Lecture 6: Oomycetes Reading: pages 409 - 433

Contents of Lecture

Order Saprolegniales

Order Peronsporales


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.

- Order Saprolegniales

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).

- Order Peronsporales

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



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