EPWS 310 - PLANT PATHOLOGY

FUNGI LECTURE 2

INTRODUCTION TO FUNGAL DISEASES AND FUNGAL CLASSIFICATION

See Classification, Fungal Relationships and Cell Wall handout

Contents of Lecture

B. Classification of Fungi

C. Introduction to Fungal Diseases

C1. Myxomycetes

C2. The Chytridiomycetes and Plasmodiophoromycetes

C3. Oomycetes

C4. Zygomycetes

C5. Ascomycetes

C6. Basidiomycetes

D. Relationships between fungi

E. Ecological roles of fungi

B. CLASSIFICATION OF FUNGI

- See pages 248 to 254 in Agrios. The first thing to note is that plant pathologists only have to deal with about 25 orders of fungi out of the 60 or so which exist.

C. INTRODUCTION TO FUNGAL DISEASES

C1. Myxomycetes

Slime Moulds; Mycelium?; Cell walls?;

Vegetative body consists of plasmodium (ameobae-like); Spores germinate to form zoospores; typical species Physarum

C2. The Chytridiomycetes and Plasmodiophoromycetes

Thallus type??; cell walls??  cause root rots and hypertrophic /hyperplasia disorders of roots; vectors of viruses; not major economic pests; Typical species: Plasmodiophora brassicae - Club root of crucifers. Olpidium.

C3. Oomycetes

Water molds; very important plant pathogens; unrelated to the rest of the fungi; coenocytic or aseptate mycelium; produce zoospores and sexual resting spores called oospores; cell wall composition??. Typical plant pathogens are Pythium and Phytophthora.

C4. Zygomycetes

Minor plant pathogens; mycelium type??; Not related to Oomycetes; Cell wall composition ??; Produce sporangiospores and sexual resting zygospores; Haploid in somatic phase; Mostly cause postharvest rots; Typical pathogens include Rhizopus and Mucor.

C5. Ascomycetes

An enormous and extremely important group of plant pathogens; Septate hyphae; Cell walls consist of Chitin--Glucan and Mannans; Reproduce by a dazzling array of asexual spores called conidia and sexual spores called ascospores; Ascospores may be produce naked, in cleistothecia, in perithecia and in apothecia; Ascospores are produced inside a sac-like ascus; Haploid in somatic phase;

These fungi are classified on the sole basis of their asexual conidia; so the same fungus may have two names, one for sexual state (Teleomorph) and the other in the asexual form (Anamorph). Conidia may be produced in many ways but the most important are: Naked, in sporodochia, in acervuli, and in pycnidia; Some species produce no spores at all; Cell walls consist of Chitin--Glucan and Mannans;

C6. Basidiomycetes

This group of fungi is the best known as the mushrooms; They are an important group of plant pathogens and culinary delights; Septate hyphae; Reproduce mainly by sexual basidiospores which are bone on the outside of a club-shaped basidium; They exist as haploid monokaryons and dikaryons in somatic phase.

D. Relationships between fungi

Many of the groups of fungi can be distinguished on the basis of cell wall chemistry and lysine synthesis.

The Oomycetes have _______ cell walls; no other true fungi have this.

The oomycetes have a Diaminopimelic Acid pathway (DAP) in common with higher plants, bacteria, and algae.

All other fungi use the Amino Adipic Acid pathway (AAA) in common with Euglenoids (Protozoan Algae).

E. Ecological roles of fungi

Fungi are highly variable and adaptable organisms; They are basically decomposers and can cause human, animal and plant disease. They mutate freely and can synthesize or decompose an enormouse array of molecules; They can mutate rapidly and we therefore see various strains, races and pathotypes. There are terms such as forms specials, special forms. The fungi are the most important group of plant pathogens and these are some of the reason why they are very difficult to control. Only by understanding the biology and life cycle as well as disease cycle of these pathogens can control be obtained.

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