) shows leaves of two
rhododendrons planted on the western foundation of my home outside Boston.
The plants have had these rust-colored spots since we bought the house six
years ago. The plants get strong afternoon sun and probably not much water
from the sprinkler system - is this heat damage or a disease? The plants
have flowered every year, but don't have many leaves.
Thanks in advance for any help.
Light green or yellowish patches on leaves sometimes accompanies by
brown spots on the back side of leaves is a sign of rhododendron
powdery mildew (Microsphaera azaleae) . One of the puzzling aspects of
this fungal problem is the fact that two different affected
rhododendrons vary in appearance. Rhodendron cultivar 'Unique,' for
instance, shows almost no upper leaf changes, other than occasional very
faint lighter yellowish areas, while the underside of the leaves will be
completely covered in brown spots. A deep green leaf may begin to show
lighter green patches, and these areas will gradually become more
yellow. Another cultivar, 'Virginia Richards,' gets brownish purple
spots on both tops and bottoms of leaves . This common disease is named
rhododendron powdery mildew despite how little the symptoms resemble the
familiar fungal disease often seen on roses and azaleas. Usually the
disease doesn't produce the familiar white powder-like spores, although
late in the summer some may become visible. The disease manifests
instead as color changes in the leaves, followed by defoliation toward
the end of the growing season. Many rhododendrons, if basically
healthy, will coexist with the disease and seem to outgrow or at least
survive the symptoms. Last year's leaves, once they have been hit by the
disease, will always have it, with symptoms persisting from year to year
until the leaves drop off. High relative humidity at night and low
relative humidity during day with 70-80 F (22-27 C) temperatures is
ideal for the disease to flourish.
Keep rhododendrons healthy to help them manage this problem. If you
notice symptoms on last year's leaves, consider protecting the new
growth with a fungicide . Apply it now to the new growth as it expands,
before symptoms appear on this spring's leaves. Fungicides won't get rid
of the existing problem on old leaves. A new fungicide, 'Remedy,' which
is a potassium bicarbonate (made by Bonide Company), is registered for
the problem. Thorough leaf coverage is necessary with all fungicides.
Fungicides containing sulfur (such as Safer Garden Fungicide RTU) are
also registered. Others are Funginex (sold as Ortho RosePride Funginex
Rose and Shrub Disease Control Concentrate.) Be very careful to read all
label instructions, and wear protective goggles and gloves. Funginex can
be corrosive to eyes. For more information see the section above on
'azalea powdery mildew'. The symptoms are different, but the organism
and control are the same.
Rhododendrons that have been hybridized with Rhododendron cinnabarinum
as one of the parent plants do get the disease severely. Two of those
are 'Elizabeth' and 'Lady Chamberlain.' The Cornish Cross hybrids,
including 'Virginia Richards,' 'Seta' and 'Mrs. G.W. Leak' seem
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