Chapter X 



ACTINOMYCETES AS CAUSATIVE AGENTS 

 OF PLANT DISEASES 



Actinomycetes are known to occur in the outer layers of roots and 

 tubers, particularly in the potato plant. According to Lutman (268), 

 the filamentous nature of irregular lines and simulating cell walls in 

 potatoes and in other plants is actually due to actinomyces mycelium; 

 the infection is said to occur throughout the leaves, flowers, and other 

 parts of the potato plant. No explanation was suggested for the effect 

 of these actinomycetes on the cell walls of plants. Other observers, 

 however, believed that the impression of filaments was not due to acti- 

 nomyces mycelium but to the staining of the middle lamella. 



For staining purposes, Richards (359) impregnated the potato 

 scab organism with carbol-auramin; when exposed to ultraviolet radia- 

 tion, the mycelium produced a bright yellow fluorescence. By his 

 method, the sections are stained for 4 minutes in a solution contain- 

 ing 100 mg. carbol-auramin, 3 ml. liquefied phenol, and 97 ml. distilled 

 water. They are washed, destained in a 0.5 per cent solution of sodium 

 chloride in 70 per cent alcohol and 0.5 ml. concentrated hydrochloric 

 acid per 100 ml., washed, and mounted in glycerin. The staining is 

 done at room temperature, and no counterstain is used. The marked 

 contrasts of the stained filaments permit the ready localization and 

 study of the micropathology of the tissue by means of a simple fluores- 

 cence microscope. This fluorescence technic appeared to confirm Lut- 

 man's conclusion that the filaments are intercellular and grow within 

 the middle lamellae. 



Comparatively few plant diseases are known to be caused by actino- 

 mycetes. These diseases are frequently known as actinomycoses. The 

 common scab of potatoes and that of sugar beet occupy a prominent 

 place. It has also been claimed (178) that a variety of other plants 

 are subject to infection either by the potato scab organism or by other 

 actinomycetes. 



An extensive literature has accumulated dealing with the nature of 

 the causative agent of scab, the course of development of the disease, 

 the effect of environment, especially soil conditions, and methods of 

 control. Only a few selected references to this literature will be cited 

 here. 



Potato Scab:— 



Method of isolating actinomycetes jrovi scab lesions. Sew ex2\ pro- 



