18 THE UNIVERSITY SCIENCE BULLETIN. 



When we turn, however, from the damage done by the 

 Cicadellidx by the mere sucking up of the sap of the plant, to 

 the possible and proved relations of the insects to the trans- 

 mission of plant diseases, we enter at once an open and a very 

 important field. We will discuss this phase of the economic 

 importance of the group under the following heads : 



1. Leaf hoppers and bacterial diseases. 



a. Leaf hoppers and curly-leaf. 



b. Leaf hoppers and fire blight. 



2. Leaf hoppers and hopperburn. 



3. Leaf hoppers as possible disseminators of fungous diseases. 



The relation of leaf hoppers to the transmission of bacterial 

 plant diseases opens at once a very large and important field. 

 Who knows but what these and related insects are responsible 

 for many of the diseases that have hitherto baffled the plant 

 pathologist and been the despair of the farmer and horticul- 

 turist? 



Doctor Ball's excellent work has opened up the way for the 

 future on this line. He seems to have proved definitely that 

 such insects may be the normal disseminators of plant diseases, 

 just as, in the case of the mosquito, they are responsible for 

 spreading animal diseases. After years of work on the beet- 

 leaf hopper and its relation to curly-leaf in the sugar beet, 

 among others, the following facts, quoted from his bulletins, 

 were proved : 



"The punctures of the beet-leaf hopper (Eutettix tenellus) 

 cause a specific disease in sugar beets called 'curly-leaf.' 



"Leaf hoppers taken from wild plants did not transmit the 

 disease until they fed on diseased beets. Three hours on a 

 beet rendered them pathogenic, but they could not transmit till 

 after an incubation period of one or two days. 



"Curly-leaf has never been produced except through the 

 punctures of a beet-leaf hopper. If a single leaf hopper is 

 applied to a beet for five minutes, the curly-leaf disease will 

 appear after about two weeks, if conditions are favorable." 



The above facts, added to the fact that the bacterial agent, 

 Bacillus morulans, has been isolated by Boncquet, show con- 

 clusively that these insects may be responsible for similar plant 

 diseases. And since the amount of damage done in such cases 

 is very large, the field should prove both interesting and im- 

 portant. 



