Eco Apple Call Summary, April 30th, 2013

Eco Apple Conference Call

Tuesday April 30th, 8:00 – 9:00

Presenters: John Aue, Threshold IPM; David Rosenberger, Cornell University

Moderator: Peter Werts, IPM Institute of North America; questions or comments, pwerts@ipminstitute.org.

Call Agenda

  • Apple scab biology and management
  • Powdery Mildew
  • Canker diseases
  • Call format and future direction
  • Feedback for improving calls

Apple scab and powdery mildew biology and management

David Rosenberger comments on use of copper at green tip:

  • David had compared performance of copper to mancozeb and Syllit (dodine) with sprays made at ½ green.  Fixed copper gives same protection as mancozeb applied at the 3lb/acre rate.  Copper and dodine had similar residual activity and after wash off scab does move and does not provide residual control, this is the same residual performance of dodine and mancozeb.  The residue can be expected to provide up to one week of control.
  •  Kocide label has changed from 2000 to 3000 and the labeled amount of copper had been reduced for applications because of a finer grind.  There is more potential for phytotoxicity in vegetables with the coarser grind, but is better environmentally to not apply lots of copper.
  • It is hard to know how much copper we are getting from newer formulations, e.g., Mastercop.  New coppers are less than 1/10th of old coppers in residues.  New products designed for vegetable crops where there are weekly applications, e.g., highly active low-rate copper.  David does not think the newer products will be as effective for fruit crops.  Copper is not systemic and if it was systemic, it would become phytotoxic to trees very fast.
  •  Copper is often known to cause russeting if residues are present during bloom.  The general rule to follow to expect two to three inches of rain, sometimes less, to dissipate copper to prevent russeting.

Scab modeling

  • The typical model of recording degree days based 33°F for predicting ascospore maturity works best when applied at McIntosh green tip.  The Northeastern Weather Association has online models using a green tip date.  These models are tough and had more value when post-infection materials used more frequently.
  •  Due to increasing fungicide resistance it is recommended to apply protective sprays prior to first infection period.  If rain is forecasted, stretching applications and waiting to apply ahead of the next predicted rain is an option.  David believes lots of problems start during infection periods that occurring early in the season at or just after green tip.
  •  Snow cover makes ideal conditions for scab development.  When there is no snow cover scab spores are often frozen or dried out and there will be no ascospore maturation.  But, when snow cover keeps leaves wet and insulated at 32°F, this can lead to early spore discharge.  Development will stop or slow when leaves and spores dry out.

Powdery Mildew

  • Powdery mildew has been an increasing problem in recent years.  It is only speculative to assume if this is due to warmer winters or resistance.  The pathogen over winters as a mycelium in infected buds and begins growing with green tissue in the spring.  These overwintering infections will not be eliminated by any sprays made this year.  Our target is to stop the spores released from infected shoots.  Infected buds begin producing conidia between tight cluster and bloom.  The timing of fungicides at tight cluster is needed because of the requirement for a protectant-based approach to management.  Powdery Mildew only infects young and unfolding leaves and protectant sprays may need to be applied one or two times again in the summer on non-bearing trees with actively growing shoots.
  •  High humidity is a requirement for growth of the pathogen and dry weather can slow it down.  Typically we expect temperatures of -5F and below to begin killing mildew infested buds and  90% kill at -10°F, is often experienced.
  •  We are expecting lots of inoculum going into 2012 and modest programs that have worked previously might not work, in high-inoculum orchards.

 How do we approach resistance management for powdery mildew?

  •  We don’t have a good understanding of powdery mildew resistance, but should expect we are shifting to some DMI resistance.  Historically growers could wait until petal fall to treat powdery mildew with Flint or Rally.  In research conducted by Rosenberger they found that Rally (myclobutanil), had tremendous reach back.  Flint (trifloxystrobin) provided excellent on new leaves, and one leaf back. but did not have the post infection activity of myclobutanil. This post-infection activity of DMI fungicides allowed us to wait until petal fall to apply and could expect Rally (myclobutanil) to eradicate it.  If you wait to petal fall to apply Rally, Flint sulfur or anything else, don’t expect control.  It is not to say Rally won’t work, but likely is not providing kickback.  An important step to help manage resistance to powdery mildew is to include the use of sulfur with materials for powdery mildew and will be ideal in cool conditions.  This approach is the same as mixing captan or mancozeb with a DMI or strobilurin for scab management.

 Additional Scab and Powdery Mildew Fungicide Restrictions and guidelines

  •  Fontelis (penthiopyrad) is only a single product and has one mode of action.  Mancozeb and captan have no activity on powdery mildew.  Due to the SDHI susceptibility to resistance we should be careful about the fungicide combinations and rotations which are applied.
  •  Where resistance is a concern, applying an SDHI with a different mode of action may be wise for powdery mildew management.
  •  Merivon (fluxapyroxad, pyraclostrobin) and Luna Sensation (fluopyram, trifloxystrobin) have restrictions on adjuvants.
  •  Sulfur not compatible with oil.  In hot weather sulfur can stall photosynthesis.
  •  Scala (pyrimethanil) or Vangard (cyprodinil) are in a different class than strobilurins or DMIs and could be better options for managing scab in cool weather.  Temperature during infection period is very important and an infection period down in the 40s will discharge far fewer spores than in the 50s.  A cold rain will cause problems but not as serious.
  •  If there are time constraints it is suggested to spray the most susceptible cultivars first.

Canker Diseases

  • Coming off of drought we may expect more white rot canker and may take a while to move in as effects of last year’s drought show up.  This spring is a good time to look for borers at graft union.  In the spring borers will resume activity when tree shoots up nutrients.