I have a rhododendron on which every bud plus 3"-4" of its stem has
turned pure white. It is  not distorted in any way. I have sprayed the
plant thoroughly with a propriety fungicde which removes (or
disguises) the condition for 24 hours. It then returns to pure white.
What is the condition and remedy?

I have spoken with my father in law who is a rhodo specialist here in
the UK; he thinks the condition is POWDERY MILDEW (Microsphaera alm)
 
The definition of this disease in our Rhodo book is:
 
'This fungus, one of the several powdery mildews is an exception to
the rule that cool damp conditions are necessary for the spread of
fungal diseases.  In this particular case, the powdery mildew thrives
in a dry climate and serious attacks only become prevalent in hot
weather.  It can, therefore, be seen that, since Rhododendron as a
genus prefers cooler, damper conditions, the majority of growers will
not experience great difficulties with this pathogen.
 
SYMPTOMS - The disease can appear on the stem, leaves and the flowers
as a superfical powdery growth during the summer months.  Later, the
white powdery mats will turn brown'

I have yet to ascertain what can remedy this and will post as an
answer if/when I do.

Regards,
Ali

One of the problems with diagnosing plant diseases over the phone (via
the web, email, etc.) is that plant pathology is as much of an art as
a science and requires the diagnostician to see the symptoms and the
pathogen.  Most of us would not expect a medical doctor to diagnose a
potential disease over the phone.  In the same regard, it is
impossible to be sure what is causing a plant disease without seeing
it.

You could have a powdery mildew but they do not usually colonize woody
stem tissue.  Powdery mildews spend most of their time in the
epidermal leaf cells, not living under woody tissue.  In addition,
symptoms vary on rhody's depending on the time of the year and
developmental stage.  In addition to powdery mildew, it could be any
number of fungi causing a tip dieback.

Determining which pathogen is involved is the first step.  Once that
is accomplished, then you determine the treatment.  If you have been
applying a protectant fungicide, you will probably get very little
effect.  I suspect the application you have been making is just
wetting down the fungal mycelium, perhaps killing some of it, but it
will easily overcome such an application if it is living inside the
woody tissue.  A more long term strategy is to apply a systemic
fungicide which will enter into the leaves an move throughout the
plant.  Unfortunatley, not all systemic fungicides are registered for
every crop in all states.  And general use fungicides are usually
harder to pick up at your local garden store in the winter. Depending
upon the severity, pruning is aanother potential option.

Your best bet, contact your state extension service and submit a
sample.  They can diagnose it and give you a management
recommendation, usuallyfor less than $20.00.