Introductory Plant Pathology

Class 8

"Higher Fungi"
Ascomycetes and Basidiomycetes

CHARACTERISTICS OF PLANT PATHOGENIC FUNGI

Morphology

Systematics

In fungi, as well as in plants, naming and systematic relationships are based on sexual reproductive morphological characters (telomorph). This causes difficulty and confusion because sexual reproduction has not been observed for many important phytopathogenic fungi. Because of this and because asexual reproductive morphological characters (conidia, mycelium, chlamydospores, sclerotia) are most often observed in the field; an entire nomenclatural system based on asexual characters (anamorphic) has been developed. This nomenclatural system designated "form-genus" and "form-species" to identify and classify fungi is based primarily on spore ontogeny and shape. Use of the word "form" recognizes that nomenclature and identification are based incomplete characters. Over time, use of the word "form" was largely abandoned and a quasi legitimate standing for the name came into common usage. When sexual reproduction is observed, a new name is based on complete morphological (sexual and asexual)characters is constructed often the specific epithet (species name) remains the same while the new genus name takes precedence. An inordinate competitive rush to rename organisms has lead to incorrect renaming, and subsequent renaming, and has resulted in both confusion and a loss in the needed communication when the transitions are made between one name and another.

As students and professionals, it is important to known both the current name and also the synonyms that have been used in the past. It is useful to know both the telomorph (based on complete characters) and the anamorph (based on asexual characters) names for each pathogen. In common usage, quite often the anamorphic name prevails because of history and causal agent-disease relationships.

A certain amount of scientific hyperbole has occurred. One finds that under some schemes Phyla have been raised to Kingdoms. This is owing to molecular research that places fungi closer to animals than to plants and thus some have chose to erect new kingdoms for protozoa, chromista, and fungi. Currently one can find any number of schemes for fungal systematics. The scheme presented below follows Hawkworth et al^R. Simply because it is published as an entire treatment of these organisms, including the now Protists and Chromistans. Dr. Agrios' descriptions are informative, useful, and in general agreement with the Berkeley School.

Another, and much simpler, system is presented by Wessels et al ^R. It is summarized here to encourage students to evaluate this approach as well. Under this scheme the true fungi are placed in the Phylum Eumycota.

Divisions:
Chytridiomycota

Example Phytopathogenic Species: Olpidium, Physoderma, Plasmodiophora
Zygomycota

Example Phytopathogenic Species: Rhizopus, Mucor
Dikaryomycota
Subdivisions

Ascomycotina

Basidiomycotina

Classes

Saccharomycetes Galactomyces, Saccharomyces
Ascomycetes---The predominance of fungal species pathogenic to horticultural and agronomic plants are Ascomycetes.

Orders Taphrinales---Leaf Curl causal agents
Example:Taphrina
Erysiphales---Powdery Mildew causal agents
Example:Erysiphe, Blumeria, Uncinula, Sphaerotheca
Hypocreales
Examples:
• Hypocrea (anamorphs Trichoderma, Gliocladium)
• Melanospora (anamorphs Phialophora, Gonatobotrys)
• Nectria (anamorph Fusarium)
• Gibberella (anamorph Fusarium)
• Claviceps
• Epichloe (anamorph Acremonium)
Microascales---Oak Wilt causal agent
Example: Ceratocystis
Phyllachorales---Anthracnose causal agent
Example: Glomerella (anamorph Colletotrichum)
Ophiostomatales---Dutch Elm
Example: Ophiostoma (formerly Ceratocystis) (anamorphs Graphium, Sporothrix)
Diaporthales
Examples:
• Diaporthe (anamorph Phomopsis)
• Gnomonia
• Gaeumannomyces (formerly Ophiobolous)
• Magnaporthe (anamorph Pyricularia)
• Cryphonectria (formerly Endothia)

Classes

Heterobasidiomycetes


Example Phytopathogenic Species: Clavicpes, Puccinia, Rhizoctonia, Sclerotium, Tilletia, Uromyces, Ustilago
Homobasidiomycetes
Example Phytopathogenic Species: Armillaria, Fomes, Polyporus

Structures of Fungi courtesy: Dr. Donald C. Erwin ; University of California - Riverside

Generic Life Cycle of a Rust

The cyclic sequence of spore production, i.e. Basidiospore - Spermatia - Aeciospore - Urediospore - Teliospore, is the same for all rust fungi. However, with this proviso; not all rusts have complete cycles as indicated on the image as microcyclic and macrocyclic.

Also notice that each spore has a particular reproductive structure , i.e. basidium for basidiospores, for each spore type. The location of these structures is predictable and diagnostic for disease identification. Typically, some are on the abaxial surfaces while others are on the adaxial surfaces.

Notice the nuclear condition of these spores. Only the basidiospore is 2n; therefore, where does sexual recombination occur?

Point to Ponder: Knowing that rusts must complete their spore cycle in order to be viable as disease causing agents and adding to the equation that some rusts require alternate hosts (some spore stages on one host and the others on another), one is able to devise a plant health management strategy for rust control. As with all diseases, if one can break the life cycle one can "control " the disease. Therefore, what are some of the obvious general management strategies that will be effective against any rust fungus.

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:

Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

Copyright (C) 2005 J.E. Partridge, University of Nebraska-Lincoln. All Rights Reserved.