Dec. i 5l 1920 Effect of Temperature and Humidity on Citrus-Canker 503 



stock has been found during the months of July and August, months 

 with the highest mean temperature. In other words, the trifoliate orange 

 is not very susceptible at temperatures of 20 , but when temperatures 

 of 30 are reached the period of incubation is as short as that of grape- 

 fruit, and the plants themselves are as susceptible as grapefruit, or more so. 

 This fact can be still more clearly shown by stating that in Japan, where 

 the temperatures are rather low and uniform during the growing season, 

 cankers on the trifoliate orange are rare, though grapefruit and navel 

 orange in the same orchard or nursery may be badly infected. 'The slow 

 growth of the trifoliate orange, then, at temperatures around 20 makes 

 it more or less resistant to canker, though when grown at temperatures 

 of 30 it becomes extremely susceptible. 



Grapefruit, which grows at a much lower range of temperature than 

 any of the other Citrus plants tested, is the first plant to become infected 

 in the spring and the last in the fall. The greenhouse experiments showed 

 that the period of incubation at 20 , 25 , and 30 C. was 4 days. How- 

 ever, the spots produced at 20 were not so large or so numerous as those 

 produced at 30 . 



Thus, it has been found that the optimum temperature for the growth 

 of the organism in culture media in the laboratory lies between 20 and 

 30 C. Since the same optimum has been found for the host plants, it 

 should be expected that the same optimum should prevail for infection 

 and development of the disease. That such is the case has been proved 

 in the experiments reported. 



At a temperature of 35 C. or thereabouts, the maximum for the 

 growth of the organism in culture is approached, especially when the 

 length of exposure is included. This same temperature also inhibited 

 the growth of some plants in the greenhouse experiment. No canker 

 was obtained on any of the plants when a culture of the organism grown 

 at 35 was used to inoculate plants kept at this temperature in the 

 greenhouse. Only one spot was formed on grapefruit when plants were 

 inoculated at 35 ° with cultures grown at lower temperatures. The tri- 

 foliate orange appears to make a good growth at 35 , and general infections 

 were obtained on these plants at this temperature. In the field, temper- 

 atures of 35 prevail for portions of some days over periods of several 

 months. The question naturally arises whether the organism can exist 

 outside the host plant for extended periods, especially if high humidities 

 prevail at the same time. On the other hand, we know that the disease 

 develops during these periods. 



The influence of temperatures below 15 C. in the field will be dis- 

 cussed more fully in a forthcoming article on the overwintering of the 

 disease. It is sufficient to state here that although a temperature of 20 

 is necessary for infection, the disease after it is once produced can keep 

 on developing at temperatures lower than 20 and is fully dependent on 

 the growth of the host plant. In other words, the canker organism is 



