Apple Pest Report: Thursday,
June 7, 2007
Vol. 15 No. 10
Apple Scab
Primary
scab ascospore releases are done for the year at Highmoor Farm and for any
orchard that reached 95% McIntosh Petal Fall by June 1 (which includes just
about every orchard south of
The
last primary scab infection period this year was unusually severe. Relative scab infection potential for an
infection period is a combination of the portion of the year’s ascospore
population released and the amount of susceptible foliar tissue. The previous date with conditions for a
full release of mature ascospores was all the way back to May 20. By June 1, all of the remaining
ascospores (and a large portion of the year’s total) became ready for
release. The final infection period
accounts for an estimated 62% of the total primary scab infection potential at
Highmoor Farm. That percentage is
lower/higher for orchards that reached Mac Petal Fall before/after May 27.
Compounding
this is that heavy rainfall. Many
locations had more than 2 inches rain on June 1-5. This made it unlikely that fungicide applied
before the rainy period started would have lasted throughout the infection
period. Depending on location, as
much as 10% of the season’s primary scab potential was not released until
June 4-5 when fungicide coverage was largely removed by the heavy rain.
If
full-dose good-coverage fungicide protection was applied shortly before the
rains began (i.e. Tuesday May 29 or later) then there probably is not too much
to worry about. But if the
fungicide protection going into the June 1-5 was applied May 28 or earlier, there
is cause to worry that protection wore off early in the infection period with
additional ascospores landing on foliage after fungicide protection had worn
off.
As
with any disease discussion, the degree of risk is driven by the amount of
inoculum in the orchard. If scab infection
was noticeable at even a low level last fall, then the combination of aging
fungicide protection, a large infection period, and high inoculum is reason to
consider following up the June 1-5 infection period with a post-infection
sterol inhibitor.
By
the time you read this and would be able to reach (if you so chose), the 72-96
hour post-infection window for a sterol inhibitor fungicide application will be
past. But sterol inhibitors also
have pre-symptom activity that can shut down developing scab infections before
they can produce secondary spore producing lesions. Strobilurins and anilinopyrimidines
(Vangard, Scala) do not have this ability.
Whether
the extra expense for one or two sterol inhibitor applications is worth it is a
tough call. Because they are not effective
for long term scab protection, or for flyspeck, sooty blotch, and summer fruit
rots, post petal fall is not the preferred time for sterol inhibitor
application. If protection was
solid through the June 1-5 infection period, or inoculum level is very low,
then maintaining protection for the next couple of weeks until you can confirm
that primary scab control was successful should be adequate.
But
for an orchard with high inoculum level and dubious protection through a
season-ending infection period that could have represented almost 2/3 of the
season’s total primary scab infection potential, a sterol inhibitor
applied in the next couple of days could prevent having to deal with numerous
scab lesions. Applying one or two
“extra” sterol inhibitor applications is not cheap, but neither are
the more frequent sprays required to shut down scab lesions and maintain strong
fruit protection if scab lesions develop from the June 1-5 infection period.
Looking
ahead, renewing fungicide protection at no more than 14 day intervals is
recommended for the next few weeks so that if you do discover scab lesions,
they will not have had chance to rain conidial spores onto unprotected fruit
and foliage before you can apply eradication treatments.
There
is a table post-primary scab fungicide interval recommendations online for
Highmoor Farm and vicinity. If the
rain amounts are similar at your location, then that table may be useful for
your location. See http://pronewengland.org/AllModels/MEmodel/me-Monmouth-SecondaryScabSpray.htm.
Scab
lesions on unsprayed trees from the season’s earliest infection periods
only started showing about 10 days ago.
It won’t be possible to verify effective control of 1st
generation lesions from the June 1-5 infection period until June 14-16.
Because 1st generation lesions clustered in the tops of trees can be hard to find, it isn’t until 2nd generation lesions have had time to develop, and scouting confirms that they are not present, that you can safely assume that scab control was effective. I learned this a few years ago at Highmoor when I gave a clean bill of health to a block of taller trees that was scab-free (from the ground) after there had been enough time for all 1st generation lesions to show. But then within about 10 days later a large number of 2nd generation lesions started appearing.
But
first things first. While
it’s never a bad time to look for scab, the best time to check for scab
is the week of June 12-19.
Fire blight
Streptomycin
application can cause marginal leaf yellowing. (see photos on right from an article by
Dr. David Rosenberger in the May 7 issue of the Cornell Scaffolds Newsletter).
Strep induced leaf yellowing has not been
found to cause any negative effect on fruit set or tree health, and so does not
require any response. Unfortunately,
the initial stage of potato leafhopper (PLH) damage also causes marginal leaf
yellowing. With PLH damage, damage
symptoms will progress with leaves cupping upward along the edges and turning
brown.
Normally PLH damage on established
trees does not require treatment.
However, you would want to prevent further PLH damage on young trees
that have not yet filled their space because the damage can interfere with
growth. Where there is ANY history
of fire blight it would be wise to not tolerate PLH on young or established
trees this year. That is because PLH
feeding may spread shoot blight infections. If fire blight does appear, you do not
want to have PLH in the orchard to spread the disease.
PLH
do not overwinter in Maine. 
They
arrive on weather fronts moving up from the south, such as the rain we had
earlier this week. Sevin (carbaryl)
used for thinning also provides potato leafhopper control. Of the insecticides used for plum
curculio control, Imidan has a “Poor” rating against PLH, but the
neonicotinoids (Actara, Calypso, Assail),
Avaunt, and the pyrethroids (Asana, Baythroid, Danitol, Proaxis,
Warrior) are all excellent against leafhoppers. PLH can show up at any time until
harvest.
If
PLH are found at a time that does not coincide with insecticide coverage for
other pests, a quarter-rate Provado applications (0.5 oz./ 100 gals. dilute)
can control them. Timing is at
least as important as rate. Be on
the lookout and ready to respond if you find more than one 1 PLH per leaf. If there is active fire blight in the
block, a lower threshold would apply.
I don’t have a number, but given the risk of fire blight spread,
it does not seem unreasonable to use detection level as the threshold, i.e. if
there are enough PLH that you can find them then treatment is warranted.
We
are still in the waiting stage before any fire blight blossom strikes will
become readily apparent.
Massachusetts had similar (but less severe) infection conditions on May
11-12, a full two weeks before
Here
in Maine, blossom blight infections are not expected to become readily apparent
until around June 15. So when you
are out looking for scab, keep an eye out for potential fire blight infections.
Insects and Mites
The heavy rain on June 1-5 washed off previously applied insecticide protection against plum curculio. Without much rain in the current Highmoor Farm forecast, renewed insecticide coverage in that area renewed on June 6 is expected to last until June 19 which is 82% of the way through the plum curculio control window (= 308 DD base 50 after Mac Petal Fall). Full-dose insecticide applied on or after June 10 should last long enough to get through 100% of the plum curculio control window which ends on June 23. Updated plum curculio re-spray intervals for Highmoor Farm and vicinity are online at http://pronewengland.org/AllModels/MEmodel/me-Monmouth-PCTable.htm.
The control window only covers 40% of the plum curculio egglaying period. It works because once a plum curculio female is killed she does not come back to finish the rest of her egglaying. But growers using Surround to deter rather than kill plum curculio should maintain protection until at least early July.
Insecticide applications for plum curculio keep codling moth in check by killing the moths before they can lay eggs, killing eggs before they hatch, or by residual control of newly hatched larvae. Where specific codling moth treatments are needed (such as in organic blocks where certain Bt products or Entrust can be used against codling moth in the absence of broad spectrum insecticides targeted at plum curculio), the ideal timing is to begin codling moth control is around June 19 for the Lewiston-Augusta area, and slightly earlier or later for orchards ahead of behind of Highmoor Farm.
This week is the optimum time for assessing European red mite populations. Treatment is advised if living hatched mites are present on more than 30% of middle-aged leaves collected from the interior canopy. Miticide options were listed in the May 28 Apple Pest Report.
Also keep an eye out for
white apple leafhopper (WAL). While
they do not spread fire blight, WAL feeding can stress foliage and probably
more importantly, if not controlled can be quite a nuisance to pickers. Less than one WAL per 4 leaves (i.e.
0.25 per leaf) is too low to require treatment, and more than 1 per leaf is
likely to result in tree stress or worker nuisance, especially by the time
second generation WAL appear in mid-August. If needed, 1st generation
control is more effective than dealing with the more spread out 2nd
generation.
Roundheaded apple tree borer is primarily a pest of home orchards where full canopy sprays for plum curculio and later apple maggot are not being used. Borers kill a significant portion of untended backyard apple tree plantings.
Lorsban (chlorpyrifos) trunk application is the most effective tactic to prevent new borer attacks. The 4E formulation is restricted use and requires a pesticide applicator’s license, but the 75WG formulation is not restricted. But Lorsban 75WG is not sold in a small container size suitable for hobby orchards. For small plantings, the best option is probably one of the premixed home fruit tree multipurpose sprays applied monthly (mid-June, mid-July, mid-August) in a soaking spray to the lower four feet of trunk. This is in addition to any canopy sprays for plum curculio, codling moth, or apple maggot.
Cornell University is providing scab fungicide resistance testing for 20 sites in the Northeast. This is a first-come first-served arrangement, but their testing lab agreed to my request to hold 3 spots for Maine sites which would lose out in a first come basis because our phenology runs later than other apple growing areas. We are going to use one of those slots for a block at Highmoor Farm.
Candidate orchards should be those where there is some reason to suspect resistance to one or more of the scab fungicides. Frequent past use of sterol inhibitors is the most likely scenario. A block where scab control has been more difficult than other similarly managed blocks would be another good candidate.
Testing requires 50 leaves with scab that has not received an eradication fungicide treatment. Unless you purposely leave tress unsprayed for resistance testing, such leaves are not going to be available. A more likely source is to collect scabby leaves from an unsprayed apple tree no more than 1000 feet from the orchard. Collect leaves in a paper bag and keep them cool. If you are interested in sending a sample, please contact me for details ahead of time because once collected the sample needs to be in the hands of the lab within 2 days. Samples must be in by June 30.
Sincerely,
Glen
Glen Koehler
Pest Management Office
491 College Avenue
Orono, ME 04473
Voice: 207-581-3882
Email: gkoehler@umext.maine.edu
Web: PRONewEngland.org
Fax: 207-581-3881
What we call the secret of happiness is no more a secret than our willingness to choose life. - Leo Buscaglia