26 BULLETIN 828, U. S. DEPARTMENT OF AGRICULTURE. 



hundred have been carried continuously in culture for considerable 

 periods — in some cases for nearly four years. 



It was early noted by Dr. Erwin F. Smith 1 that his isolations of 

 Bacillus tracJieipMlus from squash would always infect cucumber, 

 but that his isolations from cucumber usually gave no infection at all 

 on squash and at most only a slight local wilting near the point of 

 inoculation. It seemed possible from these results that there might 

 be at least two true biological strains of the organism. 



With a large number of isolations from different hosts and localities 

 at hand it seemed well worth while to investigate this question 

 further. For this purpose relative virulence tests were made of 

 large numbers of isolations grown on the same medium and in cultures 

 of the same age inoculated in as nearly an identical manner as pos- 

 sible into one of the host varieties as an indicator. Chicago Pickling 

 cucumbers were selected as being a representative variety of one of 

 the most susceptible host species; and in each series of inoculation 

 tests plants 12 to 18 inches high and of like age and similar vigor 

 were inoculated simultaneously with cultures of the different isola- 

 tions. From two to eight series of these tests were made, and in 

 each series two or more (usually two) plants were inoculated with 

 each isolation under study. Experience showed that within a 

 variety a fairly constant reaction was given to any one isolation of the 

 wilt organism, provided the plants were of comparable age and vigor. 

 Occasionally an individual plant would show a wide variation from 

 its fellows in reaction to a particular isolation, but since considerable 

 numbers of plants were used these comparatively rare aberrant cases 

 did not alter the end results of the study. 



In each series of tests careful observations were made of the abso- 

 lute and relative progress of the disease, special attention being 

 given to three cardinal points — (1) the number of days after inocula- 

 tion when incipient wilt was noted, (2) the total number of days after 

 inoculation required for actual wilting of the larger part of the inocu- 

 lated leaf, and (3) the total time after inoculation for the wilting of 

 the entire plant. By, "incipient wilt" is meant either a slight loss of 

 turgor without actual wilting or a slight indication of wilt immediately 

 around the point of inoculation. For purposes of ready comparison a 

 "virulence index" was worked out for expressing the degree of 

 relative virulence of each isolation to the indicator used. This 

 virulence index was secured in each instance by adding together the 

 three cardinal points above mentioned. These figures as they stand 

 express the relative virulence in inverse ratio, the smaller figure 

 expressing the greater virulence. In order to show the relation 

 more clearly, expressing the greater virulence by the larger number, 

 the numerator "1" was placed over each of the figures so obtained 



lOp. cit., p. 284. 



