RI Apple IPM Newsletter
May 16, 1997
From Heather Faubert and Steven Alm
We have had three apple scab infection periods since the last
newsletter: May 3, 6, and 9. Apple scab lesions take 9 to 17 days to appear
after an infection period, depending on the temperature. Many scab lesions
appeared this week in insufficiently sprayed orchards. These lesions are
probably from the April 28th infection period or the May 3rd infection
period. Check now for the velvety, dark lesions primarily on the underside
of leaves. Scab lesions were found on 2nd, 3rd, 4th, and 5th cluster leaves.
If you find scab lesions during the first two weeks after petal fall,
the safest recommendation is to apply Rubigan or Nova plus a full rate
of a protectant fungicide, through second cover. This protects against
the development of new lesions and suppresses further development of secondary
inoculum on the lesions. Read about eradicating scab in the 1996 - 1997
New England Apple Pest Management Guide on page 15.
If scab lesions are hard to find, (Cornell says if it takes more than
three minutes of scouting to find each lesion), a weekly application of
Captan or mancozeb fungicide may be adequate for keeping scab off the
fruit given average weather conditions for the year. Dave Rosenberger,
from Cornell, believes that Captan is better against a scab epidemic than
the mancozeb fungicides. Also, remember we are trying to limit the amount
of mancozeb fungicides applied to the trees because these fungicides are
believed to be harmful to predatory mites. We have seen quite a few predators
already this year, let's protect those predators!
Many orchards have high numbers of apple blotch leafminers this
year. If you look on the underside of the oldest fruit cluster leaves
with a hand lens you may be able to spot the small, round, pale, gelatinous
eggs. Some eggs have started to hatch into larvae that will be inside
sap feeding mines. The small sap feeding mines are only visible from the
lower leaf surface. By early June the larvae will have grown large enough
to cause visible damage from the upper leaf surface. This is when we can
see the mines as densely spotted mines, called tissue feeding mines.
Just how much damage trees can tolerate from apple blotch leafminers is
not clear . It depends on the variety and what other stresses the trees
are under. For example, a tree suffering from drought or red mites can
tolerate less stress from apple blotch leafminer. Generally we use a threshold
of 13 mines per 100 fruit cluster leaves. Without using a hand lens you
can't accurately determine the number of mines until about 7 -10 days
after petal fall. Seven to ten days after petal fall is too late to use
Provado or Agrimek against leafminer. The only material that would work
at that time is Lannate. The problem with using Lannate is it is very
harmful to predator mites. If you do use Lannate it should be applied
as soon as the mines start advancing to the tissue feeding stage. If you
wait for too many mines to advance the Lannate application will be less
Leafminers can also be controlled with Provado during the second generation
but this often requires two applications because the second generation
is more spread out than the first generation.
Agrimek must be mixed with something to help it penetrate the leaf surface.
This can be accomplished by mixing it with oil or mixing it with a spreader-
sticker such as Silwet or LI 7000.
European red mite nymphs are easy to find now in some orchards.
They generally complete hatching from overwintering eggs by petal fall.
Luckily it looks like we will have a new miticide to use this summer,
Pyramite from BASF. It is suppose to give good mite control for 45 - 50
days. Hopefully everyone's prebloom treatment will last until July and
then Pyramite could be used to control mites for the rest of the season.
European apple sawfly trap captures are lower this year than the
last several years. Some orchards are still above the threshold of 5 -
9 per trap by petal fall. Being above the threshold indicates a need to
spray Imidan or Guthion right at petal fall. Sawfly numbers below the
threshold means that you can delay applying a petal fall insecticide until
plum curculio are more likely to be active. With the cool temperatures
we are experiencing this spring, it isn't likely that plum curculio will
be active right at petal fall. So in locations where sawfly numbers are
low, this would be a good year to delay a petal fall insecticide application.
Plum curculio adults overwinter in leaf debris in nearby woods
and hedgerows, and in the orchard. They may begin migrating to apple trees
during bloom, but peak migration usually occurs from petal fall to 14
days after petal fall. Egglaying and feeding damage can occur as soon
as the fruit begins to form. The risk of plum curculio damage increases
after there have been 3 - 4 days of average temperature of 55 -
60 degrees, or 2 days with maximum temperatures above 75 degrees. Humid,
calm, warm evenings pose the greatest risk. If you delay your petal fall
insecticide application be sure to scout for plum curculio scars and watch
To monitor for plum curculio, check fruit on border row trees (especially
near woods) for the crescent-shaped egglaying scar. Large fruit usually
get attacked first - we usually find the first damage on Lodi, Idared,
or Liberty fruit. If hot, humid weather is predicted, put on an insecticide
before it arrives. Extensive damage can occur in a single warm night.
Use Imidan or Guthion when you first find damage and again about 10 days
later. Occasionally a third spray is needed against plum curculio, if
the plum curculio migration is slowed down by cool weather. The only way
to know is to continue scouting orchards through June for fresh egglaying
White apple leafhopper nymphs begin hatching at pink and have completed
hatching by petal fall. The small, pale nymphs can be found on the underside
of older leaves. White apple leafhopper feeding removes sap from leaves,
causing stippling that may coalesce into silvery, white patches. Early
in the season, extensive leaf damage may affect bud formation. White apple
leafhoppers are easiest to control when they are small, first generation
nymphs from petal fall to early June. White apple leafhoppers have developed
resistance to several organophosphate insecticides so Guthion or Imidan
will not control them. They can be controlled with Thiodan up until early
June. Provado or Agrimek applied at petal fall against leafminer will
also control leafhoppers. Sevin used as a thinner at 1 lb./100 gallons
of water will control leafhoppers as well.
Weekly phone message can be heard by calling (401) 949-0670 from
5:00PM to 8:00AM daily.
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