Biology Reference
In-Depth Information
Peronospora dianthicola
Carnation Downy
Mildew
, common in California on seedlings.
Leaves turn pale, curl downward; terminal
growth is checked, and plants may die. There is
a white growth on lower leaf surfaces.
Peronospora effusa
Spinach Downy Mildew
,
Chard Blue Mold
, found wherever spinach and
swiss chard are grown, absent some seasons,
nearly destroying the crop in others. Large pale
yellow spots grow together to cover all or part of
the leaf; lower leaves are infected first, and then
the blight is scattered through the plant. Gray to
violet mold forms on underside of leaves; some-
times the whole plant decays and dries. Initial
infection comes from oospores in the soil; it
requires humidity above 85 % and a mean tem-
perature between 45
and 65
F for a week. Sec-
ondary infection is from conidia. The fungus is an
obligate parasite and does not live over on hosts
other than spinach.
Control
Plant on well-drained, fertile ground;
do not crowd; if overhead irrigation is used,
water early on sunny days; practice a 2-to
3-year crop rotation. Resistant varieties such as
Califlay and Texas Early Hybrid 7 are being
introduced.
Peronospora farinosa (formerly
Peronospora
schactii
).
Beet Downy Mildew
, on beet, sugar
beet, and swiss chard. Inner leaves and seedstalks
are stunted and killed, covered with violet down.
The disease appears on the Pacific Coast during
the fall rainy season. Oospores can survive in the
soil several years.
Peronospora fragariae
Strawberry Downy
Mildew
.Peronospora grisea, on veronica, a
grayish mildew on underside of leaves.
Peronospora manshurica.
Soybean Downy
Mildew
, general. Yellow-green foliage spots
turn brown, with a grayish mold underneath;
there may be premature defoliation. The patho-
gen winters as mycelium in seed and oospores in
soil. There are at least three races.
Peronospora grisea on veronica, a grayish mil-
dew on underside of leaves.
Peronospora manshurica
Soybean Downy
Mildew
, general. Yellow-green foliage spots turn
brown, with a grayish mold underneath; there may
be premature defoliation. The pathogen winters as
mycelium in seed and oospores in soil. There are at
least three races.
Peronospora myosotidis
Forget-Me-Not
Downy Mildew
, also on lappula. Pale spots on
upper surface of leaves, with downy growth
underneath.
Peronospora oxybaphi, on sand verbena and
four-o'clock.
Peronospora parasitica,on garden cress;
P. leptosperma, on artemisia; P. linariae,on
linaria; P. lophanthi, on agastache.
Peronospora parasitica
Downy Mildew
of cru-
cifers, general on cabbage, Chinese cabbage,
broccoli, cauliflower, horseradish, radish, turnip,
cress, peppergrass, also on sweet alyssum, arabis,
arugula, stock, and hesperis. Chief damage is to
cabbage seedlings or plants grown for seed. Leaf
lesions are light green, then yellow, with downy
mold on both sides of the leaf in the widening
yellow zone but not in the dead, shrunken, gray or
tan central portion. Secondary fungi often cover
dead parts with a black sooty mold. Fleshy roots
of turnips and radishes may be discolored inter-
nally. Warm days and cool nights favor the dis-
ease. The pathogen lives between crops in
perennial plants or winter annuals. There are sev-
eral strains of
P. parasitica
; one, often reported as
P. matthiolae
, blights stock in greenhouse and
nursery. Leaves wilt; tender stems and flower
parts are stunted and dwarfed.
Control
Avoid crowding plants; keep foliage
dry. Spray cabbage seedlings; repeat two or
three times a week until plants are set in field.
Treat heading cabbage every 6 or 7 days begin-
ning 1 to 3 weeks before harvest.
Peronospora pisi
Pea Downy Mildew
. Water-
soaked tissue and white growth appear on any
aerial plant part. The mycelium winters in vetch
stems, fruiting there in spring, and spores are
disseminated back to peas. The disease is not
important enough for control measures.
Peronospora potentillae
Downy Mildew
of
agrimony and mock strawberry.
Peronospora radii
Downy Mildew
of Margue-
rite daisy,
Argyranthemum frutescens
(formerly
Chrysanthemum frutescens
).
Peronospora rubi
Downy Mildew
of black-
berry, dewberry, and black raspberry.
