THE SPREAD OF PLANT DISEASES. 123 



Pear Blight. — I will not uudertake to say who first ascribed 

 the spread of this disease to insects, but the honor of clearly 

 proving it to be so disseminated belongs to Mr. Merton B. 

 Waite. He it was who first isolated the pear blight germ from 

 the mouth part of bees which had visited blighting pear flowers ; 

 who saw bees pass from such flowers to healthy ones and the 

 blight subsequently appear in the latter ; and who afterwards 

 showed conclusively on a large scale that pear flowers covered by 

 mosquito netting always remained free from blight, while the 

 unprotected, insect-visited ones blighted freely. There is not 

 only no doubt that pear blight is spread through the agency of 

 insect visits, but so far as he has been able to determine, it is 

 never disseminated in any other way. The organism exudes 

 from the tree in the form of small sticky or gummy masses 

 which are not likely to be blown about by the wind, and is easily 

 destroyed by drying. The disease occasionally winters over in the 

 tree, and so far as Mr. Waite has been able to determine all the 

 spring outbreaks of pear blight start from these hold-over cases, 

 as a result of insect visits, and not from the soil. Indeed, we 

 have as yet no evidence that pear blight lives over in the soil. 



Bacterial Wilt of Cucumbers, 3ficsk melons, JPujnpkins, and 

 Squashes. — This is a common disease in the northern United 

 States, and often does great injury. It is due to a very sticky, 

 white micro-organism which fills the water ducts, and thus brings 

 about a sudden collapse of the plant. I have experimented Avith 

 it extensively since 1893, and find it to be readily communicated 

 by the striped cucumber beetle and sometimes also by squash bugs. 

 Wilting vines are very full of virulent, sticky germs, read}" to be 

 carried away on the beak or jaws of the first visiting insect and 

 deposited on the surface or in the interior of the next plant that 

 is bitten. On one occasion I examined nearly a thousand freshly 

 blighted leaves, and found small gnawed places inside of the 

 blighted area of every one, and in such relation thereto that the 

 wilt appeared to have proceeded outward from the bitten places. 

 Subsequently I produced the disease by allowing striped beetles 

 and squash bugs to feed on diseased leaves and then on healthy 

 plants. My greenhouse inoculation experiments with this bacillus 

 now exceed four hundred, but I have never had the disease escape 

 from inoculated plants to controls, and I believe that in the field 

 it is spread almost wholly, if not exclusively, by insects. In 



