junei,i92o Behavior of the Citrus- Canker Organisfn in the Soil 205 



From such positive results of inoculations P. citri was several times 

 reisolated; and such cultures reinoculated on leaves of Citrus maxima 

 gave positive results. There is therefore a possibility, considered how- 

 ever to be small, that the canker organism is carried on the roots. 



In digging in the soil beneath citrus trees in the Philippines, leaves 

 have been uncovered upon which cankers were found. These leaves 

 were skeletonized by the soil organisms, the lignified tissues apparently 

 resisting the action of the soil organisms while the cellulose parts of the 

 leaf blade had entirely disappeared. Canker lesions upon such buried 

 leaves also seem to resist the dissolving action of the soil bacteria. Photo- 

 graphs (PI. 37, B) show the persistence of cankers upon such buried 

 skeletonized leaves. Whether cankered leaves which have been buried 

 and subsequently uncovered may possibly furnish another means of 

 carrying the canker organism over in spite of control measures is a 

 question that deserves special experimental investigation. 



In Florida there have been many cases of seemingly thorough eradica- 

 tion of the disease followed by a new outbreak, even after considerable 

 periods of inactivity. Such outbreaks at the time have been the cause 

 for considerable conjecture and speculation. It is possible that the 

 results presented here may point to hitherto overlooked sources of new 

 infection occurring after a period of latency. 



SUMMARY 



(i) Experimental evidence is given to show that P. citri disappears 

 from unsterilized soil in tubes and boxes usually within six days after 

 they are inoculated. P. citri inoculated in sterilized soil increases and 

 multiplies. Since the main difference in. this latter case is the exclusion 

 of the normal soil organisms, the disappearance of P. citri seems to be 

 ascribable to the antagonistic effect of such soil inhabitants. 



(2) In soil under orchard conditions, the canker organism is found to 

 disappear even more rapidly than in the soil confined in boxes or culture 

 tubes. 



(3) Seeds were planted in pots of soil naturally infected with the canker 

 organism and in pots of soil artificially inoculated. The seedlings came 

 through the soil and developed normally without any canker, thus 

 corroborating the conclusion that the canker bacteria are killed out in 

 normal soils. 



(4) Cankers upon mature wood of citrus trees and positive inoculations 

 upon the roots of citrus trees are shown. Cankers upon buried leaves 

 and mature wood and roots as possible sources of holding over the canker 

 organism are suggested. 



175343°— 20 2 



