being applied under slow-drying conditions, being mixed with an adjuvant or single site fungicide that is oil based, or when it is applied with several other products within a tank mixture.  If growers choose to apply captan, use the lowest labeled application rate and limit the number of other products applied in the tank until the leaves harden off.

Most of our scab infections have occurred under cooler conditions and will expand the incubation period for apple scab.  This means it may take upwards of three weeks or more before scab lesions are observed.  When scouting for apple scab, early season infections may be visible on the underside of the first spur leaves, whereas lesions found on the top of the leaf surface may indicate secondary infection.  At this point in the season, it can be difficult to distinguish between apple scab lesions and other injury, e.g., copper phytotoxicity.

Fire blight
Fire blight epiphytic infection periods (EIPs) have dropped to zero after the 40°F night that occurred on Sunday.  If temperatures during a two to three-day period are below a 60°F average, this will also reset the EIP to zero.  Late-blooming varieties and new trees will be susceptible to fire blight when EIPs are over 100, which will likely occur over the next four days with warmer temperatures and moisture levels.   Young trees can be protected with Apogee (prohexadione calcium) applications, streptomycin sprays or by removing blossoms.  Growers should also continue to monitor the Maryblyt or CougarBlight models if varieties are still in bloom.  The CougarBlight model is available on the NEWA sites.

Apogee can be applied to new trees at king bloom petal fall and then three to four times at two-week intervals.  It takes between ten days and two weeks for the trees to produce a physical barrier (thickened cell walls) to fire blight infection.  Apogee applied at this timing and rate won’t significantly impact growth but may not need to receive the full schedule of applications to allow growth to resume earlier if desired.  The “Apogee effect” will eventually wear off and allow growth to resume in July.  Phil Schwallier, Michigan State University – Extension, has noted similar growth in Apogee treated and untreated trees.  Newly planted and nonbearing trees treated should still receive scheduled fertilization.

For more information on applications of streptomycin and other bactericides for fire blight control, refer to last week’s AppleTalk blog post:

Insect updates
Codling moth
Codling moth traps and mating disruption should be hung by now, if not already.  Ideally, one trap should be set for every 2.5 acres.  Large, flat, uniform orchards may hang one trap every five acres, but smaller, unevenly shaped blocks should try to stay within one trap for every 2.5 acres because codling moths do not tend to fly very far.

Many sites are still behind the 180 to 250 DD from January 1st that are needed prior to the first codling moth (CM) flight.  Some orchards may begin to accumulate enough degree days to have a biofix late this week.  Codling moth biofix is marked by a significant biological event, or first sustained flight, where moths are captured multiple days in a row or exceed a threshold of five moths per trap per week.  Once a biofix date is established, begin tracking degree-days (base 50°F) and monitor traps weekly.

Codling moth fly between 6pm and 11pm when wind speeds are between three and five miles per hour and when temperatures are above 62°F without rain.  Assign the biofix date for the warmest, calmest night.  When checking traps, fluttering CM had likely flown within the last 48 hours.  Most female CM can live for seven to 14 days yet will mate and deposit the majority of eggs on the evening they emerge.  Every day that a sexually viable female emerges and isn’t able to fly, egg fecundity is decreased by 20%.

For more information on codling moth lures and mating disruption, refer to last week’s AppleTalk blog post:

Plum curculio
This pest typically becomes very active when the temperatures are in the 70s and 80s and activity can be begin in orchards with average temperatures near 65°F.  Growers are advised to take a note of the McIntosh petal fall date and begin counting 308 degree-days from this date.  Plum curculio are probably delayed due to cool temperatures and can be found along the perimeter while scouting as temperatures warm up this week.  Tree phenology is slightly ahead of insect development this year, as buds develop at cooler temperatures (42°F) compared to insects (50°F).

This year, spraying perimeters more frequently is an excellent option to reduce cover sprays.  If PC injury is found past the perimeter, e.g., first four or five rows of trees, a full cover spray is recommended.  In the past, many assume that thinning sprays with carbaryl will offer protection.  Thinning with carbaryl at one pint per acre delivers a half pound of actual carbaryl, whereas if carbaryl was used as an insecticide, this rate would be delivering two to four pounds of actual carbaryl per acre.  Therefore, the rates used for thinning, e.g., one pint to one quart, are not going to deliver much insecticidal activity. Reducing rates of carbaryl may also help lessen the impact on beneficial insects in the orchard.

Spring-lepidopteran complex
The spring-lepidopteran complex is moving along slowly and the first spring canker worms were observed in Lacrosse, WI this week.  Green fruitworm and obliquebanded leafrollers have been observed across as first or second instar larvae.  Once temperatures are sustained at 65°F for several days, this will promote insect feeding and will be the optimum time for an insecticide application.  Petal fall sprays will suppress these lepidopteran species.  It may not be necessary to apply Bacillus thuringiensis (BT) or Intrepid (mehoxyfenozide) at bloom due to their slow development.  Non-bearing trees are an exception and may be treated if they have a 10-14% infestation.  A 10% threshold is achieved when small trees with only seven to ten growing terminals have an average of one larvae per tree.  Intrepid may be applied to non-bearing trees but Avaunt (indoxacarb) and neonicotinoids should be saved for plum curculio management.

Impact of adjuvants on pollinators
Adjuvants help to facilitate the spreading ability and reduce weathering of pesticides.  Non-target effects are not often accounted for or expected.  Several studies have shown adjuvants can increase the toxicity of pesticides by up to 100x.  This is true for several fungicides that agricultural experts have deemed safe.  Only three states require growers to keep a record of adjuvant usage.  The EPA does not require registration and considers adjuvants to be non-toxic.

Recent research has shown organosilicon adjuvants have been discovered to be highly toxic and have affected honeybee populations in California.  Organosilicon adjuvants include products like Kinetic, Duo Stick Select, Weather Gard and Widespread Max.  Dr. Dave Rosenberger has been suspicious of the effects of adjuvants for many years and how they damage bark, specifically with a penetrating surfactant (acidifier).  Contact with the trunk overtime will make bark easier to penetrate, which may lead to herbicide damage or cause higher susceptibility to canker causing fungi.  John recommends to only use surfactants when necessary and at the lowest effective rate.

For more information and further reading:

Fruitload stress and winter injury
Return bloom in Honeycrisp has been variable across the entire region.  It is known that fruitload stress does not affect cultivars the same.  Stress from fruitloads do generate more sustained stress when compared to water stress from draught, too much rain or phytophthora.  It may take many years for low-vigor varieties, e.g., Honeycrisp, to recover from this accumulated stress.  When assessing tree health, examine the current state of the tree and consider stress concerns from the previous year.  Remember that stress accumulates over the tree’s entire life span.  A heavy crop can be a big driver of tree stress and on Honeycrisp, a medium cropload might be most ideal.  This may mean planting more Honeycrisp to compensate for lighter fruitloads.

Herbicides for young high-density trees
The orchard floor of newly planted trees and non-bearing trees are often managed differently than established orchards.  Weed control during the establishment of new orchards is absolutely critical.  Greater light penetration will allow for more weed seeds to germinate and will require more attention.

Where smaller plantings of high-density orchards are being established, mulching with hardwood bark or hardwood mulch is an excellent option.  There are several smaller AppleTalk growers who have been doing this with great success for several years.

There are growing concerns by both consumers and growers relating to the use of Roundup (glyphosate).  Good alternatives to glyphosate for non-bearing trees include Gramoxone (paraquat-dichloride) and Prowl H20 (pendimethalin).  Chateau (flumioxazin) and Rely (glufosinate) are also options but have use restrictions for non-bearing trees.  Chateau has a supplemental label for use on all pome fruits.

Debra Breadth, Cornell, has written one of the nicest articles on managing weeds in new trees and is a must read for anyone establishing new blocks,

A more technical discussion on this topic was published in the 2012 New York Fruit Quarterly,

Petal fall thinning and use of the carbohydrate model
Developing fruit are at many different growth stages due to the variable weather conditions during bloom and timing thinning sprays at petal fall will be more challenging.  The carbohydrate model is a tool that can be used to help determine application rates of chemical thinners and achieve more reliable thinning.  The model takes some of the guess work out of thinning by using the current weather forecast to determine if thinner rates should be increased, decreased or applied at the standard rate.  The model calculates the general carbohydrate balance, which has been found to correlate well with natural drop and decreasing tree sensitivity including sensitivity to chemical thinners.  Cool, sunny periods of good carbohydrate supply leads to a natural drop and less response to thinners (increase thinner rate).  Cloudy, hot periods result in carbohydrate deficits and lead to stronger natural drop and stronger response to thinners (decrease thinner rate).  The carbohydrate model needs to be consulted at the time of thinning to help determine rates.  The grower needs to use their experience with thinning and adjust rates based on what they have applied in the past to the target variety and crop stage.  This model does not tell you what product to use or what rate to apply but is particularly helpful when using MaxCel (6-BA) or Fruitone (NAA), which are both dose dependent.  Conversely, Sevin (carbaryl) is not dependent and will generally provide the same level of thinning regardless of rate.

Beginning at petal fall and before fruit begin to put on size is a relatively safe time to thin, since thinners are much less responsive at this time.  For example, to achieve a 50% fruitload thinning when applying Sevin (carbaryl) at 10-15 mm fruit, the same application will generate only 25% fruitload thinning at petal fall.  Application timing is dependent on variety and level of desired thinning.  Since petal fall occurs at different times, Sevin (carbaryl) should only be applied as a spot application.  Applications to the entire orchard at petal fall are not advised.

Find more information in part three of Amaya Atucha’s paper published on pg.11 here,

The carbohydrate model is available on NEWA under the crop management section,