1. IB 371 – GENERAL MYCOLOGY Lecture 14 Tuesday, October 14, 2003
OOMYCOTA
PYTHIALES, RHIPIDIALES

2. PYTHIALES Occur in aquatic, amphibious, and terrestrial habitats.
Thallus is of coenocytic mycelium - narrower and more delicate than in the Saprolegniales.
Septa delimit reproductive structures and may occur in older hyphae.
Walls generally lack chitin and are mostly glucan with partially crystalline cellulose-like material and protein.

3. PYTHIALES
Asexual reproduction is by zoospores formed in sporangia. Sporangia may produce biflagellate zoospores which encyst and form a germ tube or germinate directly to form a germ tube that develops into a mycelium.
Zoospores are biflagellate and secondary.

4. PYTHIALES
Sexual reproduction is oogamous. Each oogonium contains a single egg (except Pythium multisporem).
Antheridia and oogonia are multinucleate, but only 1 egg and 1 sperm (nucleus) fuse or if more than 1 pair fuse, only 1 oospore is formed.
Meiosis is gametangial and the organism is diploid throughout its life cycle.
The cytoplasm remaining after oosphere cleavage is called periplasm. The periplasm may be deposited around the zoospore to give it a distinctive appearance.

5. PYTHIALES
May be saprophytic or parasitic, but plant parasitic species do not form haustoria.
+/- ten genera
The two genera with the greatest number of species and which have the greatest economic and ecological importance are Pythium and Phytopthora.

6. PYTHIUM Common in soil, exists as a saprophyte
Causes damping off of seedlings growing in wet or very poorly drained soils
Sporangia are not well-differentiated and in some species are hypha-like
Usually sporangia form a vesicle into which the cytoplasmic contents are discharged
Zoospores are differentiated in the vesicle and then swim away

7. PYTHIUM
Zoospores swim around for a time, throw off their flagella, encyst, and then form a germ tube.

8. From Introductory Mycology by John Webster

9. PYTHIUM Most species are homothallic
Oogonia and antheridia are multinucleate
Most of the nuclei in the oogonia degenerate while a few enlarge & undergo meiosis
Antheridia are multinucleate but all degenerate except one which undergoes meiosis
A single oogonial nucleus fuses with a single antheridial nucleus to form an oospore

10. PYTHIUM The remaining nuclei & cytoplasm forms the periplasm
Oospores must have a period of rest before they germinate
Oospores germinate either by forming a germ tube or a vesicle which cleaves into zoospores

11. Sporangia of Pythium aphanidermatum (From D.S. Barr)

12. From Introductory Mycology by John Webster

13. Oogonia & Antheridia of Pythim aphanidermatum (From D. S. Barr)

14. Oogonium & Antheridium of Pythium erythroseptica (From D.S. Barr)

17. PYTHIUM
Causes damping off of seedlings growing in wet or very poorly drained soils.
Symptoms are water soaking of stems and brown lesions on stems and eventually the seedling drops over.
The hyphal tips secrete pectolytic and cellulolytic enzymes that cause plant cells to separate from one another and this weakens the stems.

18. PHYTOPTHORA The PLANT DESTROYER P. infestans - solanaceous hosts
P. megasperma var. sojae - soybeans
P. cactorum - 40 families of flowering plants
P. erythroseptica - pink rot of potato tubers
P. fragariae - red core of strawberries
P. cinnamomi - 1,000 different hosts
P. palmivora - pod-rot & canker of cacao & many other plant hosts

21. Lower Fungi in the Laboratory
From:
Lower Fungi in the Laboratory

22. Zoosporangium of Phytopthora palmivora

27. From: Introductory Mycology by John Webster

28. Slide from B. Kendrick

29. DISEASE TRIANGLE Virulent pathogen Susceptible host
Favorable environment

30. PHYTOPTHORA RAMORUM
Introduced pathogen
Susceptable host

31. Tan oak killed by Phytopthora ramorum

32. Symptoms of Sudden Oak Death on bark of tan oak

33. Dr. David Rizzo – Investigator of Sudden Oak Death