30 BACTERIAL DISEASES OF PLANTS 



Finally he captured bees that had visited such infected blossoms, 

 excised their mouth parts, and from these, on agar-poured 

 plates, obtained Bacillus amylovorus, with colonies of which he 

 again produced the disease. These experiments were done in 

 several "v^idely separated localities with identical results. I saw 

 them and they made a great impression on me. 



The writer has since proved several diseases to be transmitted 

 by insects, notably the wilt of cucurbits, and here the trans- 

 mission is not purely accidental, but there appears to be an adap- 

 tation, the striped beetle {Diabrotica vittata) chiefly responsible 

 for the spread of the disease being fonder of the diseased parts 

 of the plant than of the healthy parts. This acquired taste, for 

 it must be that, works great harm to melons, squashes, and cu- 

 cumbers. Whether the organism winters over in the beetles, 

 as I suspect, remains to be determined. Certainly the disease 

 appears in bitten places on the leaves very soon after the spring 

 advent of the beetles, i.e., before they have had opportunity 

 to become infected from newly wilted cucurbits: 



In the summer of 1915, Mr. Frederick V. Rand, assistant pathologist in my 

 laboratory, undertook, at my suggestion, two series of experiments on Long Island, 

 N. Y., to determine the truth or error of this hypothesis. His results, which have 

 afforded a striking confirmation of my views, may be summarized in brief as 

 follows: 



In two cucumber fields where 75 per cent of the plants contracted the bacterial 

 wilt disease in 1914 and where, up to September 1, 800 plants or about one in four 

 contracted it in 1915 (later cases no doubt occurred but no further record was 

 attempted owing to the appearance of the downy mildew), 180 plants kept inside 

 of 50 large insect cages distributed at uniform distances through the two fields 

 remained entirely free from the disease, except in two cages. In one of these two 

 cages Diabrotica vittata was purposely introduced when the plants were only 2 to 

 3 inches high, and before there was any of the disease on the check plants. In 

 this cage all of the plants contracted the disease which first appeared in bitten 

 places on the leaves. In the other cage, a single beetle of that species penetrated 

 accidentally later in the season (when the disease was quite prevalent outside on 

 the checks) and gnawed and infected a single plant before it was discovered and 

 removed, the other, unbitten, plants in the cage remaining healthy. The beetles 

 were collected in one of the two experimental fields, remote from other plantations, 

 at a time when the check plants were small and all still free from the disease. It 

 is believed, therefore, that they hibernated in the vicinity and that their last 

 source of infection was diseased plants of the preceding year, i.e., that they carried 

 the wilt organism over winter in their bodies. That not all hibernated beetles 

 transmit the disease is shown by the fact that some were liberated at the same 

 time in three other cages but the plants remained healthy, and by the additional 



