";) PRESroENTIAL ADDRESS— SECTION C. 



which is usually inch:ded in the Schizomycetes, one common soil 

 organism, A. chrouiogciuis, causes the disease of potatoes com- 

 monly known as scab. 



There are also a number of non-motile, rod-like forms such 

 as Aplanohacter rathayi causing a disease of orchard grass, and 

 A. imchiganense, causing a vascular disease of tomatoes. 



Many of the bacteria, pathogenic to plants, are extremely 

 resistant to drying, but in spite of this fact very few are known 

 to form endospores. I know of only one plant parasite, Bacterium 

 seniinum on Pisum sativitiii. which readily produces endospores 

 in cultures (13). 



Others, e.g., Bac*illus tracliciphilits, which causes a wilt of 

 cucurbits, are very sensitive to dessication and to sunlight, but 

 such organisms as these ."-tand a very poor chance in this country, 

 and no one of them is known to cause serious damage in 

 South Africa. 



Plant pathogens grow without any difficulty on the ordhiary 

 culture media in use in the laboratory, and this is sufficient 

 evidence that they are facultative saprophytes ; but little is 

 known of their growth outside of the host plant in nature, bey^ )nd 

 the fact that certain parasites can survive for a considerable 

 time in the soil. It is not easy to isolate such organisms 

 directly from the soil, but it is known, for example, that tomato 

 rind tobacco seedlings i^lanted in contaminated soil become in- 

 fected with Baclier'utni solanacearum through wounds in the 

 roots (51), and that B. tiiimfacicus in soil from which plants 

 showing crown gall have been removed, readily infects fresh 

 plants put into the same soil (53). 



The soil is the ordinary habitat of Actiiiuinyccs chromogenus, 

 and it only becomes parasitic under certain conditions. 



The optimum temperature for growth usually lies between 

 20° C. and 30° C, very few grow well at blood heat, so that 

 it is not sur]:)rising that inoculations of j^lant parasites into 

 animals, and vice I'crsa, have failed to ])roduce any definite 

 symptoms of disease. 



An interesting exception to this rule is found in the cocoa- 

 nut bud rot conmion in the West Indies, and found by Dr. Pole 

 Evans to be identical with the disease occurring in Portuguese 

 East Africa. This disease is caused by an organism indistin- 

 guishable from Bacillus coli (Esch) Migula, and inoculations 

 into cocoanut seedlings wnth B. coli of animal origin, gave infec- 

 tions similar to inoculations with the organisms isolated from 

 the cocoanut (2(S). 



Agents of Transmission. 



The transmission of disease over long distances is, in the 

 vast majority of cases, through the agency of ])arts of ]:)lants 

 used for pronagation. 



There is no doubt th:it the l)lack rot of ci'uciferous plants 

 was introduced into this country through seed collected from 

 iiifected plants. I have repeatedly isolated the organism 



