36 DISEASES OF CROP-PLANTS 



by sperms produced in an antheridium. Asexual reproduction 

 is by sporangia, which release either a swarm of motile zoospores, 

 or conidia which germinate by the production of a germ-tube. 



The genus Pythium contains the well-known fungus which 

 causes damping-off of seedHngs in temperate coimtries, but has 

 not been found conspicuous in the process in the West Indies. 

 Pythiacystis is a parasite of citrus trees. Albugo is a genus of 

 parasite's producing the white rusts, as that of sweet potato, in 

 which conidia are formed in sori under the epidermis of the host, 

 and oospoies occur embedded in the tissues. The family Perono- 

 sporaceae is of great significance in plant pathology ; its most 

 important genus, Phytophthora, is separately discussed on 



pp. 21-2. 



2. Zygomycetes. 



Sexual reproduction in this group is by zygospores, formed 

 by the coniugation of two equal hyphal parts. Asexual repro- 

 duction, which predominates, is by sporangia, producing numer- 

 ous spores, or less commonly by conidia. Most of the species 

 are saprophytic, like the Mucors, which make a fluffy growth 

 on foods, some are weakly parasitic, as Rhizopus nigricans, 

 which causes soft rots of vegetables. Choanephora infundi- 

 lulifera grows on the flowers of cotton and hibiscus in the East 

 and West Indies. The Entomophthoraceaas are of importance 

 as parasites of insects, and include species of Empusa occurring 

 on the green scale and the sugar-cane froghopper. 



Class II. Ascomycetes. 



In the typical Ascomj^cetes the mycelium is freely septate 

 from the beginning of its development, there is little trace of a 

 sexual process, and the distinctive method of reproduction is the 

 formation of eight spores in a closed tube or sac — the ascus. 

 The asci are commonly produced side by side in a regular layer, 

 the hymenium, which may lie open on the fruiting body or on 

 the substratum, or be enclosed in various ways. 



In addition to the ascospores conidia are usually formed by 

 the mycelium at some stage of its existence, and in many species 

 reproduction is generally effected in this way, the production of 

 ascospores taking place only under special and often rare con- 

 ditions. 



There are a few groups of fungi of somewhat uncertain 

 affinities, of which the fructifications show more or less of an 

 approach to or analogy with ascus formation, which are placed 

 in the sub-classes Hemiascomycetes and Protoascomycetes. 

 The latter includes the yeasts, Saccharomycetaceae, which 

 typically have no hyphae, but consist of rounded cells reproduced 

 vegetatively by budding and in some circumstances forming 

 one to eight internal spores. Nematospora, found in cotton 

 bolls, etc., is at present placed in this family. The ascomycetes 



