Apr. lo, 1916 Citrus Canker 



75 



and the material was then applied by gently rubbing the leaves between 

 the thumb and fingers. In a few cases it was arranged so that twigs 

 bearing young leaves could be immersed for an hour or two in a bacte- 

 rial suspension. Leaves inoculated in this manner are shown in Plate 



VIII, figure 2. It is to be noted that the infections are so numerous as 

 to involve the greater part of the lower leaf surface. 



The period of incubation appears to vary, depending on temperature, 

 moisture, and age of the plant tissues. Very definite signs of the disease 

 have been noted within 72 hours after inoculation. In other cases 10 

 days were required before the infections were evident to the eye. The 

 longest periods were secured on Satsumas. 



An organism of the same color as Pseudomonas citri and similar in 

 appearance on certain media, but which does not exhibit the character- 

 istic growth of P. citri on potato cylinders, has commonly been isolated 

 from old cankers. This organism has not been found to be pathogenic 

 on species of Citrus, however. There 'can be little doubt of the pathoge- 

 nicity of the organism concerning which Hasse made her preliminary report 

 (6). It is to be noted that her Plate X, figures A, B, represent natural 

 infections and Plate X, figure C, artificially produced cankers. These, 

 however, are regarded as identical in appearance. Artificially inoculated 

 seedlings are represented also in Plate IX. As can readily be seen, the 

 artificial cankers are much more prominently projecting than natural ones, 

 are evidently greenish white in color, and there has been no discoloration 

 of the leaf tissue surrounding the spot. The writer has never, under field 

 conditions, seen specimens which resembled these artificial inoculations 

 represented in Hasse's Plates IX and X, and he, furthermore, has exam- 

 ined fresh specimens in various stages of development sent from Florida, 

 Alabama, Mississippi, Louisiana, and Texas. However, cankers similar 

 in appearance to Hasse's artificial cankers have been produced in the 

 greenhouse. Following her suggestion that the open, spongy type of 

 canker is due to favorable conditions of moisture and temperature, seed- 

 ling grapefruit which had been atomized with a suspension of P. citri 

 were kept continuously covered with a bell jar. They were watered 

 sufficiently often so that the air under the bell jar was maintained at a 

 high relative humidity. Within 10 days the cankers shown in Plate 



IX, figure 3, had developed. These are regarded as similar in appear- 

 ance to those previously produced by Hasse and represented in her Plates 

 IX and X. 



DESCRIPTION OF PSEUDOMONAS CITRI 



The primary cause of Citrus canker is a yellow, i -flagellate organism. 

 Its motility can be observed when taken directly from young cankers 

 and examined in a drop of water. In this case it will be found to occur 

 singly or in pairs. On solid media it may form into chains of six or 

 more elements. It is quite variable in shape and size. When taken 

 27470°— 16 2 



