FUNGI

What is a fungus?
Most people see a mushroom as a plant, but close examination shows that there are fundamental differences:

• The fungi are all heterotrophic
• They have a filamentous cell structure that is essentially undifferentiated
• They have chitin cell walls rather than cellulose

This is why they are included in their own kingdom which in some ways is closer to animals than plants. For example insect and other arthropod skeletons are made of chitin.

Fungal Hyphae

A single fungal filament is called a hypha (plural hyphae). The chitin cell wall encloses a cytoplasm that may be undivided (Zygomycetes - molds), partially divided by perforated cross-walls (Ascomycetes - morels, cup fungi, blue molds) or fully divided (Basidiomycetes - mushrooms). The cytoplasm contains one or more haploid nuclei. If there are no cross-walls the cell structure is is said to be "coenocytic". Many basidiomycetes have hyphae built up of uninucleate cells. The yeasts are exceptional among the Ascomycetes in that they are usually uninucleate and unicellular (non-filamentous).

Hyphae grow at their tips and branch to form a mass of interwoven strands that is called mycelium. This is a culture of Pythium isolated from Taxus.

In many fungi the mycelium will form special structures in which nuclei will fuse to form diploid cells. These undergo meiosis to form haploid spores which can be transported over huge distances.

Hyphae all tend to look alike and fruiting bodies are often the easiest way to distinguish different fungi. This is the stinkhorn, Phallus impudicus

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Some fungi, particularly yeasts exist predominantly as single cells and never form large fruiting bodies.

Fungal diseases
For some reason human fungal diseases (like yeast infections, toenail fungus and athletes foot) have become more common in recent years. However it is probably true to say that we suffer from more viral and bacterial diseases than fungal.

It is the other way round with plants; fungi cause many more diseases and losses from these diseases are worse than for bacteria and viruses. Fungal pathogens can get past the plant's defenses more readily than the other organisms. Their hyphae produce enzymes that can digest both plant cuticle and cell walls so that they can penetrate the surface and the internal tissues of healthy plants.

Rust (Puccinia) on hawthorn

Leaf spot (Rhytisma) on Acer

Powdery mildew on rose

Root rot (Fusarium)

Control of Diseases
Chemical control of fungal diseases has a long history, starting with inorganic compounds such as copper salts and lime-sulfur in the nineteenth century and ending with sophisticated organic compounds such as imidazoles and triazoles in the 1970s and 80s. The inorganic compounds can be genuinely fungicidal (and phytotoxic), whereas the organic compounds are more selective but only fungistatic. Some crops cannot be grown without fungicides, particularly in damp climates (like Ohio) but public opposition to their use is making it difficult to continue spraying. As with bacteria we are having to control fungi by ecological approaches.

Mycorrhizae
Not all fungi that invade plant tissues are damaging. Fungi associated with roots can help the plant get phosphate and other nutrients from the soil. These mycorrhizal associations are of two kinds:

• endomycorrhizae that invade the root tissues and often occur in herbaceous plants
• ectomycorrhizae that form a sheath of mycelium around the root and often occur in woody plants.

It seems that just about any kind of plant can form a mycorrhizal association; they do not often occur in agricultural crops and we usually have to make sure that these have plenty of nutrients from artificial sources. Many coniferous trees just do not grow well in the absence of suitable mycorrhizal fungi and most orchids have an absolute requirement for their fungal partners.

Endomycorrhizae in T.S. of orchid root. The dark dots are starch grains and short sections of fungal mycelium can be seen in cortical cells

Common mushrooms (Agaricus bisporus)

Oyster mushrooms (Pleurotus)

Fungal saprophytes
Fungi share with bacteria the role of recycling nutrients from dead organisms. For example woodlands would soon become choked with dead plant debris without fungi to break down fallen leaves and timber: