|Themes > Science > Botanical Sciences > Trees Pathology > Fungi|
This page features several beautiful drawings by Jen Starr (nee Kulis), all rights reserved.
Above is a classification of convenience, including mostly fungi of importance as tree pathogens. This is not a complete or formal classification. Fungi are less well understood than many other groups of organisms, so their relationships are obscure and there are competing classifications. For an embryonic attempt at classifying all organisms on the internet, see the Tree of Life.
Here are some other characteristics:
What role do fungi play in the forest?
You may be asking yourself, "Why do I
need to learn about fungi? I want to learn about tree diseases and what to
do about them." Good question. You will find that fungi are
overwhelmingly the most important pathogens of trees. In order to
understand the diseases they cause, how the disease spreads, etc., you
have to understand the fungi themselves to some extent.
If you are to understand diseases, it is important that you work very hard, now, to master the overall classification of the fungi and the related details. Do it now and really make sure you've got it. If you wait, you will build your knowledge of forest pathology on a shaky foundation. It's up to you.
You should know well the divisions and the subgroups.
Asexual spores are
motile, flagellate zoospores
(but some also have nonmotile chlamydospores); sexual
spores nonmotile (oospores). Sometimes called
"water molds." These are not true fungi in a taxonomic sense,
but that is not of huge importance in understanding their pathology. Here
is a nice site on the zoosporic fungi.
Sexual spores (ascospores) form in a sac called an ascus (usually in eights), and are often discharged forcibly. The group is often called "ascomycetes" informally.
Below is an ascus developing and shooting out its spores!
Sometimes called Hemiascomycetes. Ascoma absent. These are morphologically simple ascomycetes. Various species of Taphrina cause localized abnormal growth of plants, and asci are produced on the plant surface (right).
Often called Pyrenomycetes. Asci are in a flask-shaped ascoma (perithecium) with a pore (ostiole) at the top (right). This is a big group with some important pathogens.
The ascocarp (a cleistothecium, right) is spherical and closed.
Often called Discomycetes. The asci
are in a bowl- or cup-shaped ascoma (apothecium; right). These are
sometimes called the "cup fungi", and it is a large group.
Often called Loculoascomycetes. Asci with two layers (bitunicate), produced in pseudothecia (right) that look like perithecia. Pseudothecia are cavities (locules) in a usually black stroma.
The figure at left shows some asexually reproducing forms of ascomycetes (a, see Deuteromycota, below), a cleistothecium formed by some powdery mildew fungi (b), apothecia (c), and a vertical section of a perithecium (d).
This is an artificial group (form-division) characterized on the basis of asexual spores (conidia). Most are asexual stages of ascomycetes, but some apparently have no sexual spores. Often called "deuteromycetes" informally.
This group is characterized primarily by the sexual spores (basidiospores) being produced on a cell called a basidium, usually in fours. The typical basidium, sometimes called a homobasidium, is aseptate, club-shaped and usually produces four spores (a). Some basidia, often those of rust fungi, are transversely divided into 4 cells (b). Some ("tremelloid" basidia) are longitudinally divided into 4 cells (c), and others may be shaped like a tuning fork (d).
Members of the Basidiomycota are often called "basidiomycetes" informally. Many, but not all, have special septal structures called clamp connections during most of the life cycle. No other group of fungi has these!
Formation of a clamp connection on hypha
of a basidiomycete.
Basidia aseptate, spores germinate to give only hyphae. These fungi produce the familiar mushrooms, conks, puffballs, stinkhorns, etc.
texture fleshy, basidia produced on gills. These
are your basic mushrooms, a very large group. A great many agarics
are important mycorrhizal partners with trees and
shrubs; most of the rest are saprobic decomposers.
A few are pathogens.
Basidiocarp texture usually tougher, basidia usually produced on structures other than gills. This is a large group of fungi with dizzyingly diverse form. Basidia may line the interior of tubes (polypores), the exterior of conical teeth (tooth fungi), the exterior of upright cylinders (coral fungi) the underside of a surface that is smooth or variously wrinkled (crust fungi and others), or even structures that look for all the world like gills of the Agaricales (although the name of the order means without gills). Most wood-decay fungi and many important forest pathogens are in this order, but some are apparently mycorrhizal.
Basidiospores at maturity forming dry powdery mass in basidiocarp interior. These are the typical puffballs, the chief representatives of a group of orders recognized informally (in this classification) as "gasteromycetes." Some are mycorrhizal, none are pathogens.
Basidiocarp firm and cup-shaped, fertile tissue segmented into egg-shaped peridioles. The bird's-nest fungi. Another extravagant and elegant experiment of evolution. None are pathogens.
Basidia usually septate or deeply divided, spores germinate repetitively or by budding. In the two orders presented here (rusts and smuts), a basidioma is not formed, karyogamy occurs in a thick-walled resting spore (teliospore), and meiosis occurs upon germination of teliospore. These orders are sometimes put in a separate class, the Teliomycetes.
Highly specialized parasites of higher plants, life cycles typically with up to five spore stages and two alternating hosts (except by simplification). The rusts cause many serious diseases of economically important hosts, including trees. They also have perhaps the most complex and perplexing life cycles, and variations on those cycles, of any group of organisms. What fun for students!
Before mating: yeastlike, culturable, usually not infective. After mating: usually obligately parasitic. Many smut fungi have interesting disease cycles that are nicely adapted for plant infection. Infections are often systemic. None are important forest pathogens.
The life cycles of fungi vary, depending on the group. We will focus on the life cycle of the basidiomycetes, using a polypore (order Aphyllophorales) as an example.