CONSPECTUS: AGENTS OF TRANSMISSION 33 



fact that on the checks the disease first appeared on a few only of the many bitten 

 plants, and from these few was subsequently spread to many others by the 

 beetles, the disease appearing everywhere first, in bitten leaves, a few days after 

 they were gnawed by the beetles. From these experiments we may conclude: 



1. Striking confirmation of my statements respecting summer distribution 

 of this disease by Diabrotica vittata. 



2. Freedom of plants from disease when protected from this beetle by wire 

 screens, although presumably growing in infected soil. 



3. Inability of aphides and flea beetles to cause the disease, since they entered 

 the cages to some extent but did not act as carriers. 



4. Evidence that the disease is not air-borne or water-borne. 



5. Proof that the disease is not transmitted by way of the soil at least not in 

 the absence of insects. 



6. Strong circumstantial evidence that Bacillus tracheiphilus winters over in 

 "bacillus carriers," i.e., in certain beetles which function as the spring distributors 

 of the disease. 1 



In 1897 I observed and proved experimentally that molluscs 

 sometimes transmit brown rot of the cabbage, and in 1913 

 I saw indications in Southern France which lead me to think that 

 snails are responsible for the spread of the oleander tubercle, 

 i.e., I saw them eating both sound and tubercular leaves, and 

 found young tubercles developing in the eroded margins. 



Parasitic nematodes break the root-tissues and open the way 

 for the entrance of Bacterium solanacearum into tobacco and 

 tomato, as was first observed by Hunger in Java and later by 

 myself in the United States. One of the serious problems of 

 plant pathology is how to control the nematode, Heterodera radi- 

 cicola, not only because of its wide distribution on a great variety 

 of cultivated plants and the direct injury it works but also on 

 account of the often very much greater injury it causes through 

 the introduction into the roots of the plant of bacterial and fun- 

 gous parasites. The man who shall discover an effective field 

 remedy will deserve a monument more enduring than bronze. 

 Parts of our Southern States in particular are overrun by this 

 parasite. In the hothouse, of course, it may be controlled by 

 steaming the soil, and in other ways. 



Much remains to be done before we shall know to what ex- 

 tent fungous parasites function as carriers of parasitic bac- 

 teria. H. Marshall Ward sought to explain the presence of 



1 In most beetles, as shown by Rand's further studies (which will appear in 

 Phytopathology) the ingested bacilli are promptly destroyed: in a few, they per- 

 sist for a long time and are voided in the feces, which are then infectious. 



