WILT OF CUCURBITS. 281 



can not be demonstrated in every one but they occur in 61 of them; no fungi are present, 

 neither are there any insect-injuries. In most cases the bacteria are confined strictly to the 

 spiral vessels of these petioles and they do not occur in all of these, nor in all of the bundles. 

 They are not present in the phloem, the cortical parenchyma or the tissues between the 

 bundles. Summarized, the amount of bacterial infection in the basal part of these petioles 

 is as follows: (i) In a few petioles nearly every bundle is occupied and bacteria occur in 

 many vessels, with cavities in two or three cases; (2) in 5 no bacteria detected; (3) in by far 

 the greater number the bacteria are confined to a few vessels of a few bundles. In groups 2 

 and 3 the wilt can be accounted for only by bacterial occlusions lower down in the stem itself. 



From this experiment we may also conclude that White Wonder is a very susceptible 

 variety and one to be rejected in regions much subject to this disease. 



This single experiment, purposely given in much detail, is sufiicient, in my judgment to 

 establish not only the bacterial nature of this disease but also the general movement of 

 the bacteria through the spiral vessels of the plant. 



Inoculations of July 23, 1896. 



Another attempt was made in the hothouse to transfer the disease to plants by means 

 of the beetle Diabrotica vittata. The- vines used were cucumbers and muskmelons (Nos. 

 379-429). I had grown them from the seed and transplanted them some weeks before into 

 benches filled with good earth. They occupied the whole of a small greenhouse. They 

 had been well watered and the temperature had been high (hot July weather) . As a result 

 the growth had been rapid and at the time of inoculation the vines presented a very thrifty 

 appearance. There were about an equal number of each. The cucumbers were from 4 to 6 

 feet high with hundreds of leaves, some of which were 9 inches broad. There was not a 

 yellow, dwarfed, or fungus-spotted leaf in the whole house and the plants had been remark- 

 ably free from aphides. No water had been put on the foliage. The muskmelons were 

 equally healthy but were younger plants and had not made as much growth. When ready 

 to make the inoculations I collected fifty to a hundred specimens of the striped cucumber- 

 beetle {Diabroctica vittata) from squash-vines in Mr. Curtis's field south of Anacostia, 

 where no wilt had yet appeared. Most of them were taken from the interior of squash 

 flowers where they were in hiding through the day. These were colonized (July 23, p.m.) on 

 five cucumber-leaves cut from plants infected July 16 (third set of inoculations to see if the 

 disease could be cut out). Each of these leaves contained from 10 to 15 sq. cm. of freshly 

 wilted leaf surface. Only the petioles, the wilted partof the blades and a narrow border of the 

 blade surrounding the wilt was put in, so that the beetles would be compelled to feed on 

 diseased tissues or else attack the hard petioles. These beetles feed mostly by night or in 

 the early morning. At 5'' 30" the next morning the beetles had riddled the wilted parts 

 with holes. They seemed to have fed exclusively on the parenchyma of the blade and 

 must have consumed enormous quantities of living bacteria. The mouth parts of every 

 one must have been infected. They were now turned loose on the cucumber and melon 

 vines and began to feed at once. The day was cool and rainy. Several additional coloni- 

 zations were made since it was beheved that there were many chances for failure. Such 

 additional colonizations were made on July 24, 3 p.m., July 26 (12 beetles), July 28 (7 

 beetles), July 29 (150 beetles fed 12 hours on freshly wilted leaves and turned loose at 5 

 a.m., after the leaves were riddled with holes), August i (20 beetles), August 4 (200 beetles 

 fed over night on freshly wilted cucumber leaves and set free at s"" 30° a.m., when the 

 leaves were riddled with gnawings). The beetles were allowed to feed on diseased 

 material from 10 to 19 hours and were then turned loose in the greenhouse. 



On August 4 (s** 30°" p.m.) there had been signs of the wilt for some days but inas- 

 much as the plants had been sprinkled with fine tobacco dust for aphides I thought possibly 

 the injury was due to that and I neglected to examine any of the wilting leaves for bacteria 

 (a serious omission) . 



