Principles of Plant Pathology prep for test 2

Oomycetes

• Egg “fungi” not fungi

• Absorb their food

• “Water molds”

• Multicellular organisms

• Asexual and sexual spores

Stramenopiles

Clade of oomycetes (more similar to algae than fungi)

Glucans Cellulous

Composition of Oomycete cell wall

Mycelium

Mass of strands (hyphae)

Septate hyphae

Classification of most fungi

Coenocytic (nonseptate) hyphae

Classification of most oomycetes

Hypha

Tubular filament

Chitin

Cell wall structure of fungi

Sporangia

Asexual specialized structures used to produce and release spores (oomycetes)

Zoospores

A type of asexual spore that can swim because they are equipped with flagella (oomycete)

Motile spores

Characteristic of oomycete

Oogonium

Female sexual spore structure (oomycete)

Antheridium

Male sexual structure (oomycete)

Gametangia

Oomycete sexual structures as a whole

Oospores

Sexual oomycete spores

Homothallic

Self-fertilization

Heterothallic

Outcrossing

Oospores - survival structures

• Thick cell wall

• Can remain dormant for long time

• Withstand wide range of environments

• Under right conditions, oospores germinate and initiate the disease cycle

Sporangium

Terminal structure of sporangiophore (oomycete)

• Lemon shaped in Phtophthora

Oomycetes diversity

Pythium, Peronospora, Plasmopara, Phtyophthora, Aphanomyces

Zoospore chemotaxis

Encystment = attachment

Haustoria = intercellular penetration structure of oomycete to plant cell

Pythium infection strategy

Attacks plants when most vulnerable

Oomycetes broadly

• Not true fungi

• Belong to Stramenopila

• Aseptate, multinucleate hyphae

• Cellulose/glucan cell walls

• Primarily diploid (2N) vegetative stage

Oomycete reproduction

• Asexually via sporangia and motile zoospores with two flagella

• Sexually via oogonium + antheridium = oospore

  • Oospores are long term survival structure

Oomycete diversity

Very diverse with important plant pathogens in several different orders

• Phytophthora, Pythium, Peronospora, Plasmopara, and Aphanomyces

Oospores

Thick-walled, dormant survival structures that persist in soil and germinate under favorable conditions, acting as major source of inoculum

Oomycete tissue and host specificity

Very broad. Can infect many different tissues with a wide host range

Blight

General & rapid killing of leaves, flowers, and stems

Phytophthora sporangium

Sporangiophores with lemon-shaped sporangia that release many motile zoospores

Sporangia produce

Sporangium

Oomycete sexual reproduction

Rare in nature

California Bay laurel

Foliar host of Sudden Oak Death. Plays role for primary inoculum. Fallen leaves important soil level source of inoculum

Downy mildew

Oomycetes. Signs on lower surface of leaves.

• Symptoms: distortion, chlorosis, necrosis

• Prefer cooler temps and humidity above 85%, spore production and infection require standing water

Plasmopara viticula

• Casual agent of downy mildew of grape

• Defoliation, low quality to entirely destroyed grapes, weakens/kills new shoots

• From North America. When introduced to Europe caused big problems

Bordeaux mixture

First used to control downy mildew of grapes. First widely used chemical to control plant diseases

General characteristics of fungi

• Non-photosynthetic (heterotrophs)

• Most are saprobes (feed on dead things)

• Cell wall made of chitin

• Genetically more similar to animals than plants

• Unicellular (yeast) or multicellular (mold)

Filamentous fungi

Multicellular

Hypha

Elongation of apical cell produces a tubular, thread-like structure called hypha

• Usually septate (with divisions) can be non-septate

Mycelium

Tangled mass of hyphae. Fungi producing mycelia are called molds or filamentous fungi

• Main vegetative structures of fungi

Generic fungus life cycle

Spore —> spore germination —> germ tube —> hyphal growth (vegetative part of fungus) —> spore production

Ascomycetes

True fungi

• Ascus = sac

• Ascomycetes = sac fungi

Oomycetes

• Mycelia is non-septate

• Diploid (2N) organisms during most of their life cycle

• Cell wall mainly composed of beta-glucans and cellulose

• Asexual spore is motile due to the presence of two flagella

• Sexual spore known as Oospore by the fertilization of an oosphere by the antheridia

Fungi

• Fungal hyphae are septate, few members lack septa

• Haploid or dikaryotic (N + N) organisms during most of their life cycle

• Cell wall composed of chitin

• Majority of asexual spores are not motile

• Sexual reproduction results in formation of zygo, asco, or basidiospores

Reproduction in Ascomycetes

May produce both sexual and asexual reproductive spores

• Spores and structures containing spores are important identification structures

Conidia

Asexual spores. Generated in conidiophores

Ascospores

Ascomycetes sexual spores. Survival structures