EPWS 310 - PLANT PATHOLOGY
Lecture 6: Oomycetes Reading: pages 409 - 433
Contents
of Lecture
Class OOMYCETES
Major group of
Plant Pathogens - "Water Molds"
Includes Pythium
and Phytophthora and Downy Mildews.
Anisogamous
gametes (ie oogonium and antheridium) is the primary difference between the
Oomycetes and the Gymnomycota (Myxomycetes) and Chytridiomycetes.
The Oomycetes
are mostly zoosporic, and have coenocytic mycelia, The mycelium is well
developed and resting sexual spores are oospores.
These fungi can
be homothallic and heterothallic
(up to
now all the lower fungi have been homothallic).
Homothallic
means that the same individual can produce sexual oospores.
Heterothallic
means that two different compatible thalli are needed for the development of
oospores. One can be thought of as female and the other as male. (Based on
contribution of mitchondria)
Asexual
reproduction is by zoospores and sporangia.
Note that lower
fungi are the only fungi with sporangia and zoospores.
Zoospores are
biflagellate.
Cell walls
composition?.
This separates
them from all the other fungi –
Diseases caused
by Oomycetes fall into two groups: causes by fungi in two orders - Saprolegniales
and Peronsporales
Reading Pages 409
- 427 (up to downy mildews)
1) Root rots and
damping off and rots of fruit in contact with the soil
2) Diseases of
the foliar parts of plants.
The only
pathogen in this group is Aphanomyces vegetable root rot of Peas, Sugar
beets.
As with all of
these fungi favoured by wet conditions. Motile zoospores and cause watery root
rots of seedlings and mature plants. Overseason as oospores and secondary cycle
by zoospores. Very similar to Pythium and Phytophthora life
cycle. Control and disases are also very similar.
Tubular
zoosporangia and zoospores can are primary (pyriform with two whiplash
flagella) and then secondary (reniform biflagellate) after a period of
encystment. In Aphanomyces there are primary globose spore which then
encysts and then give rise to a secondary zoospore (reniform type).
These are
serious plant pathogens!.
Pythium and Phytopthora
root rots and blights, Downy mildews and white rusts.
Both biotrophs
and necrotrophs
Both homothallic
and heterothallic, - sexual reproduction is by gametangial contact in all
Peronsporales.
Introducing Pythium
and Phytophthora
Peronsporales
cause Root rots, damping off, fruit rots, collar rots, stem rot, leaf blights,
twig blights, downy mildews
We will deal
with these by the type of disease that is caused because this is the easiest
way to understand these pathogens.
1) Damping off
disease
2) Root rot
diseases
3) Phytophthora
Foot and collar rots
4) Phytophthora
Leaf and stem blights
5) The Downy
Mildews
1) DAMPING OFF