282 BACTERIA IN RELATION TO PLANT DISEASES. 



On September 22 the experiment was closed. It failed, apparently because of the 

 extreme heat. The plants were examined frequently but I could find no secondary wilt. 

 August ID was the hottest of 4 very hot days. On that day it was 37° C. for some hours 

 and might have been considerably hotter in this small house for a short time, possibly as 

 hot as 43° C. (the thermal death point of the organism). Thermometers about town regis- 

 tered 98° to 103° I^. and it would of course be warmer in the glass house exposed to the sun. 

 In this connection see thermal tests on p. 293. If I were to repeat the experiment I would 

 liberate the beetles at midnight and divide the house, keeping one part cool. 



Field Observations on August 3, 1896. 



On July 22, 1896, at Mr. Curtis's place 7 miles southeast of Washington, I tried to show 

 Mr. Henry G. Hubbard, the entomologist, my wilt disease and failed. Mr. Curtis had about 

 an acre of pumpkins and squashes of various ages, some covering the ground and in full 

 bloom, others not yet in bloom and only just commencing to "run." There were at that 

 time a very few wilted plants which Mr. Curtis attributed to borers and I to the bacterial 

 wilt but as I could not find sticky slime inside the stems and had no microscope with me my 

 diagnosis was unsatisfactory. The field was revisited 12 days later (August 3). There were 

 then numerous well-developed cases of the bacterial wilt — long shoots with all the leaves 

 drooping, and more interesting still, there were dozens of big vines which showed no general 

 wilt, but had single leaves, usually toward the base of a stem, which bore characteristic wilt 

 patches, i. e., pale-green flabby spots varying from a few square inches to areas as big as 

 the palm of my hand. In all such cases this wilt appeared to have spread from gnawed 

 places.* Moreover I saw the striped cucumber-beetle (Diabrotica vittata) eating holes in 

 such wilted spots exactly as if this part of the leaf were a little tenderer or otherwise 

 more desirable food (see page 215). There were on the vines great numbers of this beetle. 

 A few specimens of Diabrotica duodecempunctata, and a very few of Coreus tristis were 

 observed. 



My prediction that 90 per cent of Mr. Curtis's squash-vines, especially the older ones, 

 would have the wilt by September came true. They were planted on the ground used for 

 squashes 2 years before. Then nearly the whole field contracted the disease although 

 the vines looked well at a date about corresponding to this time. The chief mischief was 

 being done by Diabrotica vittata. 



This insect feeds mostly in the evening, night, and early morning. In the middle of 

 the day it hides away from the hot sun. It specially likes to take shelter inside squash and 

 pumpkin flowers which have recently opened. The older flowers are not to its taste. 



Query: Why can not squash flowers be used as traps for this insect? Hand picking of 

 the staminate flowers in the middle of the day when they inclose from one to a dgzen of the 

 beetles would greatly reduce the numbers of this pest and, in conjunction with removal of 

 wilted vines as fast as they appear, would go far toward checking the spread of the wilt 

 disease. 



This disease may certainly be controlled by destroying the insects which distribute it 

 and to that end their habits should be studied more carefully. 



Inoculations of July 26, 1897. 



Four cucumber-vines (Cucumis sativus) and two vines of a cucurbitaceous plant from. 

 Mexico were inoculated in the hothouse with bacteria from a well-clouded tube of beef- 

 broth (tube 2, July 19) made directly from the sticky interior of a diseased cucumber-stem 

 from Virginia (near Norfolk) . No statement as to the size or age of the plants. The inocu- 

 lations were made by means of numerous needle-pricks on two leaves of each vine. There 



*ln nearly a thousand cucumber leaves examined in July, 1893, on this same farm, for very early stages of the 

 wilt, the gnawings of Diabroticas occurred in the diseased areas and seemed to have preceded the appearance of 

 the wilt, i. e., the gnawed part was dried out as if older than the rest of the wilt. (See plate i, fig. i.) 



