Dec. i 5 , 1920 Effect of Temperature and Humidity on Citrus-Canker 473 



but not seeming to show very much on the twigs, except on the young growth and on 

 nursery stock. However, after the rains begin, trees send out new growth and it is 

 on this new growth the canker appears, coming into evidence in about a week. In 

 some species, it will fairly cover the new foliage, while there also appears an abundance 

 of canker on the twigs. Throughout the rainy season, the disease thrives, infecting 

 practically all the young growth. This season (1917) would seem to offer ideal con- 

 ditions as to climate, the weather being warm, the humidity varying from 60 to 88. 



Tanaka (16), quoting Abe, of Japan, states that — 



The severity of the organism is more pronounced in the wet years and spreads more 

 rapidly at such times. 



It can be clearly seen from the foregoing excerpts from the literature 

 that the greatest development of canker occurs during warm, humid 

 weather, which in some localities can be translated into the term rainy 

 season, which in turn is usually associated with high temperatures. On 

 the other hand, these same weather conditions stimulate the rapid growth 

 of Citrus plants. The relation of the development of canker to the con- 

 ditions of the host has been reported on by the various workers. 



Stevens (11) says that — 

 young and succulent growth under humid conditions is very susceptible. 



According to Wolf (17) — 



new infections appear in spring shortly after the new growth has begun. Under 

 favorable conditions, new infections may appear at any time throughout the growing 

 season of the host. 



Mackie (6), in the Philippines, says: 



However, after the rains begin, trees send out new growth and it is on this new 

 growth the cankers appear. Throughout the season, the disease thrives, infecting 

 practically all the young growth. 



Jehle (4, 5) reports: 



Citrus canker develops more rapidly on trees which are in a thrifty, healthy, 

 growing condition than it does on those which are semi-dormant, unthrifty, or un- 

 healthy. Trees in a neglected condition may harbor the disease for months before 

 it becomes conspicuous enough to be recognized. 



The vitality and vigor of the host have a marked effect upon the prevalence and 

 severity of Citrus canker as well as upon the period of incubation. The disease is 

 much more prevalent and severe upon trees which are in an otherwise thrifty, healthy, 

 growing condition than it is upon those which are unthrifty and unhealthy. The 

 period of incubation is much longer when the trees are unthrifty and unhealthy and 

 the disease may remain on such trees in a dormant condition without becoming visible 

 for long periods of time. . . . If a tree has become infected with the organisms, 

 they apparently do not die, no matter how long the tree is kept in a semi-dormant or 

 neglected condition, but persist until active growth does occur, when the canker lesions 

 become visible. 



Tanaka (16), quoting Bakura, of Japan, says — 

 it seems to attack young plants mostly. 



Tanaka (16), quoting Nishida, of Japan, says — 



I do not claim the entirely resistant nature of the Satsuma variety. It is a matter 

 which largely depends upon the environmental condition and habit of growth of the 



