Combination sprays or dusts to take care of black
spot along with insect pests are perfectly satisfactory so long as they are
geared to your climate. Strong copper sprays are injurious in cold weather,
and sulfur may burn in hot weather. Fermate is seldom injurious, and
it is effective, but in some formulations it is quite disfiguring to blooms.
A three-in-one spray (Tri-ogen) based on ammoniacal copper but containing some Fermate gives reasonably good black spot control, does not
discolor blooms, is safe to use through summer heat, and controls mildew
as well as black spot. Promising new chemicals are Manzate and captan;
Zerlate, Dithane Z-78, or Parzate are sometimes used. Avoid overhead
watering late in the day; use a mulch of buckwheat hulls or other material
to form a barrier between old leaves on ground and young foliage overhead.
BLACK SPOT OF DELPHINIUM (Pseudomonas delphinii).
black, irregular, tarry spots on leaves and stems, occasionally flower
buds. A common but seldom serious disease, it can usually be sufficiently
controlled by removing diseased leaves as noticed and cutting and burning
old stalks in fall. In a rainy season copper sprays can be used.
CHERRY LEAF SPOT or BLIGHT (Higginsia hiemalis)
also called shot hole.
Leaf spots are circular, first purple, then brown, with centers falling
out in a shot-hole effect. Leaves often turn yellow and drop, this premature defoliation reducing next year's harvest.
Control. Usually 4 spray applications are needed-at petal fall, shuck,
10 days after husks split on young fruit, and after harvest. Lime-sulfur
at a 1 to 50 dilution is often used for the early sprays and weak bordeaux
mixture, or other copper compound, for the last two sprays. Other materials, including glyoxalidine compounds, are promising.
IRIS LEAF SPOT (Didymellina tnacrospora).
Although widespread and
conspicuous on both German and bulbous iris, this disease is not of
great consequence except in very wet seasons. The spots are tan, oval,
1/4 to 1/22 inch long, with reddish borders, most often on the upper half of
Control. Cleaning up all old foliage at the end of the season is usually
sufficient. Sometimes it is wise to shear back the spotted leaves in midsummer and perhaps to spray with Fermate or bordeaux mixture.
MOUNTAIN-LAUREL LEAF SPOT (Phyllosticta kalmicola = Mycosphaerella
The spots are circular, grayish to silvery, with red or purple borders and prominent black pycnidia in the center. The disease is
much worse in shady locations where laurel is under drip of trees. Another leaf disease (Phomopsis kalmiae), going under the name of leaf
blight, has indefinite brown areas, without light centers, which grow
together to involve much of the leaf.