Rake and destroy fallen
leaves to remove as much of the source of infection as possible.
Rhododendrons are shallow rooted plants and certainly appreciate
extra water when we are not receiving sufficient rainfall.
However, drip irrigation and soaker hoses that keep water on the
ground and off the plant are best ways to water. If that is not
likely, at least use a watering wand and direct the water to the
soil as much as possible.
Although plants get wet when it rains
or when there is heavy dew, there is no need to make the problem
worse by overhead watering. It is best to water first thing in
the morning so that plants dry as quickly as possible when the
sun comes up; overhead watering in the evening guarantees that
leaves will stay wet all night, increasing the chance of disease
development. You may want to apply a fungicide next spring
to protect the new growth from infection, beginning as new
growth starts and making repeat applications as directed by the
label of the product you are using. Ortho Max Garden Disease
Control (chlorothalonil), Spectracide Immunox (myclobutanil),
Bayer Advanced Disease Control for Roses, Flowers and Shrubs (tebuconazole),
and Green Light Fung-Away Systemic Fungicide (triadimefon) are
labeled to control leaf spot on rhododendrons in Pennsylvania.
You did not mention if any of the stems are wilting in
connection to the leaf spotting and yellowing. If they appear to
be wilting, there are two other diseases that may be
responsible: Botryosphaeria canker or phytophthora root rot. Botryosphaeria canker is a fungal disease characterized by
sunken, dark brown cankers on the stems. The leaves on affected
stems wilt as this causal fungus destroys the vascular tissue in
the stem. You can see tiny black spots, which are the fruiting
bodies of the fungus, in the dead bark over the cankers. Drought
stress predisposes susceptible rhododendrons to this destructive
disease. Although you have been watering this year, dry weather
in previous years may cause an accumulated stress that made them
Continue to water as long as the weather is
dry, but stop when we are getting sufficient rain. Prune out and
destroy dead stems, and make sure you cut back to whirls of
leaves where there are buds that will begin to grow. Do not
leave large dead stubs that invite infection by this disease.
There is no chemical control for botryosphaeria canker.
Phytophthora root rot causes rhododendrons to wilt, and affected
plants may be stunted. The wilted leaves may be marked with dark
brown spots, and usually turn a dull yellow-green color before
becoming entirely brown. The shoots appear to die from the tip
back as the fine feeder roots are killed and can no longer take
up water. Phytophthora is more likely to occur in rhododendrons
planted in heavy soil that drains poorly. You can check to see
if phytophthora is to blame by examining an affected stem at
ground level. Peel some of the bark back and look at the
underlying tissue. You will see a distinct boundary between
healthy white tissue and diseased brown tissue. While fungicide
applications can protect uninfected plants, plants that are
already infected inevitably die. If phytophthora is confirmed,
infected rhododendrons should be removed.
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