May 9, AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, May 9 2017, 8:00 – 9:00 AM
Guest speaker: Patty McManus, Plant Pathology, UW-Madison
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments,

May 9th Call Stream: CLICK HERE

Regional update
Across southern Wisconsin and northern Illinois orchards at late bloom to petal fall. Orchards in Gays Mills and Richland Center are at late bloom, while Galesville is approaching full bloom. Chippewa Falls/ Eau Claire is just entering bloom with Zestar and Idared.

Disease management with Patty McManus, Plant Pathology, UW-Madison
Rapid apple decline or sudden apple decline

  • In 2013, Kari Peter first started noticing dead trees, which were planted in 2011, mixed throughout Penn State’s test orchard with unusual symptoms of decline. Rapid apple decline (RAD) or sudden apple decline (SAD) affects M.9 (and M.9 clones, e.g., M.9 337, M.9 NIC 29) and has symptoms of necrosis at the graft union, e.g., cankers, shedding or flakey bark. The canker looks similar to fire blight, yet is not the characteristics reddish-pink color, and wood is not soft and spongy like Phytophthora. The root systems are healthy with a lot of suckering. Above-ground symptoms include the leaves prematurely turning pale yellow and reddish. Within a few weeks trees can go from healthy to dead, even with a full crop load. Total collapse is observed between July and September. This timing is different from the winter injury and canker fungi when collapse is observed after bloom. Note: The characteristic symptom of girdling, i.e., reddening of leaves in fall, occur regardless of what caused it, e.g., Phytophthora, fire blight, insect, mice, symptoms are reddening of leaves in fall.
  • RAD or SAD does not seem to be caused by fire blight, tomato ringspot, or Phytophthora. Dave Rosenberger, professor emeritus – Cornell, hypothesizes the decline may be a combination of factors, e.g., winter injury, black/white rot canker injury, herbicide injury. Kari Peter is not convinced the polar vortex polar winter of 2013 – 2014 is the culprit since the trees began declining before then.
  • For more information visit: Sudden Apple Decline – Learn All About It, Patty McManus, Wisconsin Fruit News, Apr 14, 2017,
  • Growers are recommended to frequently scout graft unions, especially in new plantings, throughout the season. Send samples of diseased or declining trees to the University of Wisconsin Plant Disease Diagnostics Clinic,

Fire blight
Are there reliable signs that characterize fire blight infected new trees that growers can look for before planting, e.g., visible cankers, immediate dieback of developing buds? Could rootstock blight also cause new trees to fail?

  • Nurseries cull trees with obvious fire blight infections before they are stored and shipped, and generally do a good job at removing infected trees right away. Symptoms in the field are typically seen as discolored tissue at the terminals or cankers at the graft union.
  • It is not recommended to apply streptomycin season long to protect first year trees that may bloom throughout the season. Tank mixing copper with streptomycin will not alleviate this risk. In Michigan in the early 90s, there were a number of growers who would apply a little bit of streptomycin all season. This was one of the first signs of practices leading to development of resistance. Two or three applications during bloom is not going to lead to resistance. Problems can occur when streptomycin is applied when obvious symptoms of fire blight are present. Even in the absence of fire blight bacteria (Erwinia), other bacteria are present that can become resistant and exchange DNA with Erwinia.
  • For recommendations on managing fire blight in non-bearing trees visit: Additional Guidelines for New Plantings (1–2 years), Scaffolds Fruit Journal, Ma7 1 2017, page 4,

How do we remove flowers manually and are we risking moving bacteria around?

  • There are some risk of spreading bacteria, even if it’s dry, regardless of method.

If fire blight symptoms develop on either a whip or feathered tree in the first season is there any removal length short of the 18-24 inch standard [for shoot strikes in mature trees] considered worth a try?

  • It might be better to remove the whole tree if symptoms are found. Some growers have had success pruning down to a clean bud and retrained the tree. This only seems to work when a tree has a very established root system. Note: Pruning may not completely remove infected tissue; continue to monitor trees for recurring symptoms.
  • If new trees require heading or other training these steps are taken when black/white rots as well as fire blight can be floating around. With relatively small wounds how quickly may these suberize and are copper or captan residues likely to protect these openings from colonization if precipitation happens?
  • Captan and other fungicides will protect trees from black rot. Trees are likely susceptible to fungi infection a few days after cuts. Only prune during dry weather e.g., do not make cuts if rain is forecasted in the next 48 hours.

Additional disease questions
How vulnerable are young trees to wet feet fungi, and is there evidence that pre-plant mycorrhizal dips or bio-fumigant plantings offer benefit against pathogenic soil fungi? Is it as big a problem as it used to be? How vulnerable are these young trees to Phytophthora?

  • Geneva rootstocks have been developed to be resistant to fire blight, Phytophthora and replant diseases. Level of resistance and tolerance vary among the series. Geneva Apple Rootstock Comparison Chart v.3,
  • Mycorrhizae dips are more about enhancing nutrient uptake and less about fighting diseases. It is better to not replant into an old site, unless adequate steps were taken to prepare the site.
  • Soil drainage and structure is most critically important factor in preventing issues with Phytophthora.

Powdery mildew and extended periods of shoot elongation: Does fungicide every two weeks offer sufficient protection. Should growers alternate FRAC group after two applications (14 days apart) or every application considering spray interval. Regarding resistance management, should fungicide rate if applied every 14 days be at the maximum per acre?

  • Organic growers can use sulfur to control powdery mildew. Sulfur is a broad-spectrum protectant and conventional growers may tank mix sulfur with DMI and Strobilurins for added protection and resistance management. Do not apply sulfur within 14 days of an oil application. Do not use captan in combination with or closely following or in alternation with sulfur products. Use product label to verify use restrictions. Fruit russeting and yield reduction can occur if sulfur is applied during hot temperatures (>80°F), especially following bloom.
  • When targeting powdery mildew with single-site fungicides, i.e., SDHIs, strobilurins (QoIs), sterol inhibitors (DMIs), do not make more than two sequential applications before switching to a different mode or action.

How much danger do we have right now with scab vs. fire blight?

  • It was too cold during the first week of May for fire blight, but scab is certainly a concern during extended periods of rain. In the 40°Fs development will continue very slowly, but when it heats up, infections can spread rapidly. Leaves and fruitlets area highly susceptible right now.

Disease and insect update
Apple scab
Good weather is forecasted for the second week of May to reapply a fungicide. When we are ten days out and have had one inch of rain, it is important to reapply. Do not go beyond 14 days. If you have had less than an inch of rain, going ten days before recovering is fine.

Early season lepidoptera
Scouting orchards at pink and into bloom, may not see lots of evidence of lepidoptera activity, obliquebanded leafroller, redbanded leafroller, spring cankerworm, green fruitworm. If you have not applied an insect growth regulator, e.g., Esteem (pyriproxyfen), Intrepid (methoxyfenozide), or a pink spray of pyrethroid, you will likely of having some feeding activity. If you don’t see evidence of larvae, it may not mean they are not there. Most of the time the caterpillars have been around for 10 days before we see significant visible damage.

These pests can be controlled with petal fall sprays for plum curculio or with applications of Bacillus thuringiensis (Bt), e.g., Agree, Deliver, Dipel, when it is warm and blossoms are open. If blossoms are still closed a Bt application will not target larvae that have tunneled into closed blossoms. Bt 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. As with most insecticides, young larvae are generally more susceptible than older larvae. Consider waiting to apply an insecticide for these pests until the weather is warm (~60°F) and sunny.

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

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.

Plum curculio
It is often challenging to control spring lepidoptera and plum curculio (PC) at the same time. When we take the bees out there is an urgency to control lepidoptera larvae right at petal fall. Often if we do not control the spring lepidoptera larvae right at petal fall, they will do an increasing amount of injury. This may not be the best practice for controlling plum curculio. We are approaching petal fall but the weather has not been conducive for PC movement. Plum curculio emerge around 250 DD, base 50°F, and movement into the perimeter of orchards begins when temperatures are above 60°F for three days. This is a gradual process and requires several days of warm weather. Plum curculio are primarily nocturnal and the entire population does not migrate into the orchard at the same time and fruitlets become susceptible to egg-laying when they reach 5 mm. We would need a real turn around for the PC spray to have any impact. Plum curculio are not likely even to be presence in the perimeter, but maintaining a perimeter spray could still be a good idea. Full cover sprays this week are not advised.