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IPM News - October 27, 1999
Lorraine P. Berkett, IPM Specialist
A Look Back at the Season .....
Disease Situation:
Apple Scab: What a contrast from last year when we had so much rain
that it was very difficult to manage apple scab! This year, the dry weather was good for
at least one thing -- it helped in terms of disease management. Because of the high
levels of scab that had developed last year, the potential risk of scab was high as
we entered the 1999 growing season. However, from Green Tip through Pink, the
weather was very dry. The season was unusual in that we were almost at Bloom and had
not had an infection period but, the rains that occurred across the state on May 4 -5 (at
Pink) turned into the first infection period for apple scab. We had relatively
few infection periods ( approx. 5) during the primary scab period and they were very
discreet. It appeared that most growers were able to get into their orchards to
spray. However, in orchards were coverage was poor or timing was off, a significant
amount of scab did develop -- a reminder that even in a dry year, apple scab still
poses a serious risk.
Fire Blight: Conditions were most favorable for infection at the
very beginning of bloom (5/8) and at the very end of bloom (5/18 and 5/19).
Fire blight blossom blight infections were significant in at least five orchards -
one was so severe on Paulareds that the grower cut down a block of 12-14 year old trees to
try to reduce potential inoculum from spreading to the adjacent McIntosh trees.
Again, aided by dry weather, additional shoot infections during the remainder
of the growing season were not a problem.
Powdery Mildew: The warm, dry weather in the spring was favorable
for powdery mildew infection. Given that last year there was a higher
incidence of this disease and thus potentially more overwintering inoculum, the risk
of mildew was higher this year. Again, if we have a mild winter, more
powdery mildew will successfully overwinter which increases the risk of disease next
year. One word of caution: if you are planting the new cultivar 'Gingergold',
it is very susceptible to powdery mildew.
Sooty Blotch/Fly Speck: Again, in contrast to last year which was
so wet and favorable for disease development, the dry weather throughout the growing
season was a factor in reducing incidence of these diseases this year.
Silver Leaf: Symptoms associated with silver leaf disease
were apparent in a few orchards. The disease is caused by the fungus
Chondrostereum purpureum. Spores of this fungi land on fresh wounds caused by
pruning, broken branches, etc. Winter and early spring are the most critical times
for infection. The fungus invades the sapwood and mycelium extends both upward
and downward from the point of infection. Fruiting bodies will develop on the
outside of dead wood on the tree. They range in diameter from about 1" to
3" and are purplish in color, but will look grey when dried. Silvering of
foliage can appear on infected trees soon after petal fall. The silver leaf symptom
is due to a separation of the upper leaf layer from the rest of the leaf tissue.
This separation is caused by a toxin produced by the fungus. The silvering is not
always present each year in infected trees. If you cut through branches or the
trunk, you will often see that the wood is stained and decayed. Infected
trees can decline for years.
It seems every few years we have conditions that cause more symptom expression and this
was one of those years. If you have never seen silver leaf symptoms, the
following picture if of a tree that has the symptoms (lighter leaves in foreground)-- a
color picture which shows the symptoms better than the black and white figure below
appears in the web-site version of this newsletter:
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