WILT OF CUCURBITS. 



213 



pure culture on steamed potato or nutrient agar, or in beef-bouillon, and make a few deli- 

 cate punctures into a susceptible plant, c. g., into the blade of a cucumber-leaf or musk- 

 melon-leaf. The period of incubation in the writer's experiments has varied from 3 to 3 1 

 days and must depend partly at least on the number of bacilli inserted. Ordinarily when 

 20 or 30 needle-pricks are made the first signs appear in from 5 to 9 days in the punctured 

 part of the leaf (figs. 56, 60, 63, 74). When young cultures are used on very susceptible 

 plants such as Citaimis sativus, Cucumis melo, or Citciirbita joetidissinia, the disease appears 

 with the certainty and regularity of clock-work. It is more difficult to inoculate squashes 

 successfully, at least with some strains of the organism, and this corresponds to the observed 

 fact that they are more resistant in the field. One winter, on several kinds of squashes 

 the writer experienced repeated failures, using virulent cultures obtained from the cucum- 

 ber. The pricked cucumber-plants and muskmelon-plants contracted the disease : the 

 squashes, both summer and winter varieties, inoculated at the same time, in the same 

 way, and from the same cultures, resisted, or only showed traces of primary wilt. This 



Fig. 53.* 



resistance may be due to some extent to varying degrees of virulence on the part of par- 

 ticular strains of the organism; or to varying degrees of resistance on the part of the host. 

 Possibly the squash bacihus should be regarded as a variety. 



In the summer of 1905 the question of the identity of the squash-wilt and cucumber- 

 wilt was gone over once more. Inoculations made into four varieties of squashes, using a 

 strain isolated the previous year from a muskmelon, and proved by numerous control- 

 experiments to be virulent to cucumbers, would not infect squashes. A little later in the 

 season the same squashes were readily infected with a strain of the bacillus isolated from the 

 vessels of a wilting squash-plant found in a garden in Washington, and the same cultures 



*FiG. 53. — A- Winter squash, variety Pikes Peak, No. 215, inoculated Oct. 5, 1895, by needle-pricks on two leaf 

 blades, using viscid white slime from a cucumber-stem. Both leaves contracted the disease and shriveled slowly, one 

 of them being shown at X- Although the plant was under observation for 66 days the only additional signs of disease 

 were conspicuous dwarfing with yellowing of the foliage, especially the lower leaves. Photograph made Dec. lo; 

 plant then cut and examined under nliscroscope, bacteria being demonstrated in a few vessels of several (5) bundles. 

 B. An uninoculated plant from the same lot of seedlings. About one-sixth natural size. 



