FUNGI AND PLANT DISEASE 423 



vised. These four diseases by no means exhaust the list of Phyco- 

 mycete plant pathogens, which include such other important 

 diseases as club root of cabbage, the white rust of crucifers 

 and powdery scab of potatoes. 



The Ascomycetes vary greatly- as to their body size and com- 

 plexity, ranging all the way from the microscopic single celled 

 yeast plant to the well known fleshy, edible, morel. They all 

 have in common the sac like reproductive structure, or ascus, 

 within which are borne characteristically eight ascospores. The 

 asci are often arranged in a compact layer, either underneath 

 the epidermis of the host plant, or in special, variable repro- 

 ductive structures which may be flask, disc, or cup shaped, and 

 situated on the surface tissues of the host. Almost one half of all 

 known fungi belong to this group, the greater part of which are 

 saprophytes, but which contains many plant diseases. 



Perhaps the most recent Ascomycete disease to make its 

 appearance in this country is the so called Dutch Elm disease, 

 which did not originate in Holland, but probably in China, 

 being brought into the United States about ten years ago on 

 elm burl logs imported from France for conversion into fancy 

 grained veneers. The disease first gains entrance to the elm 

 trees by way of breaks in the bark, often caused by insect borings. 

 Two species of elm bark beetles are known to be active in 

 spreading the disease, and it is possible that other insects, as 

 well as birds, may transport the fungus spores from one locality 

 to another. Within the tree, the fungus grows rapidly in the 

 sapwood, causing a plugging of the water conducting vessels, 

 and a subsequent wilting and drying of the green leaves beyond 

 the point of attack. This wilting is one of the symptoms of the 

 disease, but drought and several native diseases of minor impor- 

 tance produce the same external symptoms. When the limb or 

 trunk is cut, a ring of brown spots is evident in the sapwood, but 

 this internal symptom is true also of other wilt diseases, so that for 

 final determination it is necessary to isolate the causal organism 

 and grow it in the laboratory in order to secure the fruiting struc- 

 tures which will give its actual identity. Spores are produced on 



