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Thorn scratches on citrus leaves or fruit form a ready 

 means of entrance for the canker organism, and many of the 

 lomons grown in the Groot Drakenstein valley become infected 

 with Bacillus ciirmiaculans through thorn scratches. 



It has already been mentioned that Ba'ctcriuiii solanacearum 

 and Bacillus irachciphilus enter the host through wounds caused 

 by leaf-eating insects, and that the punctures caused by sucking 

 insects alTord a means of entrance into pear shoots for Bacillus 

 amylovorns. 



Until recently it was thought that infection through wounds 

 was the rule, and only a few instances were known where infec- 

 tion took place through the natural openings of the plant. In the 

 light of recent research, however, it seems probable that in 

 diseases afifecting the parenchyma, at least, infection usually 

 takes place through the natural openings of the plant, and most 

 frequently through the stomata. 



Stomatal infection has been proved in the cases of citrus 

 canker {Bacterium citri), the black spot of plum (B. pruni), the 

 angular leaf spot of cotton (B. nialvacearum) and others; and 

 i:. all probability will be proved in many other diseases where 

 artificial infection of leaves and shoots can be brought about 

 without wounding. 



Smith (51) has shown that in the black rot of the cabbage, 

 due to B. campcstre, the majority of the infections begin in the 

 v.ater pores; these are grouped on the margins of the leaf, and 

 from these points the bacteria make their way into the vascular 

 system of the leaf, and so pass into the stem and other parts of 

 the plant. 



Organisms causing blossom blights enter the flower through 

 the nectaries, as in the case of Bacillus auiylovorus and Bac- 

 terium, nediarophilum. The bacteria multiply in the nectaries 

 and finally destroy the blossoms. 



Period of Latency. 



Some time elapses between the actual entrance of the 

 bacteria into the plant and the appearance of external symptoms 

 of disease. As in the case of diseases of animals this may be 

 very short or surprisingly long. 



Given suitable conditions tlie soft rot bacteria visibly afifect 

 the tissues in 1-3 days. 



In the case of leaf s])ot organisms, the first visible sign of 

 infection is the appearance of dark green areas, which are water- 

 soaked or oily in appearance. This symptom may be visible in 

 3-5 days (citrus canker), 5-10 days (angular leaf spot of cotton, 

 bean blight), or may take as long as 3 weeks (mango blight). 



Some of the vascular diseases develop even more slowly ; 

 in the case of Stewart's disease of sweet corn (52) infection 

 usually occurs in the seedling stage, but the maize may be three 

 months old and six feet tall before it finally succumbs. 



