Biology Reference
In-Depth Information
Powdery Mildews
Mildew is a disease in which the pathogen is seen
as a growth on the surface of plants. The same
word is used for the fungus causing the disease.
Mildews are Ascomycetes. Black mildews are
parasites in the order Meliolales with a dark
mycelium to give a sooty effect. They are com-
mon in the South or on tropical plants in green-
plant parasites in the order Erysiphales. They
have white mycelium, in a delicate weft or thick
felt, made up of a criss-cross tangle of hyphae.
Colorless spores borne in chains on upright
conidophores give the white powdery effect (see
Fig. 1
). False or downy mildews are oomycetes,
and the conspicuous growth is not vegetative
mycelium but fruiting structures and conidia pro-
truding through stomata or epidermis to give
a white frosty appearance in moist weather
(
inside the leaf but produces a tangle of septate
threads, hyphae, on the surface. Special sucking
organs, haustoria, penetrate the epidermal cells,
occasionally the subepidermal cells, in search of
food. The penetrating tube is slender, but, once
inside the cells, the haustorium becomes a round
or pear-shaped enlargement or a branched affair,
with greatly increased absorbing surface.
Condiophores, growing at right angles from the
mycelium, produce one-celled conidia in rows or
chains of somewhat barrel-shaped hyaline cells,
which become oval as they are dislodged from the
top of the chain and disseminated by wind. Mil-
dews known only in this anamorph state are called
by the form genus name
Oidium
.Itrequiresthe
sexual fruiting bodies, perithecia, to place mildews
in their proper genera.
Perithecia are round with a dark membranous
wall, technically cleistothecia because they have
no beak or ostiole, and rupture irregularly to free
the asci. They are held in place in the mycelium
by appendages. The form of these appendages
and the number of asci in the perithecium are
the chief characters differentiating the six genera
important in this country (see
Fig. 2
).
Sphaerotheca
and
Erysiphe
both have simple
appendages; but the former has only one ascus,
the latter several.
Podosphaera
has appendage
tips dichotomously branched and one ascus;
Microsphaera
has the same type of appendage
but several asci.
Phyllactinia
has lancelike
appendages swollen at the base; those of
Uncinula
are coiled at the tip. Both have more
than one ascus.
True powdery mildews - and in speaking of
them we usually eliminate the word
“powdery” - are widely distributed but some-
times more abundant in semiarid regions than in
areas of high rainfall, where other diseases flour-
ish. Unlike those of most other fungi, powdery
mildew spores do not require free water for ger-
mination. Some species require high humidity,
but it is usually provided at the leaf surface
when cold nights change to warm days or when
plants are grown in crowded, low, or shady loca-
tions without sufficient air circulation. Spores of
other species can germinate with very low humid-
ity. When a mildew spore lands on a leaf and puts
out its germ tube, it does not make its nearest way
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