82 presidential address section c. 



Bacterial Diseases of Plants occurring in South Africa. 



The number of plant diseases in South Africa which have 

 been actually proved to be due to the action of bacteria is as yet 

 comparatively small, but does not in any way indicate that 

 bacterial disease of plants are of minor importance in this 

 country ; since, in spite of the small number of workers in 

 phytopathology, the number of bacterial diseases known to 

 occur in South Africa somewhat exceeds the number recently 

 recorded (5) as occurrin^^ in Great Britain and Ireland. A short 

 summary of the South African diseases may be of interest. It 

 will be noticed that the majority are diseases of wide distribu- 

 tion which have in all probability been introduced into this 

 country with contaminated seed, or nursery stock. 



Citrus Canker^ Bacterium Citri Hasse. 



Citrus canker (17) is undoubtedly the most important 

 economically. This was introduced from Japan with nursery 

 stock, and was unfortunately disseminated through the country 

 before the highly infectious character of the disease was 

 realised. 



It produces raised, corky areas on the leaves, twigs and 

 fruit of most of the varieties of citrus, being particularly severe 

 on grape fruit, and not only disfigures the fruit but causes it to 

 yellow and fall prematurely. One of the most unpleasant quali- 

 ties of the canker organism is its ability to lie dormant for 

 months, or even years, in the soil or on to the bark of the tree 

 itself, and to become active as soon as suitable conditions occur. 

 In the nursery it is fatal to the buds, causing a mortality as 

 hig-h as 75 per cent. The occurrence of this disease has led to 

 the closing of Rhodesia against citrus trees and fruit i)roduced 

 in the Union, and the American market is also closed against 

 cur citrus fruit until such time as the disease is eradicated. This 

 IS not surprising, as the United States have had themselves to 

 contend with a disastrous outbreak in the Gulf States, and have 

 .spent large sums of money on the eradication of the disease. 



In South Africa citrus canker is confined to the Rustenburg, 

 Pretoria and Waterberg Districts of the Transvaal. One or 

 two outlying cases of infection which occurred, were fortunately 

 ia isolated groups of trees, which were immediately destroyed 

 by fire. The infected districts have been placed in quarantine 

 and a vip^orous campaign of eradication is in progress within the 

 restricted area; the magnitude of the operations may be guessed 

 from the fact that the canker eradication campaign has cost 

 over i6o,ooo up to the present, and that 33,737 infected and 

 contact trees, and 416,850 nursery stock have been destroyed. 



The disaster to the citrus industry in this and other parts 

 of the world caused by the epiphytotics of citrus canker, illus- 

 trates the necessity for a larger number of workers in this field, 

 and for the investigation of problems in phytopathology which 



