COBB S DISEASE OF SUGAR-CANE. 



47 



Of all varieties tested Common Green has shown itself most susceptible. Louisiana 

 variety No. 74 and Common Purple were very resistant, and all the others under the con- 

 ditions of the tests were more or less so. Unfavorable conditions of growth rendered the 

 variety test unsatisfactory, and the writer does not consider the results trustworthy. 



ACID CANES LESS SUSCEPTIBLE. 



The question of immunity is perhaps the most interesting one. To what is the im- 

 munity of resistant varieties due? or of susceptible varieties under special conditions? 

 It is a very interesting and perhaps a very complex problem, but also perhaps one relatively 

 simple. Only a few observations will be noted here. Before making the first inoculations 

 on Common Green cane, Common Purple cane, and Louisiana variety No. 74 (Feb. 1903), 

 the freshly expressed juices of well-grown, sound canes of the three varieties were titrated 

 with phenolphthalein and sodium hydroxide to determine their acidity. According to these 

 titrations the sap of the Common Green cane was the least acid. Bacterium vascularutn 

 also grew very much better on steamed cylinders of this variety than on those made from 

 the other two varieties. The cane used for this latter purpose was pared, cut into cylinders, 

 put into test-tubes containing an amount of distilled water sufficient to cover less than half 

 the cylinder, and heated a few minutes on three successive days in the steam sterilizer. 

 The cut surface of the cane was then streaked copiously and the amount of growth noted. 

 It was these unexpected results which led the writer to choose Common Green cane for 

 his first inoculations. He believed it would be more susceptible than the other varieties, and 

 such proved to be the case. The others were a hundred, yes, a thousand times more resist- 

 ant to infection. The majority showed only local signs on the inoculated leaves or stems. 

 Signs were not observed on the uninoculated canes from these stools, not even after many 

 months. A few of the inoculated canes of these resistant varieties showed dwarfing with 

 secondary signs on the leaves, but after 6 months, when the plants were cut down, there 

 were to be seen for the most part only a few red bundles without distinct bacterial ooze, 

 although in two stems there were a very few bundles containing yellow bacterial slime. 

 The red bundles appeared only in the nodes giving rise to the inoculated leaves and in the 

 adjacent internodes; they were not to be found in the greater part of the well-developed 

 stems either above or below. These facts can be tabulated as follows : 



TABLE 4. Relation of Bacterial Growth to Acidity oj Cane Juice. 



It may be noted here also that the Common Green cane is the one which is most cul- 

 tivated for eating, which would not be the case if the juice were very acid. 



On steamed cylinders of Striped Green cane and Striped Purple cane, which titrated 

 +20 and +32 respectively, the organism also grew very feebly. 



Another titration experiment was made in April 1906 (by Mr. Johnston and Miss 

 Hedges) on the nine varieties of uninoculated canes which had been growing (very slowly) 

 in the greenhouse since the winter of 1904-5. Eight of these canes were from the lot used 

 for a variety test in the inoculations of May 22, 1905, which resulted rather unsatisfactorily 

 even on the Common Green cane, due, no doubt, to the condition of the plants at the time 

 of inoculation (slow growth, hard nodes). The ninth was a Cuban variety which had been 

 growing in the greenhouse about the same length of time. The old uninoculated canes 



