202 Journal of Agricultural Research voi. xix, No. s 



upon the lansones (Lansium domesticum) , and there can be no question 

 as to its virulence. The temperatures and humidity were at all times 

 favorable for the development of canker. 



Theoretically, criticism of the results of this experiment might be raised, 

 since none of the trees, even the controls in sterilized inoculated soil, 

 showed canker. Practically, however, there is a very good explanation. 

 The seeds did not begin to sprout and the young shoots to push through 

 the soil until the first week in December — that is, 35 days after soil was 

 noculated. During this time the sterilized soil pots were exposed in the 

 Lamao woods, protected only from contamination by coarse cheesecloth. 

 Under these conditions it could be expected that a few weeks after being 

 placed in the woods the soil in the pots would be well inoculated with the 

 ordinary soil flora and the canker organism would then be killed out. 

 Another explanation might be that the normal young seedlings of Citrus 

 trifoliata are possibly resistant to citrus canker infection, in which event 

 the value of this method of testing for soil infection would be lessened. 



SUMMARY OF RESULTvS OF EXPERIMENTS 



It has been shown in tv/o separate experiments that P. citri lives and 

 may even increase in culture tubes of sterilized soil throughout a period 

 of 14 days or more. On the other hand, tubes of identical soil, handled 

 in an identical manner with the exception of not being autoclaved, showed 

 the canker organism to be entirely killed out within a period of 6 days. 



In three similar experiments, representing two distinct seasonal periods, 

 it was shown that the canker organism can be found in the soil beneath 

 a heavily infected tree on the day immediately follov/ing the rain and on 

 the second and third days following. Thereafter there is no indication 

 of the canker organism in the soil. 



In another experiment a box of soil was autoclaved and then inoculated 

 with P. citri. This box, placed in the orchard to simulate field conditions, 

 showed no decrease in the activity of the canker organism during a period 

 of 14 days after inoculation. A box of similar soil, treated in an identical 

 manner with the exception of not being autoclaved, showed the canker 

 organism to be entirely killed out within a period of 6 days. 



In the last experiment seeds were planted in nonsterile soil which 

 had been inoculated with a heavy infusion of P. citri. The seeds which 

 germinated and pushed through the soil 40 days after inoculation never 

 showed any sign of canker although they were kept for 45 days after their 

 appearance above the soil. 



The results of each series of experiments point to the dying out of the 

 canker organism in untreated soils. The indication is that the normal 

 soil organisms are antagonistic in some way to the existence of P. citri 

 in the soil. 



The soil at Lamao is a sandy loam and seems to be of alluvial origin. 

 There is little or no indication of decaying organic matter in the soil 



