Dec. 15, 1920 Effect of Temperature and Humidity on Citrus-Canker 50 1 



Because of the preliminary nature of the greenhouse experiments on 

 the influence of humidity on Citrus plants, no statements can be made 

 at this time, except to point out that there is a definite relation between 

 the development of the plant and humidity. 



The first prerequisite for infection of Citrus plants by Pseudomonas 

 citri is the presence of free moisture on the plant. The second condition 

 is a suitable temperature. However, with both these conditions fulfilled, 

 no infection can take place unless the plant is in an active, growing con- 

 dition. In other words, no infection of a dormant plant is possible. 

 This fact has been clearly demonstrated by the greenhouse experiments 

 and is borne out by observations under field conditions. During the 

 short rest periods in summer, it is infrequent that new infections occur. 

 This is due to the fact that the shoots have completed their growth and 

 the period of maturation is at hand. In other words, canker is most 

 abundant during the growth periods, the severity of the disease de- 

 creasing during the short rest period. Thus, we have cycles of infection 

 which in turn correspond to the growth periods of the plants themselves. 



In speaking of infection one must distinguish between the period of 

 initial infection and the period of incubation. By the period of initial 

 infection is meant the time required by the organism, after it reaches a 

 a leaf, to enter the stomata or, in the case of wounds, the tissue of the 

 plant. The period of incubation, on the other hand, is the period ex- 

 tending from initial infection until the disease is visible. As has been 

 stated before, experiments have clearly shown that the period of initial 

 infection is short, the organism getting into the stomata within 20 

 minutes. The period of incubation, on the other hand, may be short 

 (48 hours) or long (several months), depending on external conditions. 



The presence of free moisture is necessary for limited periods only in 

 order that initial infection may take place. Initial infection does not 

 occur at high humidities, but because of the stimulating influence of high 

 humidities on the active growth of the plant, when accompanied by 

 suitable temperatures, they are more conducive to the disease. As has 

 been noted before, all investigators agree that the greatest development 

 of canker occurs during warm, humid weather. However, in all local- 

 ities where warm, humid weather prevails, we have alarge rainfall. Thus, 

 so far as initial infection and, incidentally, the development of the disease 

 is concerned, it is not the high humidity that must be considered but the 

 frequency of the rains. The temperature factor must not be overlooked, 

 in that, even though frequent rains occur, no canker will develop unless 

 a suitable temperature for the development of the organism and growth 

 of the host is at hand. Thus, without question, even though the same 

 amount of rain occurred in the orange districts of Japan as falls in the 

 Gulf coast section, canker would not be so severe, because of the lower 

 mean temperature prevailing in that country. 



