99 



in tolal acid conUiil ol' the two is ikjI regarded as suf- 

 ficient to account for llie diflercncc in susccplihilily. 



Fig. G. Cross section of citrus canker on grape- 

 fruit leaf showing enlargement of cells 

 of mesopliyll and collapse of exposed 

 cells. 



Spread of Citrus Canker. 



Rain and dew are pro])al3ly to ])e regarded as very 

 important factors in carrying the disease to unaffected 

 parts of trees in which the disease is already present. 

 Man himself is a very important agent in efl'ecting the 

 spread of canker from diseased trees to nearhy healthy 

 ones. The bacteria may be present in drops of water 

 or in a fdm of moisture on the affected trees, especially 

 if newly formed cankers occur on these diseased trees. 

 In the cultivation and care of the groves man may 

 come in contact with these infected trees and carry the 

 bacteria to healthy ones. The spread of canker to 

 two groves which have come under observation was 

 very probably effected by the human agency. Sterling 

 (5) reports transmission of the disease through hand- 

 ling diseased leaves prior to touching healthy ones. 

 Certain birds and insects may also transfer the organ- 

 ism from diseased to healthy parts. 



Control. 



Efforts toward control have been directed along three 

 lines: exclusion, protection and eradication. 



The further introduction of the disease into the 

 United States from foreign countries and localities has 



(5) Bergcr, E. W., Stevens, H. E., and Sterling. Frank. Citrus 

 canker II. Fla. Agr. Exp. Sta. Bull 124, 27-53, figs. 6-14, 

 1914. 



