352 Journal of Agricultural Research voi. xix, no. s 



although they commonly rupture (PI. 60, D). The spots do not penetrate to the upper 

 surface. The plants are much more resistant than Poncirus trifoUata, which the 

 writers understand has to a large extent replaced Yuzu as stock for Satsuma in Japan. 

 Other conditions being equal, Yuzu is to be preferred to P. trifoUata from the stand- 

 point of canker susceptibility. 

 Citrus sp. Natsu-mikan (CPB 11184, seedlings), I, II, III, in field, 1918 and 1919. 



In some ways this plant resembles the hybrids between the grapefruit and loose- 

 skinned oranges, such as the tangerine, known in this country as tangelos. 



All plants of the Natsu-mikan in the greenhouse and field have been rather severely 

 infected. Some twig and stem infections have been found, and from 50 to 100 per cent 

 of the leaves have medium to large and scattering to many spots. The spots, although 

 larger and more corky, resemble those found on Satsuma. Some defoliation has taken 

 place, due to canker. 



If the Natsu-mikan is closely related to the mandarin orange it is very much more 

 susceptible than any of the plants so far studied in this group. Lee (3) reports the 

 Natsu-mikan as susceptible in the Philippines. 

 Citrus excelsa Wester. (CPB 11280, seedlings), I, III. 



From 90 to 100 per cent of the foliage of the two plants is infected with many 

 large spots. Some few spots on the twigs are also present. Because of the citron- 

 like texture of the leaves, the spots resemble those on the citron, except in size. 

 Apparently it is not quite as susceptible as grapefruit. 



In the Citrus fruits, where so many species and varieties were tested 

 with more or less varying results, it is extremely hard to classify the 

 susceptibility of these plants, especially where so many factors must 

 be taken into consideration. Probably the most important and vexing 

 factor is the physiological condition of the plant. In looking over the 

 notes taken approximately each month on the plants in the experiments, 

 it is found that there are certain cycles of canker infection which coin- 

 cide with the growth periods of the plants. Thus, one or two observations 

 on inoculated plants in the greenhouse or on those exposed to natural 

 infection in the field are not sufficient to detennine accurately the exact 

 susceptibility or resistance of a plant. Some of the points to be reckoned 

 with under the factor of the physiological condition of the plant are the 

 rate of growth, not only of the plant but of the leaves themselves, age 

 and size of the plant and leaves, leaf texture, and rate of maturation of 

 the leaves. All these have an important relation to canker suscepti- 

 bility and resistance. 



Leaf texture with its various ramifications probably plays an impor- 

 tant role in determining resistance in many cases. This can be best 

 illustrated by comparing an infected kumquat leaf (PI. 60, A) with an 

 old grapefruit leaf (PI. 60, B). The leaves are apparently very similar 

 in texture, and a close study of the spots produced on the two shows that 

 they are identical. In other words, while an ordinary grapefruit leaf is 

 still thin and light green in color, it is very susceptible, large corky spots 

 being produced. However, if an old leaf is taken which has apparently 

 the same texture as a leaf of the kumquat, it is as hard to infect as the 

 kumquat, and small, rounded, glistening spots are formed. 



