The first symptom is a slight curling of leaves, then a light white weft
of mycelium soon covered with the powdery spore chains. The buds may
not open or the flowers are misshapen. Leaves are sometimes somewhat
blistered under the white mycelium and have a reddish or purple cast,
becoming almost black. On canes there is a heavy felt near thorns. Perithecia are rare, the fungus wintering in buds or on canes. Mildew is
prevalent on succulent growth encouraged by excess nitrogen. Certain
floribunda varieties are particularly susceptible.
Control. Any copper or sulfur spray or dust started in time will control
mildew. When the coating is well established, it is easier to eradicate the
fungus with sprays which wet the felt than with dusts. A polysulfide
spray, Orthorix, similar to lime sulfur, is used by many West Coast gardeners. Most of the carbamate sprays-Fermate, Dithane, Parzate, etc.-
do not control mildew, but the new Manzate may be more effective. My
personal preference is still for the ammoniacal copper in Tri-ogen because
it is so very safe in hot weather as well as being very effective. Sulfur
often burns when temperatures are above 85°F., and strong coppers, as
in bordeaux mixture, injure in cool weather. Iscothan (renamed Mildex)
is a rather new chemical being used for rose mildew in greenhouses.
Actidione, an antibiotic, has possibilities.
Some varieties are relatively resistant to mildew. The Fairy, a polyantha
that becomes a large, spreading shrub, seldom requires spraying. Climbers
like Dr. Van Fleet, with shiny foliage, are mildew tolerant. The delightful pink floribunda Else Poulsen is rather subject to mildew, but Betty
Prior, a similar color, seldom gets the disease.
LIVE-OAK MILDEW (Sphaerotheca lanestris). A disease very destructive to
coast live oaks and some other oaks along the Coastal Plain in
California. Some shoots are stunted, covered with a white powdery
growth, some are swollen, fleshy, with foliage reduced to bract-like leaves
which brown and shrivel, giving a witches' broom effect. Leaves growing
from normal buds are covered with mycelium turning brown with age.
Control. A fairly new chemical-Isothan Q 15-has given promising
control. Do not prune heavily; this stimulates new succulent growth
susceptible to mildew.
Psyllids are jumping plant lice in the order Homoptera, family Chermidae. In western states the potato psyllid by its sucking produces a
disease of potatoes and tomatoes known as psyllid yellows.
BOXWOOD PSYLLID (Psylla buxi). Terminal leaves are cupped (Figure
43B), and young twig growth is checked, but this insect is not
nearly so important as the boxwood leaf miner. In spring soon after new
growth starts you can usually find, inside cupped and curled tips, small
gray or greenish nymphs covered with white cottony wax