Biology Reference
In-Depth Information
Fruit produced on infected twigs is stunted
or
Control
Sulfur dusts have been standard treat-
ment for garden roses for many years; to be
effective dusting must be started at the first sign
of mildew, before the mycelium gets too thick.
Sulfur may be injurious to roses in very hot
weather. Choice of variety of rose is important.
Shiny-leaved climbers like Dr. Van Fleet seldom
have mildew, and the shrub polyantha, The Fairy,
is very resistant. Many red roses, hybrid teas and
fluoribundas, are especially susceptible, but the
orange-red floribunda Spartan remains free from
it (in my own experience). Garden planning
avoids a lot of mildew trouble. Keep the plants
well spaced, in beds away from buildings, and not
surrounded by tall hedges or walls.
Podosphaera tridactyla Recently reported on
almond in California and the most common mil-
dew on apricot, causing large nonnecrotic lesions
on leaves.
russeted.
Jonathan variety is especially
susceptible.
Podosphaera clandestina (
P. clandestina
var.
tridactyla
).
Powdery Mildew
of cherry, occa-
sional on plum, peach, apricot, apple, pear, quince,
hawthorn, serviceberry, spirea. Budded sour
cherry is most severely attacked, but the disease
is seldom serious except on nursery stock. Young
leaves and twigs are covered with a white myce-
lium and powdery spores. Leaves are curled
upward; terminal leaves are smaller; twig growth
is stunted. Sulfur sprays or dusts will control.
Podosphaera pannosa (formerly
Sphaerotheca
pannosa
var.
rosae
).
Rose Mildew
, general on
rose; distinct from peach mildew but apparently
not confined to rose, since apricots growing near
roses have been infected. More than one strain
may be involved. Rose mildew is found wherever
roses grow. Always a problem with greenhouse
roses, it was enhanced when aerosol treatments
for red spiders and other pests were substituted
for old-fashioned syringing. Mildew increased in
garden roses when ferbam and other new
organics replaced the old sulfur and copper in
the blackspot sprays. Rose mildew is omnipresent
along the Pacific Coast and is serious in the semi-
arid Southwest. In the East, it appears on small-
flowered ramblers such as Dorothy Perkins and
Crimson Rambler in May, and may be quite seri-
ous on hybrid teas and some floribundas in late
summer, with the advent of cool nights.
The first symptom may be a slight curling of
leaves, with the mycelial growth such a light and
evanescent weft as to be almost unnoticed. Later
the white coating is conspicuous from the chains of
conidia produced lavishly over the surface. The
coating may cover buds, resulting in no bloom or
distorted flowers. Leaves often have a reddish or
purplish cast under the white mycelium and some-
times turn black. They may be slightly blistered.
On canes, the growth is heavier and more felty,
especially near thorns. Toward the end of the sea-
son perithecia may be found on canes, but they are
not common, and I have not seen them on leaves
except on a Rugosa rose at Ithaca, New York.
Mildew is prevalent on soft succulent shoots, fos-
tered by an excess of nitrogen.
Sphaerotheca
Appendages simple, flexuous, resembling
hyphae; only 1 ascus in a perithecium.
Cystotheca lanestris (formerly
Sphaerotheca
lanestris
).
Powdery Mildew
of coast live oak on
Quercus agrifolia
in California, reported also on
white, southern red, bur and post oaks. The dis-
ease is most destructive in the narrow coastal
plain. The most conspicuous symptom is
a powdery white, stunted growth developing
from certain terminal or lateral buds. The shoots
are swollen, fleshy, with much shortened inter-
nodes. Foliage on such shoots is often reduced to
pale yellow, bractlike leaves, which turn brown,
dry, and shrivel; these shoots resembles witches'
broom. On leaves developing from normal buds
and shoots, the fungus forms a dense layer on
both surfaces, more abundant on the lower side.
This species is sometimes called the brown mil-
dew because the grayish-white mycelium
changes to tan and then brown with age. Perithe-
cia are formed in the brown felt, abundantly in
some years, rarely in others. In southern Califor-
nia the fungus may winter in the conidial state,
with widespread leaf and shoot infections coming
from wind-borne spores.
