Life Cycle

In general, fungi begin their lives as a spore, then germinate and develop into a weblike network of filaments called a mycelium. Individual filaments are called hyphae.

Throughout this process, the fungus is excreting digestive enzymes and absorbing the resulting nutrients. Because of the hyphal nature of most fungi, they are especially adept at penetrating and assimilating whatever material they are determined to grow on / in. In the case of plant pathogenic fungi, the hyphae are capable of attaching to the surface and burrowing through the outer tissues with special enzymes and great physical force.

In addition to secreting digestive enzymes, many fungi also attempt to discourage competition by secreting powerful antibiotics and sometimes poisons. Oyster mushrooms, for instance, will kill nematodes with their secretions and absorb the remains as a nitrogen source. Equally, fungi may try to discourage other fungal species from sharing their turf. If the conflict zones occur amongst wood rotters, it can create spalted wood.

By default, many fungi will begin their lifecycle producing microscopic asexual fruiting bodies which bear asexual spores called conidia. These are generated quickly and easily, and help the fungus occupy the area quickly and choke out competitors. Many molds are fungi operating in this fashion.

If a fungus encounters another of its own species, and conditions are right, it may mate and begin the process of producing sexual fruiting bodies. The process of determining sexual compatibility, and the method of forming a fruiting body varies depending on the fungus. Most fungi do not have obvious male / female structures, and compatibility is determined genetically. Some fungal species can have over 100 different compatibility traits. The sexual fruiting body is a major investment by the fungus, and often it will only produce these larger structures after a long period of colonization, or when they predict impending unfavorable conditions. This is why autumn is such a good season for finding mushrooms! Sexual fruiting bodies come in great variety, from microscopic mini pockets to garbage lid size shelves, but they all have one thing in common: they are designed to maximize surface area and bear spores with genetic material from both parents.

For many fungi, the only way to visually identify the species is to examine their fruiting bodies. There is great variety in the size, shape, color, odor, and taste of these structures; while pretty much all hyphae look the same.

Major Types of Fungi

Like many of the other kingdoms, fungi come in many shapes and sizes, and perform many different roles. Below is a rough breakdown of the major forms fungi can take. Note that this is not a taxonomic breakdown. As you will later discover, a single genus / species may take more than one form. This is called polymorphism and is a major trait of many fungi.

Molds:
Molds are a bit difficult to describe, as molds are more about fungi appearing and behaving in a particular way, rather than being a particular class of fungi. In fact, many mycologists would prefer to never use the word 'mold', and just deal with each species on its own terms. Molds represent the basic structure of all fungi: a weblike network of microscopic strands called mycelium (a single strand is called a hyphae). Unlike some of the other fungal forms, molds have one primary goal: eat as much as possible in as little time as possible and reproduce like mad! Because there are no long term goals, molds produce only microscopic reproductive structures. These are almost always loaded with spores and are usually produced en-masse. When molds are in rapid attack mode, they often are only producing asexual spores called conidia. This eliminates the delay to find a compatible mate, and helps a fungus reproduce quickly before settling in for the long haul.

As mentioned above, many fungi are polymorphic, and the mold form is often half of a fungus' lifecycle. The other half may produce larger fruiting bodies such as mushrooms, but this takes time and far more nutrients. Other fungi are always mold-like, and both their sexual and asexual fruiting bodies are microscopic. There is an entire group of fungi that never reproduce sexually. These are called the hyphomycetes, or deuteromycetes, or fungi imperfecti, and among them are some well known and important molds. Species of Penicillium, Aspergillius, and Stachybotrys are in this group.

Yeasts:
Yeasts are single celled fungi that typically reproduce asexually by budding. Yeasts are not considered a taxonomic group themselves, but rather a form the fungi take. There are many fungi that will produce yeast cells when certain environmental conditions are met. For example: the pathogenic fungus histoplasma grows in hyphal form normally, but when inside the body it switches to yeast form. Some yeasts, such as brewers yeast (Saccharomyces cerevisiae) never enter hyphal form, but will mate sexually and produce minute sexual fruiting bodies. Yeasts are especially effective in food and industry as they have the greatest possible surface area of all fungal forms, and can metabolize nutrients very quickly. The study of fermentation is called zymurgy.

Lichens:
Certain fungi have adapted to incorporate algae or cyanobacteria into their tissues, and leech off the carbohydrates they generate. Because food is generated on-site, lichens can grow anywhere there is sunlight, even on bare rock! The interesting thing about lichens, is that they are genetically very similar to non-lichen counterparts. In the lab, a lichen-capable fungi will grow just like its non-lichen counterpart, until it encounters algae. When the two meet, the shape and chemistry of the fungi changes dramatically and it develops into its lichen form! Lichens produce two different type of spores, one that contains both algae and fungi, and another that is just a fungal spore. The hybrid spores are generated on special structures called isidia. The fungal spores are generated on structures very similar to its non-lichen counterparts, which are often cup shaped. Some lichens are related to fungi that produce mushrooms, and wouldn't you know it, the lichen also produces small mushrooms! Many lichens are especially cold and drought tolerant, but can be very sensitive to air pollution. The study of lichens is called lichenology.

Macrofungi:
Mushrooms, puffballs, truffles and shelves are the reproductive structures of fungi which have lived a pleasant life, had tasty meals, found love, and are ready to make more mushrooms. Fungi that generate reproductive structures that are large enough to be seen by the naked eye are called macrofungi. Because the reproductive structures can take many forms besides the typical mushroom / toadstool, it is better to use the generic term fruiting body (this term applies to any spore-bearing structure, even microscopic ones).

As you have probably gathered by now, the fruiting body is a special structure developed specifically to release spores. The vast majority of a fungus' body is spread out as mycelium within whatever materials it is busy digesting. Large fruiting bodies are always the result of sexual pairing, and represent a major investment of nutrients and energy by the fungus.

Rusts and Smuts:
Rusts and smuts are obligate plant parasites / pathogens that have very unique lifecycles and are often adapted to a very specific host range.

Rusts remain microscopic throughout their lifecycle, and tend to produce spots on leaves and stems from which they release their spores (hence, rust). Unlike all other fungi, rusts will produce up to four different sporulating structures with a different spore type in each phase. In some rusts, the phases alternate between two different host plants. For example, wheat rust requires both wheat and barberry to complete its lifecycle. So, as you can imagine, one way of controlling wheat rust is to remove all barberry from the area! Rusts can be the greatest threat to plants in both natural and agricultural settings. They usually do not kill their hosts outright, but can severely limit the host's ability to photosynthesize and reproduce. Although a rust may be vulnerable in its specificity, it is especially effective in its ability to infect, spread, and survive harsh conditions.

Smuts are also very host specific. So much so, that they incorporate themselves into the host tissues and take over locations where the plant invests most of its resources: the seeds / fruit. When ready, the smut will produce plant growth hormone, forcing the plant to invest even more resources into the area, creating nutrient-rich galls which the smut will convert into fruiting bodies. The spores that smuts produce are full of nutrients and are designed to survive through whichever season the host does not grow (usually winter). When ready, the spores then produce asexual fruiting bodies from the stored nutrients, and the asexual spores spread through the air to infect new hosts. A well known smut is corn smut, which is edible and called huitlacoche in Mexico.

Mycorrhiza:
Mycorrhiza is latin for 'fungus root'. It is a relationship between the roots of many plants and their fungal partners in the soil. This relationship is truly symbiotic; fungi procure nutrients from the soil that are difficult or impossible for the plant to absorb, such as phosphate, and the plant donates sugars to the fungus. Mycorrhiza not only aid in nutrient uptake, but also massively increase the surface area of the root system, which facilitates hydration and uptake of other minerals.

There are two main divisions of mycorrhiza: the Glomerales, which are a microscopic fungi that associate with herbaceous plants; and the ectomycorrhiza, which are a group of many different fungal families which associate with trees and are responsible for producing a wide variety of tasty mushrooms!

We will go into more detail about mycorrhiza in the ecology section; the most important thing to know about mycorrhiza is that the vast, vast majority of plants rely on mycorrhizal partners for their success!