This page features several beautiful drawings by Jen Starr (nee Kulis), all rights reserved. They were featured in a chapter in the book, Structure and Dynamics of Fungal Populations (®).

Three chief characteristics of fungi:Hyphae and spores

Other important 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.

Angry professor 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 phyla and most of the subgroups.

Phylum Subgroup Some major genera
Oomycota Oomycetes Phytophthora, Pythium (water molds and downy mildews)
Glomeromycota Glomeromycetes Acaulospora, Gigaspora, Glomus (arbuscular/endomycorrhizal fungi)
Zygomycota Endogonales Endogone (ectomycorrhizal fungi, saprobes)
  Mucorales Mucor, Rhizopus (mostly common saprobes)
Ascomycota Classification of the Ascomycota is so complex, it is better for the beginning student to focus instead on forms of fruiting as presented here
naked asci Taphrina
perithecia Nectria, Ceratocystis, Ophiostoma, Valsa
cleistothecia Microsphaera
apothecia Peziza, Rhizina, Rhytisma (saprobes, root pathogens, ectomycorrhizal fungi)
pseudothecia Apiosporina, Phaeocryptopus
  Asexual forms of Ascomycota
simple conidiophores Verticillium
coremium Graphium
sporodochium Fusarium
acervulus Gloeosporium
pycnidium Sphaeropsis
Basidiomycota Agaricales Mushrooms (Armillaria), puffballs and bird's nest fungi (Lycoperdon, Crucibulum), and coral fungi (Clavaria)
Boletales Poroid mushrooms (Boletus), gasteroid fungi (Pisolithus, Scleroderma) and merulioid crust fungi (Coniophora, Serpula)
Corticiales A small residual group of mostly corticioid fungi; not important in forest pathology (Corticium, Vuilleminia)
Hymenochaetales Polypores (Inonotus, Onnia, Oxyporus, Phellinus, Trichaptum) and crust fungi (Hymenochaete, Hyphoderma, Hyphodontia)
Polyporales Polypores (Fomitopsis, Ganoderma, Oligoporus, Phaeolus, Polyporus) and corticioid fungi (Phanerochaete, Pulcherricium)
Russulales Polypores (Albatrellus, Bondarzewia, Heterobasidion), tooth fungi (Echinodontium, Hericium), crust/stereoid fungi (Peniophora, Scytinostroma, Stereum) and mushrooms (Lactarius, Russula)
Pucciniomycetes Cronartium (rusts), Helicobasidium

Above includes mostly groups with important tree pathogens and others that forest pathologists should be at least minimally familiar with. This classification is based on a new higher-level phylogenetic classification of the fungi (®). Fungi are less well understood than many other groups of organisms and their relationships are just beginning to come into focus, so there are many different classifications in various books. Unfortunately, in many cases, morphology does not correspond with phylogenetic relationships based on molecular data. For an embryonic attempt at classifying all organisms on the internet, see the Tree of Life.

Oomycete cycle


Although these are not in the kingdom Fungi, they are fungi (with a small “f”). The word is used here in the sense of life form, much as maples and spruces are both trees. Depending on one’s kingdom-level classification, this phylum may be placed in kingdom Chromista, Straminipila (=Stramenopila), Protoctista, or Protista.

Unlike true Fungi, Oomycetes (the class of interest in this phylum) have these characteristics:

With about 800 species, many important plant pathogens are in this group. Sometimes called "water molds." 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!

         Animated ascus
Taphrina asci

Naked asci

Sometimes called Hemiascomycetes, now classified in the subphylum Taphrinomycotina. 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, now most are in the class Sordariomycetes. 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. These are distributed in various modern classes, including Eurotiomycetes, Sordariomycetes and Leotiomycetes.



Often called Discomycetes, most are now in the class Pezizomycetes. 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, now placed in the class Dothideomycetes. Asci with two layers (bitunicate), produced in pseudothecia (right) that look like perithecia. Pseudothecia are cavities (locules) in a usually black stroma.

Ascomycete asexual stages

The figure at left shows some asexually reproducing forms of ascomycetes (a, see asexual forms, below), a cleistothecium formed by some powdery mildew fungi (b), apothecia (c), and a vertical section of a perithecium (d).

Asexual forms of Ascomycota

Many ascomycetes produce asexual spores (conidia), and often we find the fungus in this state without the sexual form. Some apparently have no sexual spores. Often called "deuteromycetes" informally. They have traditionally been classified in form-taxa from phyla down to species. Many fungi have two names, one for the sexual stage or the "whole fungus", and one for the asexual stage.



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 holobasidium, is aseptate, club-shaped and usually produces four spores (a). Some basidia, such as 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.
Clamp connection

Various basidiomycetes


The fungi in this subphylum, about 20,000 described species, have gone through huge changes in classifications in recent years. Although classification now hopefully reflects evolutionary relationships, often there is little in terms of morphology or ecology to help define the groups. In the Agaricomycotina, morphology certainly does not recapitulate phylogeny!

In orders important in forest pathology (class Agaricomycetes, except for Auriculariales, Sebacinales and some members of Cantharellales), the basidia are aseptate and spores germinate to give only hyphae. Basidia are organized in exposed, defined layers (the hymenium) or in an enclosed mass (the so-called gasteroid forms). This class contains fungi that produce the familiar mushrooms, conks, puffballs, stinkhorns, etc.

Of 21 orders currently recognized in the Agaricomycotina, we cover 6 below, all in the class Agaricomycetes. See the outline table above for examples of genera.


More than 9,000 described species, 350 genera and 30 families. Basidioma texture fleshy, basidia usually produced on gills. Most of these are your basic mushrooms, a very large group. A great many agarics are important mycorrhiza partners with trees and shrubs; most of the rest are saprobic decomposers. A few are pathogens. In the bird's nest fungi (no pathogens), the basidioma is firm and cup-shaped, and fertile tissue is segmented into egg-shaped peridioles: another extravagant and elegant experiment of evolution. Puffballs are modified by production of basidia and spores in a mass inside and enclosed basidioma (gasteroid). There are also some poroid, corticioid, and club/coralloid forms.


Most of these are soft, fleshy mushrooms, but with tubes instead of gills (known as boletes). However there are some gasteroid fungi (with basidia and spores in an enclosed mass during development), and some fungi that are flat on the substrate with a wrinkled surface lined with basidia. Most of these fungi are mycorrhizal, but some of the flat ones are important wood decayers, some in living trees and some in wooden buildings.


Mostly corticioid fungi (mostly flat on the substrate with a more or less smooth fertile surface). Traditionally the family Corticiaceae held almost all corticioid fungi and was more important. Now most are distributed in other orders according to their evolutionary relationships with fungi having other forms. No major pathogens remain in this order.


The family Hymonochaetaceae was one of the earliest recognized groups that broke with the traditional classification according to fruitbody form. There are various anatomical and chemical features that clearly unite the members, although the fertile surface in some cases lines tubes (polypores), teeth, and smooth surfaces (corticioid). That family has largely held together under the harsh glare of molecular phylogeny. However, other families, without obvious related features, were formed and added to the order Hymenochaetales. Most members of the order are either polypores or corticioid fungi, and many important pathogens and other wood-decay fungi are found here.


This is still a major group of polypores, including many important pathogens, although other important polypores are found in orders such as Hymenochaetales and Russulales. Some large genera of corticioid fungi are here also. Apparently all members of this order are decayers of wood.


Well over 4,000 described species, 80 genera and 12 families. This order now contains just about every configuration of fruitbody. Two very large genera that produce gilled mushrooms form the core of the order, but all the morphological forms found in the other orders are found here as well. Even many of the 12 families within the order have varied forms. Ecologically, they are saprobes, mycorrhizal partners, root parasites and insect symbionts.

Pucciniomycotina and Ustilagomycotina


Basidia usually septate, spores germinate repetitively or by budding. In these two subphyla (rusts and smuts), a basidioma is not formed, karyogamy occurs in a thick-walled resting spore (teliospore), and meiosis occurs upon germination of teliospore.


Rusts and relatives, more than 8,000 species. Rusts (order Pucciniales, about 7,000 species) are 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!


Smuts and relatives, about 1,500 species. 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.

Life Cycle

The life cycles of fungi vary, depending on the group. We will focus on the life cycle of the basidiomycetes, using a polypore (order Polyporales) as an example.

Spores are wind disseminated. Each has a haploid nucleus. We designate the nuclear state as "n." The spores germinate if they are lucky enough to land on a suitable substrate and conditions are favorable. Germination of spores
When two sexually compatible germlings grow nearby, their hyphae fuse. Mixing the cell contents of the two individuals as a step of the sexual cycle is called plasmogamy. Then there are two different kinds of nuclei together, a state we call the dikaryon ("n+n"). Clamp connections are usually formed in the dikaryon. The fungus exists in this state for most of its life. Life cycle - plasmogamy
When it has had enough to eat and conditions otherwise allow, the fungus produces a fruitbody, the function of which is sexual reproduction and dispersal. In the polypores, we call them conks informally. Life cycle - conk
In the lower part of the conk are tubes that end in pores on the lower surface. The tubes are lined with basidia. The basidia are busy little cells, for three important events of the life cycle take place there: karyogamy, meiosis, and spore production. Life cycle - inside tube
This is roughly what a tube looks like in cross section. Notice that the spores are forcibly discharged from the basidia, but just far enough to reach the middle of the tube. Then they fall out and are carried away in the wind. Life cycle - pore

Funny picture "I think I got it!!!!"

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Last modified 18 Aug, 2012