April 30, 2019 AppleTalk Conference Call

AppleTalk Conference Call Summary

Tuesday, April 30, 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 30th Call Stream: CLICK HERE

Regional update and freeze assessment
Most apple varieties across southern Wisconsin and northern Illinois are at tight cluster.  Orchards in central Wisconsin and central Minnesota, e.g., La Crosse, Twin Cities and Chippewa Falls, are at ¾” green to tight cluster.  In northeastern WI and the Door County Peninsula, trees are at silver to quarter inch green tip.  Wet and cool weather will persist through the week.  After a brief warm up into the upper 60s on May 4 and 5, temperatures will fall back into the mid-50s and low 60s and many orchards in the region may hold at pink for a week or more.

On April 27 and 28, a freeze event occurred across the region.  The coldest temperature was recorded in Woodstock, IL (23.6°F) and the average low temperature across the region was 29.3°F.  Critical temperatures for fruit damage to trees at tight cluster are 27°F for a 10% bud kill and 21°F for a 90% bud kill.  If bud growth has reached pink, 28°F will result in a 10% kill and 24°F will result in a 90% kill.

Temperatures did not fall as low as predicted and cloud cover along with high relative humidity may have protected developing blossoms and reports from growers now suggest losses are not significant as predicted.  Most growers had temperatures at or near 27°F that occurred for three to five hours.  In some locations, temperatures varied depending on the direction and slope of the orchard.  It is important to consider the location of the weather station, when interpreting data from NEWA stations.  For more information on critical temperatures for bud and blossom development, visit: https://www.canr.msu.edu/uploads/files/PictureTableofFruitFreezeDamageThresholds.pdf

Apple scab biology and management
There is an apple scab infection event occurring through mid-week for all 24 NEWA stations in the region.  Development of overwintering ascospores may be tracked by monitoring growing degree days base 32°F from 50% McIntosh green tip.  Primary ascospores are considered completely developed after 900 to 1000 DD, base 32°F.  The ascospore model is not linear and most spore maturation occurs around or after 900 DD.  Most growers are between 300 – 325 DD from mid-April green tip.  As of April 29th, NEWA stations were showing about 12-20% spore maturity with 2-3% daily ascospore discharge, during rain events.

Ascospore discharge usually begins within 30 minutes after the start of the rain and is largely completed within three to six hours.  When infections occur at temperatures below 40⁰F, the number of days for scab lesions to become visible is unknown.  Lesions become visible at or after 17 days for infections between 40⁰F and 50⁰F and when temperatures are above 50⁰F, lesions become visible in 10 – 14 days.

There has been minimal leaf expansion and enough rainfall to redistribute copper applied at green tip.  Orchards seven days or less from their last fungicide application should have adequate coverage through the weekend.  Orchards which received a fungicide ten days ago may have more unprotected leaf tissue and should apply a protectant fungicide and consider including a single-site fungicide.  High inoculum orchards or varieties more susceptible to scab should be covered as soon as possible.

Sufficient fungicide residues are required during infection periods to eradicate the developing Venturia inaequalis.  Reapplication is necessary if more than two inches of rain was received since the last application.  Fungicide performance is dependent on the quality of spray coverage, application rate, total rainfall, and the amount of new leaf growth since the last application.  The EBDC fungicides and Captan will both redistribute during light rains.  To assess fungicide performance, monitor rainfall and relative humidity.  A small amount of rain can be enough to generate spore release and high relative humidity can result in long infection periods, as leaves tend to dry very slow when relative humidity is high.  Most orchards have experienced two or three scab infections since green tip.  Short rain events still release spores, even if there is no infection period.  When spores are released without a successful infection, fewer mature spores will be discharged and infections which do occur may be less severe.

Organic scab management
Liquid-lime sulfur (LLS) will provide the most post-infection control of apple scab when applied within 24 – 36 hours of an infection and may be applied as late as 72 hours from the start of the infection.  Conventional producers have used a variety of fungicides post-infection, however, this practice is not advised due the resistance concerns for DMI, SDHI and QoI fungicides and should also consider LLS.

Liquid lime sulfur should not be applied when temperatures exceed 80°F or within 14 days of an oil application.  Liquid-lime sulfur is incompatible with many other pesticides, especially oils and other emulsified materials.  Liquid-lime sulfur can induce a thinning response when applied during bloom, and caution is advised when making an application during this time.  Do not mix copper or sulfur with biologicals like Serenade (Bacillus subtilis), as both are general biocides.

Other options for scab management include Serenade (Bacillus subtilis), Double Nickel (Bacillus amyloliquefaciens strain D747) or Regalia (Reynoutria sachalinensis).  Hold off on applying Regalia if precipitation is predicted as it requires absorption.  Once the temperature is 60⁰F and above, Regalia will be more effective.  Trials completed by Patty McManus, University of Wisconsin Madison plant pathology found that Regalia performed “Fair” on apple scab and most other biological fungicides performed fair to poor on scab.

John’s scab management recommendations

  • IPM growers could apply two back to back applications of an SDHI, e.g., Aprovia (benzovindiflupyr) in a tank mix with a full rate protectant. Apply the first round between tight cluster and late pink.  Aprovia (benzovindiflupyr) will provide protection after the warm weather on Friday through Sunday and will be effective for seven to ten days, regardless of rain.
  • Organic growers should apply liquid-lime sulfur or LLS plus micronized sulfur two to three times before bloom rather than after bloom.
  • Dormant oil is most effective when applied at temperatures over 60 F⁰ and the 1 – 2% solutions normally applied will not suppress many insects if applied during the cool temperatures this week and next. Using a lower solution of dormant oil, e.g., 0.5%, mixed with a single-site fungicide will slow the drying of the material and help with absorption.  However, it is best to wait until warm temperatures return before using higher rates of dormant oil.  Single-site fungicides must be absorbed into the leaf to be effective and function properly, therefore mixing in a lower solution of dormant oil may help absorption.

Tips for newly planted trees
Cooler temperatures over the past week will help mitigate the risk of fire blight infections.  This would be a good time to complete pruning or training of new trees, since temperatures above 65F⁰ are needed for the fire blight bacteria (Erwinia amylovora) to develop.  After the warm weekend that is forecasted for Friday-Sunday, there will be enough DD accumulation to pose a threat if additional rain accumulates.  Rain is forecasted throughout next week and it will be beneficial to apply copper to new trees over the weekend.

Avoid planting new trees into wet soils.  Mycorrhizal dips will help prevent infections of phytophthora or other root diseases.  Incorporating compost or aged manure as a less water-saturated backfill is recommended where trees are planted into wetter soils.  Fertilizer will not be required for the first few weeks after planting.

Insect management reminder
Most growers have moved away from applying a synthetic pyrethroid at pink, something that was a common practice for decades.  Pre-bloom insect management generally targets San Jose scale and mite eggs and spring lepidopteran that emerge between tight cluster and bloom.  It is doubtful that many overwintering insects have emerged due to cooler temperatures.  The first redbanded leafroller (RBLR) flight is expected around 75 DD; green fruitworm (GFW) should emerge around 100 DD and obliquebanded leafroller (OBLR) can be expected shortly after at 105 DD base 50°F.  Growers can expect to find larvae of GFW, OBLR, RBLR, spring canker worm and variegated leafroller all feeding on growing terminals and blossom clusters as temperatures begin to warm.

The insect growth regulators Esteem or Intrepid and the bio-pesticide Bacillus thuringensis (Agree, Deliver and Dipel) may be applied between tight cluster and petal fall, after careful scouting reveals insect larvae have emerged and began feeding.  Insecticides should be applied when it is warm (~60°F) and sunny.  If an application is warranted, manage these pests in the early larval stage while they are actively feeding on leaf tissue and before trees reach petal fall.  After petal fall, these species will be harder to manage, since many of them may be nearing the end of their life stage as a larva.

Note on pollinator exposure: Intrepid (methoxyfenozide) is an insect-growth regulator that only works on lepidoptera and does not have effect on honeybee larvae.  It is not recommended to apply Esteem (pyriproxyfen) during bloom.  Bacillus thuringensis products are also safe to apply when bees are active in the orchard.

Additional tips for using Bacillus thuringensis
If blossoms are still closed a Bt application will not reach larvae that have tunneled into closed blossoms.  Bacillus thuringensis must be eaten by the insect to be effective and warm temperatures are needed in the 72-hour period following an application for good mortality.  It is essential to scout for larvae following the first application of Bt, as an additional application may be needed (early petal fall).  Early detection of a pest is critical for good control.  The spray deposit may only last one to three days before it is washed off by rain or broken down by sunlight.  Sticker substances that promote adherence to leaf surfaces and UV light inhibitors that protect Bt from photo-degradation may enhance efficacy.  For more information on application timing and use of Bt products visit http://nysipm.cornell.edu/organic_guide/apples.pdf