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The organism which appears to be the first bacillus to be described as 

 attacking Cactaceae, and which we name B. cacticidus, will grow on all ordinary 

 laboratory media, but when grown in bouillon or agar its virulence is very con- 

 siderably diminished. The greatest virulence is manifested by organisms which 

 have been grown on a prickly pear decoction for two days at room temperature 

 20-25° C, and in such fluid there is a production of abundant gas. The decoction 

 was made by boiling sliced fragments in distilled water until a very viscid extract 

 was produced; it was then strained and sterilized. 



If organisms from such a culture medium be inoculated into the parenchyma 

 in quantity, incubation at Z7° C. for 48 hours commonly results in Jhe almost 

 complete disintegration of the segment, a very pronounced effect being obtain- 

 able even within 24 hours. On the other hand, inoculations from agar or 

 bouillon cultures failed to cause the disease in Opuntia inermis, 0. strict a, and 

 O. megacantha, even after a long interval, whereas segments of O. tomentosa 

 inoculated at the same time from the same cultures and kept under the "same 

 conditions, became completely disintegrated in from seven to fourteen days, 

 while the slimy liquid produced by the decomposition of the last-named species, 

 when inoculated into the other three Opuntias, produced a rapid decay which 

 destroyed the infected segments in about seven days during midsummer. Dis- 

 eased material desiccated for months retains its capability to reproduce the 

 malady if inoculated into fresh segments of prickly pear. 



Attempts to produce infection through the stomata either by smearing or 

 by spraying organisms from active cultures over the uninjured surfaces of 

 segments failed, while detached cladodes, if allowed to dry sufficiently to heal 

 the scar at the joint where broken from the parent segment, did not become 

 infected when the lower end of such segment was immersed for weeks in the 

 liquid from decomposed material, though such liquid readily produced disease 

 if an injury were made in the submerged part of such segment. 



The organism is an actively motile, gram-negative, aerobic, and facultative 

 anaerobic bacillus. When grown on agar it is almost coccoid, measuring about 

 8 /A in diameter, but in liquid media it forms short rods occurring singly or in 

 pairs and measuring 13 /a by '8 //,. Neither spores nor capsules were observed. 

 Smooth greyish-white colonies are produced on +15 acid agar slopes (Ayres' 

 scale), while on poured plates they are round, raised, wet, shiny, and dirty white, 

 inclining to yellow. 



Inoculation from a 24 hours' broth culture gave an acid reaction on the 

 first day, and persisting for at least four days, in glucose, saccharose, mannite, 

 and salicine, but none in maltose, lactose, dulcite, and arabinose. The reaction 

 in litmus milk caused an acid clot on the second day, the clot remaining un- 

 changed at the end of a week. 



The optimum temperature was found to be between 28° and 30° C, 

 which is strong evidence in favour of the view that its native home is the tropical 

 portion of America, probably in the vicinity of the Carribean Sea. 



It was ascertained experimentally that the cactus "moth borers" Melitara 

 prodenialis, from Florida, and M. junctolineella, from Texas, as well as another 

 Pyralid (Phycitid) moth, Mimorista flavidissimalis, from Texas, were all able 

 during their caterpillar stages to transmit the organism from diseased to sound 

 portions of the same or other plants, the bacteria probably being carried on the 

 hairs and skin of the larvae and deposited in the injuries or tunnels made by 

 them. Under such circumstances, the lesions occasioned by the insects are 

 greatly aggravated; in fact, in the case of very young larvae (first instar) of 

 Melitara, the destruction and liquefaction of plant tissue may be so rapid as to 

 imprison or even drown them while in their tunnels. This does not happen with 

 older caterpillars which migrate from the decomposing segment and carry the 



