EPWS 310 - PLANT PATHOLOGY
LECTURE 9: The Oomycetes continued
Content
of Lecture
Class: Oomycetes
Order:
Peronosporales
Family:
Peronosporaceae
Pages 427 - 433
Family:
Peronosporaceae (Different family from the Pythiaceae)
Differ from
Pythiaceae in being biotrophic and completely specialized to an aerial existence.
They are homothallic.
These are
foliage blights attacking tender green tissue such as leaves, buds, twigs and
fruit. They are highly specialized pathogens a logical step from the Phytophthora
infestans type of foliar blight.
Historically
very serious diseases - eg. The grape Downy Mildew story
Downy mildews
continue to be serious pathogens in humid parts of the world both tropical and
temperate. They are very serious greenhouse pathogens and can cause total
losses of young seedlings and seed plants.
There are many
genera and species of downy mildew but here are a few of the more common and
serious: See page 428.
Bremia
lactucae
- Downy Mildew of Lettuce
Peronospora destructor - Onion
Peronospora
tabacina
- Blue mold of Tabacco
Peronospora
trifoliorum
- Downy mildew of alfalfa and clover
Plasmopara
viticola
- Grape
Peronosclerospora - Sorghum, Maize and
Sugarcane
Pseudoperonospora - cucurbits and hops
Sclerospora
graminicola
- Downy mildew of grasses and millets
Downy mildews over
season as oospores and sporangia can germinate directly (at higher
temperatures) or indirectly at lower temperatures. Some downy mildews (eg Peronospora
ONLY germinate directly.
Common symptoms
include white downy mycelium on plant tissues, necrotic spots and lesions are
common. Distinguish from powdery mildews? Very characteristic sporangiophores
(cf P. infestans) make these about the easiest of all fungal diseases
to identify. Branching distinguishes the different genera.
Require Lots of
moisture...and cool to warm conditions (depending if temperate or tropical.
The classic
example is downy mildew of grape caused by Plasmopara viticola.
Dry areas are
usually free of the disease, which is why California and Australia even New
Mexico are good grape/wine areas.
Downy mildew
affects the leaves, fruit and herbaceous stems of grape causing at worst
defoliation and stunting of the vine.
DISEASE CYCLE 11-32
page 431
First observed
as small chlorotic spots on the top of leaves with a downy growth of sporangiophores
directly under these spots on the bottom of leaves. (Difference with Powdery
Mildews). These spots turn necrotic on older leaves and may result in complete
defoliation, the sporophores turn dark grey. The fruit and shoots may become
infected and covered with sporangiophores.
The pathogen is Plasmopara
viticola. It is biotrophic and grows intercellularly and sends haustoria
into cells. Sporangiophores come out of stomata on under surface of leaf in
humid weather. These sporangiophores are the basis of identification.
Overwinters as
Oospores in dead leaves and as mycelium in infected (not dead) shoots and
twigs.
Control?
ZYGOMYCETES
Covered on Pages
434 - 438.
Last of the
lower fungi only two pathogenic species - postharvest rots
Rhizopus - soft rot of fruit and
vegetables - postharvest...very, very common fungus a good saprophyte and
sporangiospores arrive at a wound and enter to rot the underlying tissue.
LIFE CYCLE
(Figure 11 - 37 page 437)
Choanephora - attack sensing floral
parts of squash and pumpkin.
Also Mucor
- Bread mold
Glomus and Gigaspora
are zygomycetes which form VAM. Only five plant families are known which DO NOT
have mycorrhizal associations.
Sum up the lower
fungi
Show - fruiting
bodies and overview.