8 



large numbers of twig infections both on apple and pear trees. 

 The spread of the blight in nursery stock has been shown by 

 Stewart (49, 50) frequently to be due to the tarnished plant 

 bug, Lygus pratensis; and while he proved many infections 

 to be caused by aphid punctures, he does not believe that aphids 

 play such an important part in the dissemination of the blight 

 as was thought by Jones. 



It Avas early conjectured by Smith (45) and by Kussell (41) 

 that the black rot of cabbage due to Pseudomonas campestris 

 may be transmitted by various insects. Evidence in support 

 of this supposition has been brought forward by several writers ; 

 Brenner (5) showed that aphids may cause new infections, 

 and Smith (46, pp. 306-307) succeeded in transmitting the 

 disease by means of the larvae of a cabbage butterfly, Plusia. 



Insects have been accused of spreading several other bacterial 

 diseases of plants. Smith (43) cited experiments indicating 

 ^hat the wilt of Cucurbits due to Bacillus tracheiphilus may be 

 (Spread by several insects. The same writer (44) also showed 

 that the Colorado potato beetle Leptinotarsa (Doryphora) 

 dccemlifieatay could produce new infections of Bacillus solan- 

 acearum, the cause of a disease of the tomato, eggplant, and 

 Irish potato, and was led to the conclusion that insect enemies 

 are largely responsible for the spread of the disease. J ohnston 

 (27) presents some evidence to show that the coconut bud 

 rot may be carried by insects, and Horne, Parker, and Daines 

 (25) suggest that the dissemination of the olive knot disease 

 may be due to insects. 



Some interesting experiments conducted by Berlese (4) show 

 that not only are yeast cells still viable after passing through 

 the intestine of flies, but that they even multiply and enor- 

 mously increase in numbers while in the alimentary tract. 

 His experiments lead him to the conclusion that ants and flies 

 play a great part in the distribution, preservation, and multi- 

 plication of yeasts. 



A much greater number of instances can be cited of the 

 dissemination by insects of the spores of fungi than of bacterial 

 diseases of plants. One of the oldest and best known ex- 

 amples is that of the ergot of rye. It has long been known 



