46 DISEASES OF CROP-PLANTS 



diseases the information on these subjects already given for 

 fungus diseases. 



The bacteria which give rise to plant diseases form but a 

 minute fraction of the species known to exist, and with the ex- 

 ception of those involved in the rotting of ripe fruit, of storage 

 organs like the potato tuber, or of masses of soft tissue such as 

 the heart of coconut palms, they are parasites specific to the 

 particular plant or disease with which they are associated. A 

 single species may, however, as in the case of a fungus, give rise 

 to several forms of affection on the same plant (as in angular 

 spot, black arm, and boll disease of cotton) or produce the same 

 type of disease on a variety of plants (as in crown gall). 



Apart from the parasites there is evidence of the existence 

 of specific epiphytic bacteria on the surface of leaves or other 

 organs. A good instance is afforded by the organism which 

 appears to be always present, in the West Indies as in the United 

 States, on the cotton plant, and takes some part in the infestation 

 of the contents of the boll when bug punctures permit its access. 



One of the main objections to the possibility of bacterial 

 diseases was the supposed absence of any means of penetration, 

 and this was so far justified that the ability to penetrate cuticle 

 possessed by some fungi does not seem so far to have been 

 demonstrated in bacterial parasites. The latter can, however, 

 find their way through apparently uninjured cell- walls, for they 

 are often very evident inside, but it is not clearly known how 

 they do it. Some species invariably enter the plant by way of 

 wounds, so that insect injuries are frequently of great importance 

 in the incidence of bacterial diseases, others are capable of entry 

 by way of the stomata (angular leaf spot), the water pores of 

 leaves (black rot of cruciferae), the nectaries of flowers (blossom 

 blight of pears), and possibly the pistil (bacterial boll-disease). 



The tissue of the living plant most liable to attack is the 

 parenchyma of storage organs or of young developing shoots, 

 and two types of infestation may be recognised. The tissue may 

 be destroyed by the action of secretions which dissolve the middle 

 lamella (the material which cements together the walls of the 

 separate cells) or the bacteria may enter into relations with the 

 cells which induce abnormal growth and proliferation, resulting 

 in the formation of galls or swellings. The latter type may be 

 compared with the most advanced form of fungus parasitism, 

 in which the parasite derives its food from cells which continue 

 their existence. 



The most generally destructive type of bacterial disease is 

 caused by the vascular parasites, of which Bacterium solanace- 

 arum on the tomato is an example met with in the West Indies. 

 In this type, although there is some amount of invasion of the 

 parenchyma, the principal seat of infestation is the vascular 

 bundles, the conducting elements of which are affected and often 

 blocked by the parasite. 



