IB 371 – GENERAL MYCOLOGY Lecture 14 Tuesday, October 14, 2003

1. IB 371 – GENERAL MYCOLOGYLecture 14Tuesday, 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. 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