Here are three chief characteristics of fungi:

• basic structural unit is almost always hypha (a, b)
• reproductive propagule almost always spore (c, usually single-celled)
• nutrition is heterotrophic and absorptive (they secrete enzymes that break down organic matter, then absorb soluble products)

Here are some other characteristics:

• mostly multicellular
• mostly non-motile
• rigid cell wall usually containing chitin
• indeterminate growth

What role do fungi play in the forest?

• saprobes
• pathogens
• symbionts (mycorrhizae, lichens)

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.

Oomycota

Oomycetes

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.

Ascomycota

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!

         

Naked asci

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).

Perithecia

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.

Cleistothecia

The ascocarp (a cleistothecium, right) is spherical and closed.

Apothecia

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.

Pseudothecia

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).

Deuteromycota

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.

• 
• In some the conidioma is absent and conidia are produced on simple conidiophores (right).
  
• 
• Conidiophores may also be grouped into columnar coremia (not shown), or cushion-shaped sporodochia (right).
  
• 
• Conidia may also be produced in an organized, covered fruitbody (conidioma). In one group they are pycnidia (right). They look superficially like perithecia, but of course have no asci.
  
• In another they are acervuli.

Basidiomycota

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.

Homobasidiomycetes

Basidia aseptate, spores germinate to give only hyphae. These fungi produce the familiar mushrooms, conks, puffballs, stinkhorns, etc.

Agaricales

Basidiocarp 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.

Aphyllophorales

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.

Lycoperdales

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.

Nidulariales

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.

Heterobasidiomycetes

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.

Uredinales

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!

Ustilaginales

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 Aphyllophorales) 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.
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.
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.
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.
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.

Information provided by: http://www.forestpathology.org