Glossary

abscission — the detachment of structures, such as spores, leaves, and leaflets. acervulus (pl. acervuli) — an asexual fruiting body (1–2 mm) of some fungi, which is saucer-shaped and initially produced in host tissue; conidia are produced on acervuli and bristle-shaped setae may be present, e.g. Colletotrichum spp. adventitious root — a root that arises from another structure, such as a stem. aeciospore — a microscopic fungal spore produced in an aecium; in wheat stem rust, aeciospores are produced on the alternate host in spring. aecium (pl. aecia) — a cup-like fruiting structure (1–2 mm) of some rust fungi formed after fertilization between pycniospores or by hyphal fusion; produces aeciospores. air-borne — movement of inoculum such as spores through the air, usually by wind, rain splash, or insects and other vectors. alkaloid — a secondary metabolite which contains nitrogen and is basic (alkaline); some fungal alkaloids are highly toxic. alternate host — a plant species other than the main crop host required by some rust fungi to complete their sexual life cycle; compare alternative host. alternative host — a plant species that may serve as a host for a pathogen, but which is not required for the life cycle to be completed; compare alternate host. anastomosis group (AG) — a group of similar isolates of a fungal species (especially Rhizoctonia solani) in which the hyphae of all isolates can fuse with each other, but not with isolates of other AGs; AGs may be specialized to attack certain host species. anther — a male structure of a flower that contains pollen. anthesis — the maturation of a flower to the point at which pollen is shed and fertilization can take place; in wheat, usually the time when anthers are just visible outside the florets. anti-oxidant chemical — a compound capable of counteracting the damaging effects on living cells (especially cell membranes) of strong oxidizing agents, such as ozone and peroxyacetyl nitrate. aphid — a small insect belonging to a group that feeds by sucking plant sap; may transmit diseases. apothecium (pl. apothecia) — an open, cup-shaped, sexual fruiting body of some ascomycete fungi; produces asci with ascospores on the open surface; with or without a stalk. ascocarp — a sexual fruiting body of fungi that produce ascospores. ascospore — a microscopic fungal sexual spore produced in a sac-like ascus. ascus (pl. asci) — in certain fungi, microscopic sac-like cell in which ascospores are produced after sexual fusion. asexual state — nonsexual stage of a life cycle, producing spores without fertilization. avirulent — being nonpathogenic, usually with respect to a particular cultivar of host plant. awn — bristle-like extension of the outer bract of a floret of barley and other cereals. axil — the angle formed between a petiole (leaf stalk) or branch and a stem.