Downy Mildews - Westcott's Plant Disease - page 173

Plasmopara viburni ViburnumDowny Mildew .
Plasmopara viticola Grape Downy Mildew ,
general on grape, also on Virginia Creeper and
Boston ivy. This is a native disease, endemic in
eastern United States, first observed in 1834 on
wild grapes. It appeared in France after 1870,
imported with American stock resistant to the
Phylloxera aphid, and in a few years had become
as ruinous to the wine industry of Europe as the
potato blight had been to Ireland. The efficacy of
bordeaux was first discovered in connection with
this mildew.
In this country downy mildew is most destruc-
tive on European varieties of grape. Pale yellow
spots, varying in form but often nearly circular
and somewhat transparent, appear on upper leaf
surfaces, and a conspicuous white coating
appears on lower surfaces. The spots turn brown
with age; in dry weather the downy growth is
scanty. Young canes, leafstocks, and tendrils
may be infected; flowers may blight or rot;
young fruits stop growing, turn dark, and dry
with a copious grayish growth. Older fruits have
a brown rot but lack the mildew effect. Fruits
from diseased vines have less juice; bunches are
very poorly filled.
Initial infection comes from a swarm spore
stopping on the lower side of a leaf, putting out
a germ tube and entering through a stoma. In 5 to
20 days the mycelium has spread through the leaf
between cells, obtaining food through thin-
walled, globular haustoria. The hyphae mass in
compact cushions just beneath the stomata; under
humid conditions a few grow out through the
openings and develop into branched conidio-
phores (sporangiophores). Each has three to six
main branches, and they branch again. The ter-
minal branches end in two to four short, slender
sterigmata, each of which produces a single
multinucleate spore. With moisture, each nucleus
with adjacent protoplasm is organized into
a swarm spore, motile with two cilia. They
swim around for a while, then settle down, absorb
their cilia, and put out a germ tube. If they happen
to be on the upper side of a leaf, nothing happens;
if on the lower surface, the germ tube may reach
a stoma and start an infection.

Toward the end of the growing season thick-
walled resting spores, oospores, are produced in
intercellular spaces of the infected leaves. These
are set free in spring by disintegration of host tissue,
are rain-splashed to other vines, and germinate by
production of a short, unbranched hypha bearing
a single large sporangium, to start the cycle anew.
Control Copper sprays are effective. Apply bor-
deaux mixture immediately before and just after
blooming; repeat 7 to 10 days later and possibly
when fruit is half grown. Destroy fallen leaves by
burning.

Pseudoperonospora

Oomycetes, Peronosporales

Like Plasmopara but with branches of sporan-
giophores forming more or less acute angles; tips
more acute.
Pseudoperonospora celtidis Downy Mildew of
hackberry.
Pseudoperonospora cubensis Downy Mildew
of cucurbits, destructive to cucumber, musk-
melon, and watermelon, particularly along the
Atlantic seaboard and the Gulf Coast, occasional
on gourd, pumpkin, and squash. The disease was
first noted in 1889 in New Jersey, and in 1896
destroyed most of the cucumbers on Long Island.
Irregular yellow spots appear on upper leaf sur-
faces, often on leaves nearest the center of the
hill. The lesion is brown on the opposite side,
covered with a purple growth in rain or dew.
The whole leaf may wither and die, with the
fruit dwarfed to nubbins and of poor flavor. The
fungus does not live in the soil and is not preva-
lent in the North until July or August. It winters in
greenhouses or comes up from the South by
degrees. Sporangia are spread by wind and
cucumber beetles. The disease is favored by
high humidity, but temperatures need not be as
cool as for other downy mildews.
Control Resistant cucumbers are of rather poor
quality. Cantaloupe varieties Texas Resistant
No. 1 and Georgia 47 combine resistance to
aphids with resistance to downy mildew.