9.6 Conidiomata

Individual conidiophores are obviously complicated, but in some fungi several to many conidiophores can orchestrate their activities to produce coordinated tissues that form asexual fruiting bodies. Of course, these (all called conidiomata (= fruit bodies that produce asexual conidia; singular = conidioma) have all been described by specific terms and given specific names. One type of arrangement is a synnema; this is a cluster of conidiophores that adhere to each other along some of their length, forming an elongated stem or bristle-like structure (called a fascicle, this being Latin for ‘a bundle’). Synnemata (that’s the plural form) may have many hyphal strands in their structure and be fleshy, hard or brittle in structure. The apical portions of the conidiophores that make up the synnema are separate and radiate outward, often producing a mucus slime in which the conidia are trapped (perhaps awaiting insect dispersal) (Fig. 10).

Several other sorts of conidiomata are produced, in particular, by pathogens and parasites of both animals and plants. A tightly clustered collection of conidiophores, shorter than those composing a synnema, and arising from a central cushion-shaped (= pulvinate) stroma made up of tightly woven hyphae (a pseudopamchymatous tissue, see below) is called a sporodochium.

Drawings of one of the synnemata (bunched conidiophores) of Podosporium elongatum
Fig. 10. Conidiomata. Drawings of one of the synnemata (bunched conidiophores) of Podosporium elongatum. The species grows on decaying bamboo stems. The synnemata are about 1 mm tall. Here we show a habit sketch at bottom left and two successive magnifications, with a view of some separated-out conidiophores at top left. Redrawn after Chen & Tzean, 1993.

An acervulus (Fig. 11) is a distinct mass of conidiophores that develops just beneath the surface of a host plant and bursts through as the spores mature, pushing aside flaps of host tissue as they emerge. It is rather less structured than a stroma (the name is derived from the Latin acervus, meaning ‘heap’), and is produced by species of the plant pathogen Colletotrichum, cause of anthracnose diseases of citrus, banana, cucumber, and other crops.

Diagram illustrating the structure of an acervulus
structure of a pycnidium
Fig. 11. Conidiomata. Diagram illustrating the structure of an acervulus. The fungal pathogen forms a stroma of interwoven hyphae (shown in black) in the epidermis of the host plant (shown in blue) and then bursts through the plant cuticle to release conidia. Redrawn after Moore-Landecker, 1996.
Fig. 12. Conidiomata. Diagram illustrating the structure of a pycnidium. The fungus forms a chamber with walls of hyphae woven into a pseudoparenchymatous tissue (shown here in black) that may be dark and tough or brightly coloured and fleshy. Conidiophores produce conidia within the pycnidium. Some pycnidia are enclosed in stromata, others in host tissues (shown here in blue), still others are surface structures. They may be disc-shaped, globose, flask-like or cup-shaped; and may be entirely closed or open to the air through a hole (ostiole), slit, or tear to release conidia. Redrawn after Moore-Landecker, 1996.

A pycnidium is a highly organised structure, basically a chamber, lined with conidiophores, constructed from a distinct wall layer of pseudoparenchymatous tissue (Fig. 12) that may be dark and tough or brightly coloured and fleshy. Some pycnidia are enclosed in stromata, others in host tissues, still others are surface structures. They may be disc-shaped, globose, flask-like or cup-shaped; can be entirely closed or open to the air through a hole, or ostiole, slit, or tear. Fungi that produce pycnidia often occur as saprotrophs on plant litter, or as plant pathogens, such as the leaf spot diseases caused by species of Septoria on wheat, celery, azaleas, and gladiolus.

A pycnidium (Fig. 12) superficially resembles the ascoma known as a perithecium (CLICK HERE for reminder), except that the former produces conidia and the latter produces sexual ascospores.  We will describe and illustrate ascomata below, but the basis of the similarity between pycnidia and perithecia is that both are constructed from multihyphal tissues. The majority of the macroscopic fungal structures are formed by hyphal aggregation into either linear organs (strands, rhizomorphs and fruit body stipes) or globose masses (sclerotia and the familiar fruit bodies, as well as less-familiar sporulating structures, of the larger Ascomycota and Basidiomycota. The general term plectenchyma (Greek plekein, to weave, with enchyma = infusion, meaning an intimately-woven tissue) is used to describe organised fungal tissues. There are two types of plectenchyma: prosenchyma (Greek pros, toward & enchyma; i.e. approaching or almost a tissue), which is a rather loosely organised tissue in which the components can be seen to be hyphae; and pseudoparenchyma (Greek pseudo = false with parenchyma = a type of plant tissue) which, when seen in microscope sections, seems to be comprised of tightly packed cells resembling plant tissue. In pseudoparenchyma, the hyphae are not immediately obvious as such, though the hyphal nature of the components can be demonstrated by reconstruction from serial sections or by scanning electron microscopy.

When viewing sections of these (and other) fungal tissues (such as in Fig. 12) it is essential that you remember that the structure is made up of tubular hyphae; what appear to be plant-like cells are just sectioned profiles of those hyphae. The majority of the lower fungi have coenocytic hyphae, but lower fungi do not form multicellular (multihyphal) structures. Some authors have argued that the term cellular element should be used in preference to ‘cell’ in fungal tissues, because fungal cells are always hyphal compartments and consequently different from the concept of the cell which emerges from elementary biological education. However, we think this is an unnecessary complication and will use the word ‘cell’ in this book.

Updated December 17, 2016