IB 371 – GENERAL MYCOLOGYIB 371 – GENERAL MYCOLOGYLecture 14Lecture 14

Tuesday, October 14, 2003Tuesday, October 14, 2003

OOMYCOTAOOMYCOTAPYTHIALES, RHIPIDIALESPYTHIALES, RHIPIDIALES

PYTHIALESPYTHIALES Occur in aquatic, amphibious, and Occur in aquatic, amphibious, and

terrestrial habitats.terrestrial habitats. Thallus is of coenocytic mycelium - Thallus is of coenocytic mycelium -

narrower and more delicate than in the narrower and more delicate than in the Saprolegniales.Saprolegniales.

Septa delimit reproductive structures Septa delimit reproductive structures and may occur in older hyphae.and may occur in older hyphae.

Walls generally lack chitin and are Walls generally lack chitin and are mostly glucan with partially crystalline mostly glucan with partially crystalline cellulose-like material and protein. cellulose-like material and protein.

PYTHIALESPYTHIALES Asexual reproduction is by zoospores Asexual reproduction is by zoospores

formed in sporangia. Sporangia may formed in sporangia. Sporangia may produce biflagellate zoospores which produce biflagellate zoospores which encyst and form a germ tube or encyst and form a germ tube or germinate directly to form a germ germinate directly to form a germ tube that develops into a mycelium.tube that develops into a mycelium.

Zoospores are biflagellate and Zoospores are biflagellate and secondary.secondary.

PYTHIALESPYTHIALES Sexual reproduction is Sexual reproduction is oogamousoogamous. Each . Each

oogonium contains a single egg (except oogonium contains a single egg (except Pythium multisporemPythium multisporem). ).

Antheridia and oogonia are multinucleate, Antheridia and oogonia are multinucleate, but only 1 egg and 1 sperm (nucleus) fuse or but only 1 egg and 1 sperm (nucleus) fuse or if more than 1 pair fuse, only 1 oospore is if more than 1 pair fuse, only 1 oospore is formed.formed.

MeiosisMeiosis is is gametangialgametangial and the organism is and the organism is diploid throughout its life cycle.diploid throughout its life cycle.

The cytoplasm remaining after oosphere The cytoplasm remaining after oosphere cleavage is called cleavage is called periplasmperiplasm. The periplasm . The periplasm may be deposited around the zoospore to may be deposited around the zoospore to give it a distinctive appearance.give it a distinctive appearance.

PYTHIALESPYTHIALES

May be saprophytic or parasitic, but May be saprophytic or parasitic, but plant parasitic species do not form plant parasitic species do not form haustoria.haustoria.

+/- ten genera+/- ten genera The two genera with the greatest The two genera with the greatest

number of species and which have the number of species and which have the greatest economic and ecological greatest economic and ecological importance are importance are PythiumPythium and and PhytopthoraPhytopthora..

PYTHIUMPYTHIUM Common in soil, exists as a saprophyteCommon in soil, exists as a saprophyte Causes damping off of seedlings Causes damping off of seedlings

growing in wet or very poorly drained growing in wet or very poorly drained soilssoils

Sporangia are not well-differentiated Sporangia are not well-differentiated and in some species are hypha-likeand in some species are hypha-like

Usually sporangia form a vesicle into Usually sporangia form a vesicle into which the cytoplasmic contents are which the cytoplasmic contents are dischargeddischarged

Zoospores are differentiated in the Zoospores are differentiated in the vesicle and then swim awayvesicle and then swim away

PYTHIUMPYTHIUM

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

From From Introductory Introductory Mycology by Mycology by John WebsterJohn Webster

PYTHIUMPYTHIUM Most species are homothallicMost species are homothallic Oogonia and antheridia are multinucleateOogonia and antheridia are multinucleate Most of the nuclei in the oogonia Most of the nuclei in the oogonia

degenerate while a few enlarge & undergo degenerate while a few enlarge & undergo meiosismeiosis

Antheridia are multinucleate but all Antheridia are multinucleate but all degenerate except one which undergoes degenerate except one which undergoes meiosismeiosis

A single oogonial nucleus fuses with a single A single oogonial nucleus fuses with a single antheridial nucleus to form an oosporeantheridial nucleus to form an oospore

PYTHIUMPYTHIUM The remaining nuclei & cytoplasm forms The remaining nuclei & cytoplasm forms

the periplasmthe periplasm Oospores must have a period of rest Oospores must have a period of rest

before they germinatebefore they germinate Oospores germinate either by forming a Oospores germinate either by forming a

germ tube or a vesicle which cleaves into germ tube or a vesicle which cleaves into zoospores zoospores

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

From Introductory Mycology by John Webster

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

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

PYTHIUMPYTHIUM Causes damping off of seedlings growing in Causes damping off of seedlings growing in

wet or very poorly drained soils.wet or very poorly drained soils. Symptoms are water soaking of stems and Symptoms are water soaking of stems and

brown lesions on stems and eventually the brown lesions on stems and eventually the seedling drops over.seedling drops over.

The hyphal tips secrete pectolytic and The hyphal tips secrete pectolytic and cellulolytic enzymes that cause plant cells cellulolytic enzymes that cause plant cells to separate from one another and this to separate from one another and this weakens the stems.weakens the stems.

PHYTOPTHORAPHYTOPTHORA The PLANT DESTROYERThe PLANT DESTROYER P. infestansP. infestans - solanaceous hosts - solanaceous hosts P. megaspermaP. megasperma var. var. sojaesojae - soybeans - soybeans P. cactorumP. cactorum - 40 families of flowering - 40 families of flowering

plantsplants P. erythrosepticaP. erythroseptica - pink rot of potato - pink rot of potato

tuberstubers P. fragariaeP. fragariae - red core of strawberries - red core of strawberries P. cinnamomiP. cinnamomi - 1,000 different hosts - 1,000 different hosts P. palmivoraP. palmivora - pod-rot & canker of cacao - pod-rot & canker of cacao

& many other plant hosts& many other plant hosts

From:From:

Lower Fungi Lower Fungi in the in the

LaboratoryLaboratory

Zoosporangium of Phytopthora

palmivora

From: From: Introductory Introductory MycologyMycology by by John WebsterJohn Webster

Slide from B. KendrickSlide from B. Kendrick

DISEASE TRIANGLEDISEASE TRIANGLE

Virulent pathogenVirulent pathogen Susceptible hostSusceptible host Favorable Favorable

environmentenvironment

PHYTOPTHORA PHYTOPTHORA RAMORUMRAMORUM

Introduced Introduced pathogen pathogen

Susceptable hostSusceptable host

Tan oak Tan oak killed by killed by

Phytopthora Phytopthora ramorumramorum

Symptoms of Symptoms of Sudden Oak Sudden Oak

Death on bark Death on bark of tan oakof tan oak

Dr. David Dr. David Rizzo – Rizzo –

Investigator Investigator of Sudden of Sudden Oak DeathOak Death