Fungi are small, generally microscopic, branched, spore-bearing organisms that lack chlorophyll. Since they don't have the ability to make their own food (as plants can through photosynthesis) or the ability to take in complex foods (like animals do), they must take in simple foods from the breakdown and decay of other living organisms (plants and animals) in order that they can grow and reproduce. Most fungi have a vegetative body called a mycelium that branches in all directions. The individual branches or hyphae are generally of uniform thickness and the mycelial growth progresses from the tip of the hyphae. Some fungi have cross-walls or septa at regular intervals within the mycelium which effectively partition the hyphae in to discrete parts.
Fungi generally reproduce by means of spores which may be produced asexually or sexually. The spores can be regarded as the 'fingerprint' of the fungus since the size, shape, colour, number of cells and method of production are unique to a given fungus and allow the identification of the causal organism down to its species level.
Some of the more general groups of fungi, for example the basidiomycetes, tend to have characteristic structures associated with their mycelium which can allow for a basic level of identification.
Some fungi have the ability to produce structure that allow them to survive periods of adverse environmental conditions. There structures may allow the fungus to remain viable for many years.
Fungi can be spread in rootzone or turf material which is moved from one site to another, or the fungi can be moved by dispersal of their spores. Spores are generally moved by wind or rain (irrigation) but other animals and insects can also disseminate these microscopic propagules.
All fungi, as well as turfgrass plants, have what we call 'optimal' growth conditions. If the prevailing conditions are closer to those which are optimal for fungal growth, the fungi will be able to 'out-compete' the plant and disease becomes more likely.
Some of the above text is based on information in Plant Pathology 4th ed. Agrios , 1997.