DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 53 1 



ing leaves and mosses may be coated with a mealy powder derived from 

 the gills of several sporophores directly over them. 



Oat (A vena saliva, Linn.) 



Rust {Puccinia coronifera, Kleb) .■ — The oat rust, or crown rust, affects 

 oats and also several other grasses. The summer stage appears on 

 oats just prior to the period of ripening where it forms an elongated 

 uredinium of an orange color on the leaves and sheaths. The globular 

 spores germinate readily. The teliospores are formed later as black 

 spots around the edge of the uredosori. As the teliospores bear at 

 their apex a crown of blunt projections, or processes, the common name 

 of "crown rust" has been applied. Such winter spores remain in a 

 resting condition until the following spring, when they germinate in the 

 usual way. The basidiospores, which are formed from the basid- 

 ium, or promycelium, begin growth on the leaves of the buckthorn, 

 Rhamnus cathartica, where within eight to ten days cluster cups 

 (yEcidium catharticce) appear. The aeciospores germinate readily and 

 are blown to the oat and other grasses, such as perennial rye grass, 

 Yorkshire fog, so that at least eight forms of the species limited to 

 certain hosts have been distinguished. The measurements of its spores 

 are as follows: ^ciospores, orange, vermiculose, 16 to 25/x by 12 to 20^1; 

 Uredospores globose to obovate, echinulate yellow, 18 to 2 7yuby 16 to 

 24/i; teliospores brown, two-celled, crowned with rough projections; 

 approximately 35 to 60^1 by 12 to 22)u. 



Smut (Ustilago avencB and U. levis). The a-ppearance of this dis- 

 ease is illustrated in the figures (Fig. 191). 



Onion {Allium cepa, L.) 



Smut (UrocysHs ceptdcB, Frost). — This fungus, probably of Ameri- 

 can origin, is found in the onion growing districts of the eastern United 

 States where it has been known for about 50 years. The smut fre- 

 quently appears soon after the first leaf appears, and is first in the form 

 of dark spots at the base of the first leaf and on succeeding leaves, as 

 they make their appearance. These spots are followed by longitudinal 

 cracks, which show, a granular spore powder associated with threads 

 of fibrous tissue. The spore powder under the microscope is found to 

 consist of the spore balls, which number several compacted cells, the 



