38 DISEASES OF CROP-PLANTS 



fruiting bodies without special openings, the spores escaping by 

 the rupture or decay of the wall. 



The largest number of important parasitic Ascomycetes are 

 included in the group known as the Pyrenomycetes, in which the 

 hymenium is enclosed in a spherical or flask-shaped fruiting body, 

 the perithecium, provided with an aperture through which the 

 spores escape. There are three orders of Pyrenomycetes. 



1. Hypocreales. 



The members of this order are broadly distinguished by the 

 light or bright colours of the perithecia, as distinguished from 

 those which are brown or black, and their softer consistency as 

 compared with the hard and often carbonaceous condition 

 common in the following orders. The Nectriae contain the 

 genus Nectria, species of which are very common as saprophytes 

 on dead bark in the West Indies and were at one time held 

 responsible for cacao canker. Some species are wound parasites, 

 but none with this character is known locally. Neocosmospora 

 occurs on the roots of leguminous herbs but is not usually re- 

 garded as a parasite. Ophionectria coccicola and Sphcsrostilbe 

 coccophila are of great importance as parasites of scale insects, 

 and species of Sphserostilbe cause serious root diseases under 

 certain conditions (see Limes and Banana). 



2. Dothideales. 



This relatively small order is characterised by the aggregation 

 of the mycelium into a black stroma in which the perithecia 

 occur as cavities. It is represented among the local parasites by 

 Phyllachora spp. on grasses and sedges. 



3. Sphceriales. 



This is an enormous order with perithecia mostly carbon- 

 aceous in consistency and black in colour. Rosellinia, Melano- 

 psammopsis, Leptosphseria, Gnomonia and Glomerella are ex- 

 amples to which reference is made in Part 2. 



Class III. Basidiomycetes. 



The typical Basidiomycetes are distinguished by the presence 

 in a hymenial layer of crowded sporophores of special type 

 (basidia) each terminating a hyphal branch, usually club-shaped, 

 and bearing on short stalks two or four unicellular spores 

 (basidiospores). 



Parallel with the classification of the ascomycetes, there are 

 recognised in addition to the basidiomycetes proper (Eubasidii) 

 two somewhat anomalous sub-classes, the Hemibasidii and the 

 Protobasidii. The Hemibasidii contain two families only, 

 made up entirely of obligate plant parasites, the Ustilaginaceae 

 (see p 19) and the Tilletiaceae, both giving rise to smut diseases. 

 The mycelium produces the sooty masses of chlamydospores 

 direct, and these on germination produce a short filament on 



