|
Mountain
laurel (Kalmia latifolia) is prized as an ornamental shrub
not only for its exquisitely beautiful flowers but also for its
attractive evergreen foliage, which adds color to the landscape
year-around.
The
foliage of mountain-laurel can be infected by at least 14 different
species of fungi and suffers from an assortment of environmentally
caused leaf disorders. This report describes two of the most common
fungal leaf diseases and two environmentally caused leaf disorders.
|
Leaf
Spot
Leaf spot, caused by the fungus Mycosphaerella colorata
(Phyllosticta kalmicola), is characterized by round
to irregular, greyish white to silvery spots, 1/8 to 1/2 inch
in diameter, with reddish to purple borders (Figure 1). Black
specks, the fruiting bodies (pycnidia) of the fungus, form
in the centers of older spots. Leaf spot is most severe under
crowded, shaded, and excessively moist conditions.
|
Click
on image for larger version
Figure 1. Leaf Spot
|
|
Click
on image for larger version
Figure 2. Leaf Blight
|
Leaf
Blight
Leaf blight, caused by the fungus Diaporthe kalmiae
(Phomopsis kalmiae), can be distinguished from leaf
spot by the larger, circular, brown lesions that often develop
a zonate pattern (Figure 2). The lesions frequently first
appear near the margin or tip of the leaf, gradually enlarge,
merge with other lesions, and result in the death of the entire
leaf blade. The fungus may also spread from the leaf through
the petiole into the twig, causing twig blight.
|
|
Leaf
Browning and Death
These symptoms are due to drought or winter injury. They differ
from those of leaf blight in that individual lesions and a
zonate pattern are not apparent (Figure 3).
|
Click
on image for larger version
Figure 3. Winter Injury
|
|
Click
on image for larger version
Figure 4. Iron Chlorosis (Connecticut Ag. Exp. Station,
New Haven photo)
|
Iron
Chlorosis
Iron chlorosis, caused by a lack of sufficient iron in the
plant, is typified by a mottling of the younger leaves; these
leaves later turn pale green to yellow except for the veins,
which remain green (Figure 4). Iron chlorosis is caused by
a lack of iron in the soil, or by a soil reaction above pH
6.0 that interferes with the uptake of iron by the roots.
|
Control
- Maintaining vigorous, healthy shrubs is of utmost importance.
Soil that is acid (pH 4.2 to 5.2), high in organic matter, and
well drained but moist is ideal for mountain-laurel. If the soil
reaction is alkaline, slightly acid, or neutral (above pH 5.5),
addition of sulfate as iron sulfate or ammonium sulfate to the
soil will lower the pH. Aluminum sulfate is not recommended. If
the soil reaction is proper and symptoms of iron chlorosis appear,
add iron to the soil either as iron sulfate or chelate. Follow
directions on the package label.
- Watering during dry periods in summer and fall and applying
an organic mulch will help prevent drought and winter injury.
Recommendations for reducing winter injury include protecting
bushes exposed to winter sun and wind by erecting burlap windbreaks,
or growing bushes in a more protected location.
- Dead or diseased leaves should be removed from the bushes and
soil surfaces and destroyed by composting or burning. If only
a light infection of leaf spot or leaf blight is detected, removal
and destruction of diseased leaves is often sufficient.
- Heavy or recurrent infections of leaf spot and leaf blight
can be controlled by applying the fungicide benomyl (Benlate)
50 percent wettable powder, at the rate of ½ pound of product
per 100 gallons of water (2 teaspoons per gallon). Because of
the waxy coating on the leaves, a spreader-sticker should be added.
A teaspoonful of good-quality liquid detergent per gallon of spray
mix is often useful in reducing surface tension and improving
foliage coverage. If leaf spot or blight was serious last year,
spray when new growth starts, and repeat applications at approximately
two-week intervals throughout the spring growing season. If the
weather is very humid or rainy, shorten the spray interval to
7 or 10 days.
For
further information contact Nancy R. Pataky, Extension Specialist
of Turf and Ornamentals, and Director of the Plant Disease Clinic,
Department of Crop Sciences, University of Illinois at Urbana.
|
