Apr. 10, 1916 Citrus Canker 79 



new infections are known to have appeared abundantly under field con- 

 ditions during November, 1914. Old leaves on the ground may possibly 

 harbor the organism and there it may remain viable for a long time. 

 Unsuccessful attempts, however, have been made to recover the organ- 

 ism from leaves kept in the laboratory from September, 1914, to May, 

 1 91 5; nor has recovery been possible in the case of twig cankers kept 

 under laboratory conditions from March to October, 191 5. 



It is believed, moreover, that the organism survives the winter in 

 fallen leaves and that these fallen leaves constitute a very important 

 source of infection in the following spring, especially in the case of nursery 

 trees which have been planted between diseased grove trees. 



There is every reason to believe also that the organism can remain 

 alive in soil. This is evidenced by numerous instances in which new 

 sprouts have come up from the roots of diseased trees which had been 

 burned. A large percentage of these sprouts are early found to be diseased. 

 Furthermore, the leaves on the lowermost branches or those in actual con- 

 tact with the soil are commonly the first to become diseased. 



The fact that the stomata, or breathing pores, on species of Citrus 

 occur only on the lower leaf surfaces and that infections developed only 

 on the lower surface of the leaves in all of the inoculation experiments in 

 which the plants had been sprayed with bacterial suspensions led to the 

 inference that the canker organism must gain entrance to the leaves 

 through the stomata. That such is the case was established by leaf 

 sections which were fixed 72 hours after inoculation and which were sub- 

 sequently properly infiltrated, cut, and stained (fig. 4). Lenticels very 

 probably serve as portals of entrance for the organism into the stems. 

 A film of moisture on the surface of the leaf, twig, or fruit enables the 

 organism to move about and thus to gain entrance into the substomatal 

 cavity. Under ordinary conditions inoculation will be successful only 

 in the presence of moisture. Wounds or abrasions from any cause may 

 afford an entrance to the bacteria. Inoculations not infrequently occur 

 through wounds made by thorns. Inoculations on leaves made by thorn 

 scratches are shown on Plate X figure 3. Thorns which come in con- 

 tact with limbs near by may inflict wounds which have subsequently 

 been observed to be the point of origin of limb cankers. Cankers have 

 also been found at the point of contact of limbs which rub together 

 through movement by the wind. 



When once the bacteria have passed through the stomata into the 

 substomatal cavities, they multiply rapidly and effect a passage between 

 the host cells to the intercellular spaces which become filled with solid 

 masses of bacteria. As the bacteria continue to multiply, the cells 

 farther away from the substomatal chambers become involved seriatim. 

 In this way an area circular in outline and extending entirely through the 

 leaf comes to be invaded. Various stages of invasion of the leaf tissues have 



