138 Diseases of Economic Plants 



similar to those on leaves, but usually they become more 

 corky and have a tendency to girdle the twig or branch. On 

 the fruit the old cankers are superficial and very corky. 



Infection, in the absence of wounds, takes place through 

 the stomata. The period of incubation varies from two or 

 three days to several weeks. The factors favoring infection 

 are a high temperature, a relatively high humidity, and a 

 rapidly growing plant. The organism survives the colder 

 months in the spots on the twigs and branches. Under cer- 

 tain conditions the organisms may lie dormant in the bark 

 for several months. 



Pseudomonas citri has a wide range of Rutaceous hosts, 

 including all the plants and hybrids of the genus Citrus, 

 although some show considerable resistance. The order of 

 susceptibility of the more important commercial citrus 

 fruits and stocks is provisionally as follows: grape fruits, 

 trifoliate oranges, lemons, sour oranges, sweet oranges, limes, 

 citrons, mandarin oranges, and kumquats. Several new 

 introductions of citrus fruits and hybrids, originated by 

 Swingle, show considerable resistance and promise to super- 

 sede the more susceptible fruits in some instances. 



Infected nursery stock was wholly responsible for the 

 spread of citrus canker from state to state. However, owing 

 to rigid quarantine measures, further danger from this source 

 appears to be eliminated. The combination of wind and 

 rain is most important in the spread of canker from tree 

 to tree; other agents are dust, insects, birds, man, and other 

 animals. 



Owing to the rapidity with which citrus canker spreads and 

 its menace to the citrus industry of the Gulf states, drastic 

 measures for its control were adopted in 1914-15 in all the 

 Gulf states. These consisted in burning all infected trees 

 as soon as they were found. 



The number of infected citrus trees found in Florida in 

 August, 1914, was 1313. During 1918 only 15 were found, 

 and in 1919 only 4 infected trees. Few possible centers of 

 infection remain in the United States. 



