216 



BACTERIA IN RELATION TO PLANT DISEASES. 



to be full of bacilli. That this diluted culture was virulent is also shown by the fact that 

 out of 10 large cucumber-plants inoculated from it the same day by needle-puncture, 8 

 promptly contracted the disease. Up to this time therefore, the weight of the evidence 

 favors the view that aphides do not play any part in the dissemination of this disease. 

 Further experiments should be made. The fact that one check-plant contracted the dis- 

 ease in some unknown way shows that at least occasionally the disease may be induced by 

 simple spraying in the absence of suctorial insects, and this is what invalidates the experi- 

 ment with the squash-bugs. Some of the sprayed plants on which they were colonized 

 contracted the disease, but the additional inference is of the post hoc sort. 



The disease seems to be worse in moist, warm weather than in dry cool weather, at 

 the same time excessively hot weather seems to be unfavorable to its spread. A soft 

 watery condition of the tissues is believed to be favorable to the spread of this disease. 

 In a number of instances it has been observed to do most injury in wet seasons, but it is 

 not restricted to such seasons. Possibly, the greater injury during rainy periods is attri- 

 butable chiefly to the greater number of infections, favored by cloud-screens and the 

 moisture of the air. In a dry air many infected wounds probably dry out before the 

 bacillus has secured a foothold, or are rendered sterile by sunshine. The bacillus is so well 



distributed that if it were not for some such re- 

 straining circumstances it is doubtful if ordinary 

 cucurbitaceous plants could be grown at all in the 

 Northeastern United States. 



Aside from suitable weather-conditions and 

 the propagation of extra sensitive varieties, which 

 should of course be avoided, the conditions most 

 favorable to the spread of this disease, so far as yet 

 known, are the multiplication of insect-depredators, 

 particularly the leaf-eating beetles. Probably 

 puncturing insects do less harm. Among growers 

 of these plants thereis, however, a widespread belief 

 that Coreus tristis, the squash-bug, "poisons the 

 plant, " and this poisoning, as it is called, might well 

 be the transmission of this bacillus. Further observations and experiments are necessary. 

 The extent of the vascular infection soon after the first secondary wilt supervenes was 

 studied in plant No. 18 a diagrammatic sketch of which is shown in fig. 59. This plant was 

 inoculated by needle-pricks on the blade of one leaf. The second day after secondary wilt 

 appeared, the entire plant was fixed in alcohol. Subsequently, portions of this plant were 

 infiltrated with paraffin, cut, stained and studied for the presence of the bacteria at the 

 different levels indicated in the figure. These sections show that in the course of the 15 

 days which intervened between the needle-pricks on the leaf-blade at X and the fixing of 

 the tissues in alcohol, the bacteria had penetrated into some portion of the vascular system 

 of nearly every organ of the plant, the only exceptions being one lower leaf, certain tendrils, 

 and a few centimeters of the undeveloped stem at the extreme top of the plant. This plant 

 was inoculated October i, 1894; wilt first appeared October 9 (in the pricked leaf) ; a trace 

 of secondary wilt appeared October 14 and was well developed on October 15 in the ist leaf 

 above and the ist below the inoculated leaf; on October 16 the plant was put into alcohol. 

 When pricked the inoculated leaf was large and was near the apex of the vine. The infec- 

 tious material came from vine No. 2. 



The foregoing conclusions respecting the etiology of this disease are drawn largely 

 from the following: 



Fig. 55.* 



*FlG. 55. Stages in life history of Diabrotica vittata, the striped cucumber-beetle: a, mature insect; b, larva, c, 

 pupa; d, egg; e, sculpture on egg. a, b, c, enlarged; d, more enlarged; e, highly magnified. After Chittenden. This 

 beetle is the principal disseminator of Bacillus tracheiphilus. 



