Biology Reference
In-Depth Information
Downy Mildews
Downy mildews, sometimes called false mil-
dews, are Oomycetes, in the order Peronosporales
and all in the family Peronosporaceae except
Phytophthora
in the Pythiaceae. They form
mycelium in higher plants and produce sporan-
giophores that protrude through stomata in great
numbers, their sporangia making white, gray, or
violet patches on the leaves. The downy effect
distinguishes these mildews from the true or pow-
dery mildews that form white felty or powdery
patches.
The sporangiophores are often branched; they
bear a single sporangium at the tip of each branch
simultaneously, or successively in
Phytophthora
.
Sporangia germinate by swarm spores or with
a germ tube as a conidium. An oospore, resting
spores with external ridges or knobs, is formed in
an oogonium, large globular multinucleate
female cell, after it is fertilized by the antherid-
ium, a smaller male cell. The oospores are set free
by weathering and decay of host parts.
Basidiophora entospora
Downy Mildew
of
aster, China aster, goldenrod, and erigeron.
Aster losses are reported by commercial growers
in the South, but apparently this is not an impor-
tant garden problem.
Bremia
Oomycetes, Peronosporales
Dichotomous branching of sporangiophores; tips
enlarged into discs bordered with sterigmata
bearing sporangia; swarm spores rare; germina-
tion usually by a germ tube protruded through an
apical papilla (
Fig. 1
).
Bremia lactucae
Downy Mildew
of lettuce and
other composites, endive, cornflower, centaurea,
celtuce, escarole, romaine, and various weeds.
First noticed around Boston in 1875, the disease
is serious in greenhouses and in states where
outdoor winter crops are grown. Light green or
yellowish areas on upper surface of leaves are
matched by downy patches on the under surface.
Affected portions turn brown, and leaves die, the
older ones first. Entrance is through stomata. The
disease is worse in damp, foggy, cool weather
(43
to 53
F).
Control
The pathogen has numerous physiolog-
ical races so that lettuce varieties like Imperial 44
and Great Lakes that are resistant in some local-
ities may not be so in others. Avoid excessive
irrigation; eliminate crop residue and weeds.
Basidiophora
Oomycetes, Peronosporales
Sporangiophore a single trunk with a swollen
apex from which short branches grow out, each
bearing a nearly globose sporangium; germina-
tion by swarm spores; oospore wall not confluent
with that of oogonium. Mycelium is intercellular,
haustoria small, knoblike (
Fig. 1
).
