Seems like a good day to flaunt my ignorance in public
*1) Flyspeck*
Among some of us, what passes for conventional wisdom with regard to
flyspeck on apple has been that there is a lag period between the end of
primary scab and the risk of flyspeck infections such that fungicide
protection in the first few weeks after fruit set is not important.
I'm not sure that's true after reading Ismail, Batzer, Gleason
Harrington "Phenology of Infection on Apple Fruit by Sooty Blotch and
Flyspeck Species in Iowa Apple Orchards", Plant Disease
<https://www.researchgate.net/journal/0191-2917_Plant_Disease>
100(2):PDIS-02-15-0137 ยท December 2015,
Dave Rosenburger showed that maintaining protection in the weeks after
fruit set made a difference at harvest, which lines up with the Phenology
paper. Plus Dave said that fungicide stops FS development while the
fungicide is active, but that after it wears off, FS resume progress
towards becoming visible. So if you allow a portion of development time,
stop it with fungistat (vs. -cide), then you have a much shorter lag time
between last spray and possible emergence of visible flyspeck at harvest.
So allowing unprotected growth hours in early summer is at the cost of
early FS progression in the fall.
I am sharing this in hopes of irrefutable brilliance, or at least
observations from people who have kept up the literature or field trials to
shed light on this. Bottom line I think the idea that we can wait until
2nd generation flyspeck spores are available (if there even is a second
generation cycle, which for some FS species at least does not seem to be
the case) before you need to spray idea is a hypothesis not supported by
evidence.
That all said, uncertainties around development time and residual
fungicide activity ends up saying that growers should maintain 21-day spray
interval for continuous protection, which does not conflict with a view
that FS risk starts early and just takes a long time to show because during
mid summer it's too hot, and that what really kicks development into gear
is leaking nutrients as the fruit sweeten and exude sugars in the weeks
leading up to harvest. This is another idea Dave pitched and which is
mentioned in the Ismail et al article.
So regardless of the logic, it may not matter if the spray
recommendation comes out the same. But I'd like to have a less foggy idea
about how this mob of fungi operate. Make for more interesting thought
process instead of a blanket 14 (captan alone) or 21 day vs. 2" rain rule
(2.5" for Pristine). Understanding the mechanics may not make a difference
under normal weather, but if we get either a long drought or prolonged wet
period, understanding the target organism could be useful for deciding how
to adjust spray intervals.
The only evidence I bring to this is a couple of incidents in past years
when heavy hurricane rains wiped out fungicide coverage in late August, and
subsequent to that flyspeck showed up right on time with a 270 temperature
mediated growth hour estimate for when they would appear in trees with FS
history/pressure. Not exactly Nobel Prize worthy replicated science, but
kind of made me feel smart for a day or two and that I had some idea what
was going while flyspeck was busy growing from invisible to visible.
*2) Fire blight*
Why is Regulaid specified as a penetrant adjuvant for use with
streptomycin applications during bloom? Does it have unique
characteristics that make it better for this purpose than other penetrants
such as LI700?
If grower is applying captan at same time, has much strep efficacy is
lost by not adding the penetrant to the mix to avoid captan phytotoxicity?
Does the marginal leaf discoloration frequently seen after strep
application have any long term consequences? We saw a lot of that this
year but I don't think it matters. Does not seem to have affected fruit.
If it does matter, would spraying during coolest part of the day reduce the
phytotoxicity risk?
If there is none or even minimal active fire blight in the orchard, is
there reason to spray strep after hail damage?
Is Regulaid or other penetrant important for tankmix if using strep to
suppress shoot blight after hail damage?
Does bringing in honeybee hives that were recently in a location with
high fire blight inoculum create risk of importing fire blight bacteria
with the bees? I've seen it said that the risk exists, but only last for
about 3 days.
The perennial question of whether you should sterilze blades between
sanitation cuts to remove fire blight strikes still brings different
answers. My take on it has been to not bother as long as you are just
cutting fire blight out, and that it is more important to remove fire
blight as soon as possible. But to certainly sterilize the blades before
using on any other trees. Agreed?
Do we all agree that strep provides protection for a flower for about
2-3 days? But that seems like a moot point because even if strep only
acted as non-residual sterilizing agent, if a flower is thus protected, by
the time it could be reinoculated with fire blight bacteria and for those
bacteria to accumulate enough heat units to generate a threshold
population, then that flower would be too old to become infected anyway.
Though I can imagine scenario with temps around 90F where a single cohort
of flowers could be vulnerable to a second fire blight infection period
after receiving a strep application.
*3) Pheromone trap thresholds*
I stopped using redbanded leafroller traps years ago because I
could catch dozens or even hundreds per trap per week and not see any
damage in blocks that received insecticide for apple maggot. If the RBLR
trap weren't changing any decisions, why bother? I'm almost at that point
with lesser appleworm. We catch 20-30+ LAW per week per trap, but except
in organic/low/or no spray orchards, don't see damage. So why bother?
For codling moth, LAW, obliquebanded leafroller, and oriental fruit
moth, there is a lack of agreed upon threshold. Some sources say 5 moths
per week per trap indicates a problem, others say 10. And as with LAW, we
often find well over 10 per week per trap without subsequent damage in
blocks receiving even rather sparse insecticide protection against apple
maggot. And the correlation between captures of male moths of these
species and egglaying females is not strong. The traps are still useful for
detecting flight timing, but as a quantitative tool are they a reliable
indicator?
If useful for indicating threshold abundance of these species to
require treatment, what threshold value to use?
Thanks for any replies.
- Glen
Glen Koehler
University of Maine Cooperative Extension
Pest Management Office
Voice: Office 207-581-3882, Cell 207-485-0918
491 College Avenue, Orono, ME 04473
UMaine Apple IPM https://extension.umaine.edu/ipm/programs/apple/
Ag-Radar https://extension.umaine.edu/ipm/ag-radar-apple-sites
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