72 PRESIDENTIAL ADDRESS — SECTION C. 



The most strikinof example of the transmission of disease by 

 means of nursery stock is afforded by the introduction and dis- 

 semination of citrus canker, both in this country and in the 

 Southern States of America. Citrus canker was introduced 

 from the East, where it is now known to have occurred in Japan 

 as early as 1863 (33) ; the South African outbreak has been 

 traced to nursery stock introduced from Japan in 1905 or 1906 

 (17 b), and nearly every case of canker in a new locality can 

 be traced to the introduction of infected nursery stock (young^ 

 trees or budwood). 



Other cases of dissemination of disease from the nursery can 

 be quoted ; apple g"rafts are known to have transmitted crown 

 gall ; the yellow disease of the hyacinth is carried in the bulb 

 (53), s"d in the case of the black rot of cabbage and cauliflower, 

 the organism is disseminated in seedling plants by nursery-men 

 who supply small town gardens. 



The bacterial diseases affecting potatoes are carried in the 

 tuber, and those which occur in South Africa (Bacterium sola- 

 naccarum and Bacillus atroscpticiis), being of world-wide distri- 

 bution, have no doubt been introduced into this country with 

 infected " seed " before any adequate inspection was organised. 



The soil around infected plants may serve for years as a 

 source of infection. This is particularly the case in orchards 

 and nurseries where citrus canker has occurred. When a tree 

 is destroyed, small pieces of root are almost unavoidably left in 

 the ground, and from these sucker shoots develop, which usually 

 show a large percentage of citrus canker lesions soon after they 

 push through the soil; it is also not uncommon in badly-di?cased 

 orchards to find branches touching the ground, or even partially 

 covered with soil, very heavily infected. I have myself 

 succeeded in infecting lemon seedlings from soil taken from an 

 orchard one year after all citrus trees had been destroyed ; and 

 recentlv sucker shoots pushing through the soil eighteen nnjiiths 

 after the trees had been destroyed have developed the disease. 



The crown gall organism (B. tumefaciens) survives in the 

 soil for a considerable time. Smith (53) cites the case of a 

 grower of nursery stock who found part of a block qf apple 

 trees badly affected with gall. The trees were dug up, and 

 the ground left to rest a year; then peacli trees were planted; 

 in that portion where the apple trees had been diseased, most of 

 the i>each trees became affected with galls, and were worthless. 



Bacterium solanacearmn in the soil enters the roots of 

 tomato and tobacco plants through wounds such as might be 

 caused in transplanting, but some organisms (52), e.g.. B. amylo- 

 vorus appear to die out quickly in the soil, and no case of pear 

 blight by soil infection is known. 



Cases of infection by water used for irrigation are reported 

 by Honing (27) in the case of the tobacco wilt {B. solanacearum) . 

 In Sumatra the fields are watered from wells, some of which 



