47 2 Journal of Agricultural Research vol. xx.no. 6 



Since the literature on the influence of temperature can not be discussed 

 separately from that of humidity, a brief review of the literature on the 

 relation of weather conditions and the condition of the host plant on in- 

 fection and development of citrus-canker will be given at this point. 



Both Hasse (2) and Doidge (1) found that the disease developed most 

 rapidly on inoculated plants in a saturated atmosphere kept at 30 C. 



Stevens (11, 12) makes the following statements: 



In this experiment, it was found that considerable moisture must be present before 

 infection took place, and in many cases, the small trees thus treated had to be kept 

 drenched and under bell-jars for two or three days. Infections developed slowly 

 under greenhouse conditions, and were fewer in number than those obtained in the 

 open. 



Warm humid weather favors rapid development of the disease and thus it is more 

 destructive during the rainy season. 



The disease develops and spreads rapidly during rainy weather but it is more or less 

 retarded during periods of drought or in dry weather. 



High temperatures and high humidity favor a rapid development and spread of 

 Citrus-canker and these are the prevailing factors of the Florida climate. 



Stirling (15) states that — 



during warm, wet periods, the disease infects quickly and matures in a few days. 



Further that— 



during a time when the atmosphere is humid, in the rainy season, it spreads rapidly. 

 I have found that during the early part of the season, it requires two or three months 

 for the canker to infect and mature so as to reproduce itself, owing, no doubt, to the 

 dryness and coolness of the weather. Under favorable conditions, however, the canker 

 will infect and mature in a much less time. 



Wolf (17) observed that — 



the most rapid development of the disease occurred under humid conditions. 



Jehle (4, 5) in a number of articles states : 



Citrus-canker is one of the most destructive diseases of citrus plants . . . and espe- 

 cially where the climate is warm and moist during part or all of the year, — as the 

 disease develops most rapidly when the humidity is high ... it was most severe 

 and the incubation period shortest during warm moist weather. The disease does net 

 develop as rapidly in cool, dry weather as it does in warm, damp weather. 



He finally summarized his observations as follows — 

 it is much more prevalent and severe, and the incubation period is much shorter 

 during the summer than during the winter. In Florida, the humidity and tempera- 

 ture are usually high during the summer, humidity averaging from 50% to 95% 

 and temperature from 65 to 95 degrees F. at the Tropical Laboratory. Local 

 showers are very prevalent and frequently follow one another with such rapidity that 

 the trees do not dry off for long periods of time. During the winter, the opposite 

 conditions prevail, the air being dry and cool and showers few with long intervals 

 between them. At Redland, the temperature usually ranges from 45 to 85 degrees F. 

 and the humidity from 20% to 90%. Swingle learned that the disease was much 

 more destructive and prevalent in Japan during warm moist seasons than it was 

 during cool dry ones. 



In discussing citrus-canker in the Philippines, Mackie (6) states that — 

 during the dry season, which occurs from January until the monsoon changes in June, 

 the disease is apparently more or less quiescent, cankers being numerous on the leaves 



