WILT OF CUCURBITS. 215 



In the hothouse, the writer has succeeded in spreading this disease readily by means of 

 leaf-eating beetles {Diabrotica vitatta). Moreover, numerous field observations seem to 

 indicate quite clearly that this is a common method of dissemination. Leaf-eating insects, 

 and especially Diabrotica vitatta (fig. 55), are, I believe, the chief agents in the spread of this 

 disease. They feed readily, and sometimes the writer has thought preferably (fig. 7), on 

 wilted leaves which are swarming with this organism. In this way their mouth-parts can 

 not fail to become contaminated and to serve as carriers of the sticky infection. No other 

 means of dissemination is known to the writer, and this is believed to be the common way 

 in which the disease is distributed.* 



Seasonally the disease does not manifest itself until the leaf-eating beetles have put 

 in their appearance, and this has led to the suspicion that the organism might pass the 

 winter inside the bodies of these hibernating insects {Diabrotica vitatta). As to this nothing 

 definite is known. The greater part of the bacilli as they occur in bouillon are easily killed 

 by freezing, but it is likely that some winter over in the vegetative form or in some more 

 resistant form, in suitable places in the soil. The writer has attempted to plate the organ- 

 ism from the Diabroticas several times but always unsuccessfully, other organisms having 

 speedily occupied the plates. 



Possibly the squash-bug {Coreus tristis) is also responsible for the distribution of this 

 disease, but the evidence on which the writer formerly made this statement does not seem 

 to him as conclusive as it did, i.e., the results obtained may be interpreted in another way, 

 checks in sufficient number not having been made. The subject is open to further experi- 

 ment, with the probabilities in favor of this bug being a carrier of the disease (see page 235). 



The large lady beetle, Epilachne borealis, is a greedy feeder on squash foliage but I 

 have not seen it feeding on the bacterially wilted foliage. 



One experiment only was made with aphides, and this yielded negative results. Four 

 cucumber-plants were sprayed thoroughly on both leaf-surfaces with one part of a potato- 

 broth culture 2 days old diluted with three parts of water. On one of these plants 70 aphides 

 {Aphis gossypii Glover) were colonized, and the other three were held as checks, all under 

 bell-jars. The aphides (which were taken from watermelon-plants) crawled about on the 

 wet surface and immediately began to puncture the plant in many places. In the end they 

 injured it greatly, but no bacterial wilt appeared. This plant was under observation 32 

 days. Two of the check-plants remained free from the disease. The third was free from 

 signs for the first 3 weeks, but lost all its leaves by wilt between the twenty-third and 

 twenty-ninth day, and on examination of its stem at various levels the vessels were found 



*The above mentioned field observations were made by me several years ago. In the summer of 1905, accident 

 enabled me to strengthen these conclusions. Several cucumber-leaves were inoculated late one afternoon in a hot- 

 house containing about 20 well-grown plants. This house unknown to me contained a very fewspecimens of Diabrotica 

 vitatta, and next morning it was observed that the punctured parts of the leaves (those parts wetted by the bouillon 

 culture) had been gnawed out by this beetle, while the remainder of these particular leaves remained intact. It was 

 also noted that leaves on various other plants had been eaten somewhat during the night, and as a result their infection 

 was anticipated. This inference proved to be correct. About a week or 10 days later numerous cases developed. In 

 every instance the wilt began in the leaves bitten by the Diabroticas immediately after the date of placing the infectious 

 material on the leaves which were punctured. The latter also contracted the disease. One of the plants infected by 

 gnawings of the Diabrotica is shown in vol. I, plate 23. The strain used for these inoculations was that mentioned as 

 non-infectious for the squash. 



Two months later in a hothouse remote from the preceding (north part of the grounds U.S. Dept. Agric.) squashes 

 were infected by needle-puncture with the strain already referred to as plated from a squash-stem and found infectious 

 to cucumbers. Wilt of the leaves resulted. These wilted leaves were bitten by Diabrotica vitatta, of which the house 

 contained a very few only. On a side bench about 15 or 20 feet away stood 16 fine young cucumber plants which I had 

 propagated for a second set of experimentswith cucumbers. These were bitten at the same time as thewilting squash 

 leaves, or soon after, and 15 of the 16 plants contracted the wilt within a few days, and in every case it began in the 

 bitten leaves. The earliest (primary) stage of the disease in these insect-inoculated plants is shown in plate 15, fig. 2. 

 In this case also there was no likely source of infection except the inoculated squash-leaves as there were no other 

 cucurbits in the vicinity, and again the beetles were the carriers. Some muskmelons on the same bench were also 

 bitten and inoculated by these beetles. Microscopic examinations were made, demonstrating the bacillus in the vessels, 

 and typical poured-plate cultures were obtained. 



In the 3rd set of squashes inoculated in 1905, after wilted areas had developed on the leaves, it was twice observed 

 that these areas were the only parts bitten by the Diabroticas on those particular leaves. See also pp. 281, 282, and 284. 



