82 DISEASES OF CULTIVATED PLANTS 



instance infection effected by the conidial form that causes 

 an epidemic, or widespread disease. The higher or ascigerous 

 form, on the other hand, usually produces what are termed 

 winter-spores, which remain in a period of rest during the 

 winter, and infect the host-plant the following season. 



There are three principal groups of fungi, Phycomycetes, 

 Ascomycetes, and Basidiomycetes, along with some connect- 

 ing groups that cannot be considered here. 



The Phycomycetes may be considered as the pioneers of 

 fungi as a definite group of plants that evolved from the 

 Algae, and retain many features characteristic of the last 

 named family of plants. Sexual reproduction is present in 

 many of the members, and the reproductive bodies in many 

 instances consist of zoospores or motile bodies, that require 

 the presence of water to enable them to reach their destina- 

 tion and inoculate the host. Most of the species are minute, 

 and when visible to the naked eye come under the category 

 of moulds. As examples may be mentioned, Phytophthora 

 infestans, the cause of the well-known potato disease, and 

 Pythium de baryanum, the cause of ' damping off' in seedlings. 

 The resting-spores are the direct result of sexual action. 

 Conidial forms are numerous. 



The Ascomycetes are characterised by having the spores 

 produced, usually in a definite number, in specialised cells or 

 asci. This group consists of four families, Perisporiaceae, 

 Pyrenomycetaceae, Discomycetaceae, and Hysteriaceae. In 

 the Perisporiaceae the asci are contained in a special, minute, 

 more or less globose structure or perithecium, entirely devoid 

 of an opening, hence the perithecium has to decay before the 

 spores can escape. These fungi are popularly known as 

 mildews, of which the mildew of the hop and the rose are 

 familiar examples. The mildew is the conidial stage of the 

 fungus. Black mildews are common in tropical countries. 

 In the Pyrenomycetaceae the perithecia have a distinct open- 

 ing or mouth, through which the spores escape at maturity. 

 Many species are very minute, forming black dots on stems, 

 wood, and leaves. Others, as the candle-snuff fungus, are 

 large. Many species are destructive parasites. Conidial 

 forms numerous. In the Discomycetaceae the fungus fruit 

 takes the form of a cup or a saucer, sometimes seated flat on 

 the host, at others supported on a more or less elongated 

 stalk. Many species are brilliantly coloured, and the size 

 varies from that of a mere point to two or three inches in 



