Powdery Mildews - Westcott's Plant Disease - page 274

Uncinula circinata On maples, Virginia
creeper, western soapberry.
Uncinula clintonii General on American linden.
Uncinula flexuosa (see Uncinuliella flexuosa ).
Horse-Chestnut Powdery Mildew ,on Aesculus
spp., including red, yellow and Ohio buckeye,
widespread in central and eastern states.
Uncinula macrospora General on American
and winged elms.
Uncinula necator Grape Powdery Mildew ,
general on grapes, also on Ampelopsis ; common
in late summer on eastern grapes but not serious;
a major problem in California. Leaves, canes and
young fruits are covered with white patches;
growth is often distorted. Late in the season the
white mycelium disappears and the spots appear
brown or black; berries are russeted or scurfy,
failing to mature.
Control Keep California grapes covered with
a light coating of sulfur dust. Apply when new
shoots are 6 to 8 inches long; when they are 12 to
16 inches, 14 days later; when shoots are 2 to
3 feet; when fruit is half. grown; when fruit
begins to ripen. If some of the applications are
omitted, and mildew gets a head start, wettable
sulfur is used as an eradicant spray. Karathane is
also effective. Copper sprays are often used in the
East, if any are necessary. Bicarbonate sprays are
excellent as protectant and eradicant.
Uncinula parvula; U. polychaeta Widespread
on hackberry and southern hackberry.
Uncinula prosopidis (see Pleochaeta prosopidis ).
On mesquite.
Uncinula salicis (see Uncinula adunca ). Willow
Powdery Mildew , also on pussy willow and pop-
lar, sometimes causing defoliation but not often
serious.
Uncinuliella australiana (not Erysiphe
lagerstroemiae as reported earlier in U. S.).
Crape-Myrtle Powdery Mildew on crape-myrtle
only, from Maryland to Florida and Texas, the
most serious disease of this shrub. The perithecia

have been found only in Florida, but presumably
it is the same species throughout the host range.
The disease appears on young shoots in early
spring, later infecting leaves and different parts
of the influorescence. Affected parts are covered
heavily with a white mealy to dusty growth;
young leaves are stunted, often less than one-
third normal size but abnormally thickened.
Internodes are short, flower stems stunted; buds
often fail to develop flowers. Infected portions
often have a reddish discoloration under the
white coating. Diseased leaves and buds drop in
a week or two, but stems may sprout again and
sometimes produce normal growth in hot
weather.
The fungus winters as mycelium in dormant
buds and in spring covers such buds with a dense
white coating of conidia, the source of primary
infection, which starts as small, circular white
patches on young leaves. Spores produced in
abundance on these patches account for rapid
spread of the disease until midsummer heat.
Control Spray with lime sulfur when buds burst
in spring and repeat 2 weeks later. If the initial
infection is not checked, spray wettable sulfur or
dust with sulfur.
Uncinuliella flexuosa (formerly Uncinula
flexuosa ). Horse-Chestnut Powdery Mildew ,on
Aesculus spp., including red, yellow and Ohio
buckeye, widespread in central and eastern states.
This mildew gives a very thin coating on the leaf
surface, supposedly mostly on the underside
although I have seen it on the upper. Perithecia
are numerous, small, barely discernible with the
naked eye. Control is usually unnecessary except
in nurseries. A copper spray used for blotch will
also control mildew.
Uncinula adunca (formerly Uncinula salicis ).
Willow Powdery Mildew , also on pussy willow
and poplar, sometimes causing defoliation but
not often serious. The growth is in diffused or
circumscribed patches on both leaf surfaces.