Scaffolds 99 index
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HUDSON VALLEY DISEASE UPDATE
(Dave Rosenberger dar22@cornell.edu,
Plant Pathology, Highland)
Apple Scab
As a result of several extended wetting periods during the
last week, the supply of apple scab ascospores in the Hudson Valley has
been depleted and the primary scab season is over. However, secondary
scab could still cause serious problems in orchards where primary scab
was not completely controlled. Fruit and foliage are still very susceptible
to infection. In our unsprayed check plots, scab symptoms from infections
that occurred May 8-9 became apparent toward the end of last week.
Cedar apple rust
The rains of last week and this week are providing ideal conditions
for cedar apple rust infections on terminal leaves of susceptible apple
varieties. Teliohorns on cedar apple galls (the fruiting structures on
cedar trees) become gelatinous during rains and appear as colorful orange
masses on infected cedar trees. Basidiospores that are produced on these
fruiting structures are blown from cedar trees to apple leaves where they
infect the leaves and ultimately cause yellow-orange rust lesions.
Where cedar trees are present to provide inoculum, apple trees will
remain at risk for cedar rustinfection until about June 10. Apple fruit
become resistant to infection at or soon after Petal Fall, but newly unfolding
apple leaves remain susceptible. By mid-June, the cedar apple galls will
have exhausted their spore-producing capabilities for the season and the
rust season will be over. Cedar apple rust does not have any secondary
infection cycle. Infections that may appear on terminal leaves of apple
do not pose any threat for further spread of the disease in apples.
The importance of spraying to control rust after the first cover
spray is debatable because fruit will not become infected. Failure to
protect susceptible varieties, however, will result in severe infection
of two or three terminal leaves during each infection period in locations
where inoculum is abundant. The bright yellow-orange lesions can make
a tree look rather sick during early July when symptoms are most apparent,
and severely affected leaves will probably drop from the tree prematurely.
Susceptible cultivars such as Rome Beauty and Golden Delicious that are
left unprotected after first cover may lose so many terminal leaves that
fruit size could be affected. Where inoculum is less abundant, trees left
unprotected after first cover will only develop an occasional rust lesion
on terminal leaves. Light rust infection on terminal leaves may be unsightly
(except to plant pathologists!), but occasional lesions are unlikely to
affect fruit size or quality.
Captan will suppress rust (about 50% control), but only Polyram,
mancozeb, or SI fungicides (including Bayleton) will provide complete
control. If heavy rains remove protectant fungicides and trees are left
unprotected during a subsequent infection period, then the SI fungicides
or Bayleton will provide excellent eradicant activity when applied within
96 hours of the start of the infection period.
5/24 Insects
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