290 Disease-Producing Microorganisms 



Comparison of a large number of strains of actinomycetes isolated 

 from scabby potatoes shows that one is dealing here not with a 

 single species, but with a large group of organisms. As many as 

 thirty species of actinomyces have been described; these are be- 

 lieved to be causative agents of potato scab, the type of lesion being 

 influenced by the species. The existence of more than one type of 

 actinomyces capable of causing mangel-beet scab has also been 

 suggested. 



The formation of "pox" on sweet potatoes was shown to be due 

 to an actinomyces found in the soil. This organism has been desig- 

 nated as Actinomyces poolensis. 



Bacteria capable of causing plant diseases include Bacillus tiime- 

 faciens, the crown-gall organism; B. campestris, causing black rot of 

 cruciferae; and a number of other bacteria causing bacterial rot of 

 potatoes and other plants. 



Among the plant and animal pests present in the soil, we find also 

 protozoa, nematodes, wireworms, crustaceans, myriapods, and in- 

 sects. The plant-parasitic nematodes include Heterodera schachtii, 

 causing the disease of mangels; Tylenchiis tritici on wheat; Het. 

 radicicola, causing swellings or knots on roots of tomatoes and 

 cucumbers; Tylenchiis dipsae, causing the root knot on oats, tulip 

 root, and clover; Aphelenchus olesistus, causing leaf blight; and 

 Tylenchiis dipsaci, capable of causing galls on stems, leaves, and 

 tubers of potatoes. 



The plant-parasitic insects include a number of Coleoptera, Lepi- 

 doptera, and Diptera. Wireworms may cause considerable damage 

 to crops, as when old meadows are plowed under and planted to 

 corn or potatoes. These worms are capable of traveling considerable 

 distances below the surface of the soil. The nematodes and other 

 worms, as well as the various insect pests, are favored by the addi- 

 tion of organic matter. 



There is an association between the intensity of plant disease 

 caused by Het. schachtii and the cyst content of the soil, in those 

 cases where the disease was observed recently. The nematode dis- 

 ease may be due to an association between the fungus Rhizoctonia 

 solani and Het. schachtii. Infestation of soil with Phylloxera has 

 been found to be most frequent in heavy-textured soils and less 

 frequent in sandy soils. 



There is also a possibility that the soil harbors organisms which 

 are parasitic upon plant and animal parasites, as was shown to be 



