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18 CHAPTER 2

Olpidium brassicae; Figs. 2.1c, Fig. 2.3), of small animals

(e.g. Coelomomyces; see Fig. 15.5), of algae, or of

fungal spores. But very few chytrids can be considered

to be economically important – the most notable

example is Synchytrium endobioticum, which causes

potato wart disease, where the potato tubers develop

unsightly galls that render them unmarketable.

Having said this, the significance of chytrids lies

mainly in their fascinating biology. Anybody who has

watched a chytrid zoospore crawling like an amoeba

along the body of a nematode, searching for the

best site to encyst, and then winding in its flagellum,

encystingand penetrating the host will never forget

the experience (Fig. 2.2). All these events can be

Fig. 2.1Chytridiomycota. (a) Life cycle of Allomyces, which alternates between haploid (n) and diploid (2n) genera-

tions. The haploid thallus produces male and female gametangia that release motile gametes. These fuse in pairs and

encyst to produce 2n zygotes, which germinate to produce a 2n thallus. The 2n sporangia release zoospores for

recycling of the 2n phase. Thick-walled resting sporangia (rs) are formed in adverse conditions; after meiosis these

germinate to release haploid zoospores. (b) Rhizophlyctis rosea, a common cellulolytic fungus in soil. It grows as a single

large cell, up to 200mm diameter, with tapering rhizoids. At maturity, the large, inflated cell converts into a sporangium,

where the cytoplasm is cleaved around the individual haploid nuclei,and large numbers of zoospores are released through

the exit papillae. (c) Olpidium brassicaegrows as naked protoplasts in root cells of cabbages. At maturity, the proto-

plasts convert to sporangia, which release zoosporesinto the soil. These spores encyst on a host root, germinate, and

release a protoplast into the host. Thick-walled resting spores are produced in adverse conditions and can persist in soil

for many years (see Fig. 2.3).

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