Could be winter burn, but it sure sounds like Phytopthora to me. The
coalescing of the spots is unlike the leaf-spot of Rhodo I see here.
--
Mike LaMana, MS
Heartwood Consulting Services, LLC
Toms River, NJ
www.HeartwoodConsulting.net
"Stephen M. Henning" wrote in message
news
Luca wrote:
Quite soon I saw that some of the leaves were developing brown spots.
The brown area always progresses from the edge (often the tip) of the
leaf inward. Eventually the all leaf turns brown. More affected by
this disease are the leaves at the base of the plant, and particulalry
the ones directly attached to the woody (older) stems.
I doubt if it is phytophera. If it was, there is nothing you could do.
If a leaf has brown areas with white spots, it probably has a local
fungal infection of Pestalotia leaf spot. This is seldom controlled with
fungicides and is best mitigated by good sanitation and avoiding
excessive moisture
Another possibility is rhododendron mildew. 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
vulnerable also.
--
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