PRESIDENTIAL ADDRESS SECTION C. 75 



accoriIin|y, to Burrill (9), aphides and leaf hoppers arc also 

 responsible for transmitting^ pear blig^ht. 



Instances of transmission 'by insects mio;lu l)e multiplied ; 

 curly top of beet (6) is carried by the leaf hopper, Eutcttix 

 tcnclla; it has been shown (57) that leaf-eatin"^ insects may 

 transmit the wilt disease of solanaceous plants, although 

 possibly in the case of the tomato and tobacco wilt more damage 

 is done by parasitic nematodes, which break the root tissues and 

 prepare the way for invasion by organisms present in the soil; 

 and Smith (51) has succeeded in transmitting the black rot of 

 cabbage by means of the larvae of the cabbage butterfly (Plusia). 

 Another well-established case of transmission by insects is that 

 of the wilt of cucurbits (51, 52), where there appears to be an 

 adaption. The striped beetle (Diabrotica znttata), which is 

 chiefly responsible for the spread of the disease, shows a marked 

 preference for the wilted parts of the plant, a preference which 

 is apt to prove fatal to melons, scjuashes, and cucumbers in the 

 vicinity of infected plants. 



It has been shown that slugs {A^i:;rioIimax) carry the 

 cabbage rot (B. campcstrc), and there are indications (52) that 

 molluscs are responsible for spreading the oleander tubercle. 

 There is also circumstantial evidence that birds are capable of 

 transmitting- such diseases as the ])ear blight and the coconut 

 bud rot (28), but this still lacks confirmation. 



Factors Favouring Infection. 



The bacteria i:)athogenic to plants are most active in the 

 f)resence of abundant young and rapidly growing tissue. In one 

 or two instances the place is most liable to attack in the seed- 

 ling stage, e.g., Stewart's disease of sweet corn caused by Bac~ 

 Irrmni Steivarti, and the wilt of tomato and tobacco due to 

 />'. solauaccarum. 



In most of the diseases affecting the parenchyma, the 

 severity of the attack is dependent on the presence of rapidly- 

 growing shoots and fruit, or young leaves. Very numerous 

 cankers can be produced on a young and rapidly growing shoot 

 of a citrus tree by atomising with a culture of Bacterium citri, 

 but it is comparatively difficult to infect a mature leaf. The 

 same is true of various other parenchyma diseases such as the 

 mango blight (Bacillus mangiferac), the angidar leaf spot of 

 cotton { Bactcrimn mnlvaccariuin) . and the bean blight ( Bac- 

 icriuui Phascoli). In this connection some interesting observa- 

 tions have been made on the infection of walnuts with Bacterium 

 Juglandis (57A). The most common and virulent form of 

 infection is at the blossom end near the stigma ; bacteriosis very 

 seriously aft'ects small and rapidly-growing nuts, and when once 

 the organism has penetrated the tissues it spreads rapidly until 

 the nut is seriously weakened and falls. In less severe cases the 

 rut does not fall, but the lesions extend through the hull and 

 shell-forminsr tissues into the kernel, and it becomes deformed 



