The mycology of the Basidiomycetes

A number of significant tree and plantation root diseases in tropical Asia are caused by certain species of basidiomycetes. This important group of fungi, which includes such familiar forms as mushrooms, toadstools, bracket, shelf, and crust fungi, puff balls, earthstars, and coral fungi, is characterised by the production of sexual spores on the outside of a microscopic structure called the basidium. Basidiomycetes occupy many niches in the environment, including decomposing litter, decaying wood and soil organic matter, in a variety of habitats such as forests and open countryside. Some species form beneficial mycorrhizal relationships with the roots of host trees, but others are important pathogens of the foliage, stems or root systems of different tree species. Basidiomycete species are traditionally identified by the form and microscopic structure of their fruitbodies, and by their appearance when isolated in laboratory culture. Important root diseases of trees in Indonesia are caused by the basidiomycete fungi Rigidoporus microporus, Junghuhnia vincta, Phellinus noxius, and certain species of Ganoderma. Mycology is the scientific study of fungi. It is distinguished from, but related to plant pathology, the study of plant diseases, a large proportion of which are caused by harmful fungi (fungal pathogens). Fungi are a very large, diverse group of living organisms found in nearly all ecosystems. Fungi are eukaryotes, that is, they have microscopic organelles within their cells called nuclei which contain genetic material in the form of thread-like chromosomes, enabling hereditary characters to be passed on to subsequent generations. Other eukaryotes include all members of the plant and animal kingdoms, whereas life forms such as bacteria and blue–green algae, whose genetic material is not held in a nucleus, are called prokaryotes (Table 1). Recent study has shown that some of the organisms we call fungi actually belong to other groups. Those no longer recognised as true fungi, include fungus-like organisms such as the myxomycetes (slime moulds) and the oomycetes (which feature the downy mildews and microscopic root-infecting fungi such as species of Phytophthora and Pythium). Most of the true fungi exist and grow in the form of a spreading network or mycelium of living tube-like threads called hyphae (though yeasts develop as budding cells). The hyphae branch as they grow from the tip, and feed heterotrophically. This means that fungi cannot photosynthesise like plants and algae, which are autotrophic, but live either saprobically on dead organic matter or symbiotically in association with other living organisms. The latter relationship (symbiosis) may be harmful to the living plant or animal host (parasitic), mutually beneficial (as in lichens and mycorrhizas), or neutral (commensal, as in fungi that grow as endophytes within living plant leaf, root or stem tissues without causing harm). Fungi reproduce and spread by means of sexual and asexual spores, both of which may be released in large numbers. In one of the two largest groups of true fungi, the ascomycetes, sexual ascospores are produced within a sac-like structure called the ascus. Among the larger ascomycetes are the disc and cup 1 Ensis – the joint forces of CSIRO and Scion, Te Papa Tipu Innovation Park, Private Bag 3020, Rotorua, New Zealand. Email: .