PLNT2011: Fungal and Oomycete Pathogens Summary

Fungal and Oomycete Pathogens

Introduction

• Dr. Anthony Young, Snr. Lecturer in Crop Protection, School of Agriculture & Food Sciences.
• Contact: anthony.young@uq.edu.au, Room 230, 8117a.
• Focuses on Fungal and Oomycete Pathogens.

Classification

• Diverse groups including:
  • Basidiomycetes
  • Ascomycetes
  • Zygomycetes
  • Chytridiomycetes
  • Oomycota
• Also includes related kingdoms and organisms:
  • Kingdom Animalia, Kingdom Plantae
  • Diatoms, Brown Algae, Red Algae
  • Cellular Slime Molds (Acraslomycetes), Plasmodial Slime Molds (Myxomycetes)
  • Giardia, Bacteria, Archaea

What is a Fungus?

• Typically filamentous but can be unicellular (yeasts).
• Heterotrophic with chitin in the cell wall.
• Often forms relationships with autotrophs.
• The largest organism on Earth is a fungus (~10 km^2, Oregon, USA).
• Can form anastomoses.
• Great decomposers.
• Reproduce via spores; fungi ‘fly’, bacteria hitch-hike.

Evolution of Fungi

• Likely evolved over a billion years ago.
• Fungi facilitated the terrestrial invasion of plants.
• Without fungi, there is no plant life on Earth.

Broad Groups of Fungi

• ‘Zygomycota’:
  • Examples: Mucor and Rhizopus.
• Ascomycota:
  • Examples: Alternaria, Penicillium, Rhizoctonia.
• Basidiomycota:
  • Highly specialised, e.g., rusts, smuts, ‘mushrooms’, Phellinus, Ganoderma.

Fungal Taxonomy

• Based on fruiting bodies.
• Different stages have different spore types, e.g., conidia, teleospores, pycnidia.
• Teleomorph = sexual.
• Anamorph = asexual (Deuteromycota or fungi imperfecta).

Anamorph and Teleomorph Examples

• Colletotrichum (Anamorph) - Glomerella (Teleomorph)
• Fusarium (Anamorph) - Nectria/Gibberella/Cyanonectria/Cosmospora/Albonectria/Haematonectria (Teleomorph)

Plant Pathogens: Rusts

• Fully obligate plant pathogens (Basidiomycetes).
• Often have multiple hosts for different life stages (heteroecious).
• Sometimes just one host (autoecious).
• Used as biological control agents.
• Examples: myrtle rust, frangipani rust, wheat rust, Agathis rust (Aecidium).

Plant Pathogens: Smuts

• Almost totally obligate plant pathogens (can form a culturable yeast stage).
• Basidiomycetes.
• Mainly Graminaceous hosts (grasses).
• Highly host-specific.
• Move quickly on wind currents.
• Hijack host’s reproductive system.
• Examples: sugarcane smut, couch smut, boil smut.

Smut Spore Production Example

• An example from an inspection of ratoon Q205 with a smut rating of 9 stated:
• Estimated thousands of smut whips in the field.
• Each smut whip produces billions of spores each day for about 3 months.
• Calculation:
  • 1,000 whips x 1 billion spores/day x 90 days = 9 x 10^{13} spores.
  • 90,000,000,000,000 or ninety trillion spores.
• Comparison: More than 200 times the number of spores from this field than there are stars in the Milky Way (approximately 400 billion).

Wood-rotting Basidiomycetes

• Ganoderma and Phellinus can be major pests in plantations.
• Invade woody tissues and ultimately strangle the host.
• Reward for solving the Phellinus riddle for oil palm.

Plant Pathogens: Ascomycetes

• Most commonly encountered plant pathogens.
• Contain many opportunistic and spoilage pathogens (e.g., Penicillium).
• Contain some of the most important (e.g., TR4 Fusarium).
• Many leaf spot pathogens:
  • Mycosphaerella/Cercospora
  • Alternaria
  • Colletotrichum/Glomerella
  • Cladosporium
  • Bipolaris

Powdery Mildews (Ascomycota)

• Obligate biotrophs: cannot be cultured.
• Feed via haustorium.
• Evolved in Laurasia: new Australian acquisitions.

Oomycetes

• Previously called ‘water molds’.
• Filamentous like fungi but not fungi at all.
• Cell walls contain cellulose.
• Examples include Pythium, Downy mildews, Phytophthora, Phytopythium.

Slime Moulds

• Not fungi.
• Best known pathogen: Brassica club root.

Diagnosis of Fungi

• Mainly molecular.
• Spore trapping and sentinel plants can assist.

Fungus and Oomycete Control

• Cultural
• Host resistance
• Chemical: contact/systemic
• Biological