Generally, Fungi are microscopic organisms whose vegetative body consists of threadlike structures called hyphae, that are usually aggregated into branched systems, the mycelia. The mycelium is the cottony growth on decaying tissues, and its single strand is a hypha. A fungus has cell walls and a true nucleus but lacks chlorophyll.
A fungus reproduces mainly by spores that are equivalent to plant seeds. A spore may consist of one or more cells. It may be formed asexually by arising from modified mycelial portions (like buds on a twig) or sexually, through the fusion of unlike cells (gametes). Some fungi, such as those that cause rice diseases, produce asexual spores called conidia derived from terminal or lateral cells cut off from special hyphae called conidiophore.
Ascospores are the sexual spores within a saclike zygote cell (ascus). Asci (plural of ascus), in turn are contained in a fruiting body called perithecium. Sexual spores of other fungi may develop outside the clublike zygote cell, the basidium, and they are called basidiospores.
Any of the structural parts of a fungus such as spores, sclerotia (i.e., compact masses of mycelia), or fragments of mycelium can initiate infection. When any of these structures come into contact with the plant, it will germinate by producing a germ tube that may form an appressorium at the tip, bearing an infection peg through which penetration into the plant cell wall can take place. Once inside the cell, the fungus continues to grow by producing more hyphae, invading more cells, and intruding into the plant tissues until the plant exhibits symptoms.
Most of the fungi causing rice diseases have asexual and sexual structures. Asexual structures are referred to as the anamorph while the sexual structure is referred to as the teleomorph. These are two independent but coexisting classification systems used to classify fungi. In cases where both the teleomorph and anamorph structures of a fungus are known, teleomorph is generally used. However, if the teleomorph is known but rarely encountered or rarely observed, then it is acceptable to use the name of the anamorph.
Most of the information relating to the history of all fungal diseases in this module were obtained from Ou (1985).