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Pathogen Biology

Sexual reproduction

Mycosphaerella fijiensis is the name given to the sexual form (teleomorph) of the pathogen. The fungus was first described in 1969 by Morelet on specimens from Fiji.

To produce the sexual form, the fungus first develops many spermagonia on the lower surface of the leaf as the lesions collapse. A spermagonium (Figure 10) is dark, somewhat erumpent, and pearlike in shape. In moist conditions these structures may ooze large quantities of the male reproductive cells (spermatia). Spermatia are tiny and cylindrical and will fertilize neighboring female receptive hyphae called trichogynes.

Figure 10

Once fertilization is complete, pseudothecia are formed within the mature lesions, with their ostioles poking through the tissues (Figure 11). The oblong to club-shaped sac-like structures (asci) have two cell walls (bitunicate) and contain eight sexual spores (ascospores) that are lined up two-by-two. Pseudoparaphyses or sterile elements are absent from the pseudothecium. The ascospores are colorless and have one septum. One cell of the spore may be slightly broader than the other cell, and the spore may be slightly constricted at the septum. A squash mount of pseudothecia is shown in Figure 12.


Figure 11	Figure 12

Asexual reproduction

The asexual form (anamorph) is called Pseudocercospora fijiensis. Conidia are borne on the conidiophore singly and terminally. The spores are pale to a light olive-brown, and are smooth, long, and have three or more septa (Figure 13).


Figure 13	Figure 14

Conidia germinate during periods of high relative humidity (92-100% relative humidity) and infect the leaf through a stoma, usually on the underside of a leaf. Under humid conditions, hyphae can emerge from the stomata, grow along the leaf surface and penetrate through other stomata, thus enlarging the lesions. Conidiophores emerge through the stomata, sometimes on erumpent, compact masses of mycelium (stromata). Stromata may also develop on young spermagonia.