42 DISEASES OF TROPICAL PLANTS CH. 



Ascomycetes 



The Ascomycetes contain more species than any other 

 class of fungi. The following orders are the most 

 important in connection with plant diseases : Saccharo- 

 mycetes, Exoascales, Aspergillales, Perisporiales, Hypo- 

 creales, Dothideales, Sphaeriales, Phacidiales, Pezizales, 

 Lichens. 



Order 1. SACCHAROMYCETES. The yeasts are the 

 simplest of the sac fungi, and, in fact, it is very doubtful 

 if they can be properly classed as Ascomycetes. Each 

 plant consists of a single spherical microscopic cell. 

 They grow in or on materials containing sugar, and 

 reproduce by the formation of bud-like cells which from 

 time to time become separated from 

 the parent plant and become indepen- 

 dent plants. The asci are seldom 

 produced, and when so, it is by the for- 

 mation of four cells or spores within 

 the parent plant or cell (Fig. 26). 



The yeasts are used in the manu- 

 facture of bread, and in the brewing 

 of beer. There are a number of species of yeast, some 

 of which grow upon ripe fruits, and no doubt aid to some 

 extent in their decay, but are of little or no import- 

 ance in connection with plant diseases. 



Order 2. EXOASCALES. This order includes a number 

 of fungi which are parasitic on plants, especially those 

 which bear drupaceous fruits. They attack the leaves, 

 causing them to curl, and finally to wither and die. 

 The mycelium of the fungus works within the host 

 plant, but eventually produces a great mass of elongated 

 cells placed at right angles to the surface of the leaf. 

 Each of these cells or sacs contains eight spores from 

 which the plant can be reproduced. The most common 

 species is Exoascus deformans, (Berk.) Fckl., or leaf 

 curl of the peach (Fig. 27), which is probably as widely 

 distributed as the peach itself. 



Certain species also attack plums, cherries, and 



