junei,i92o Decline of Pseitdomonas citriin the Soil 221 



across the plates ; in others it was planted at the center and at four spots 

 near the circumference; in still others it was planted abundantly over 

 the plate. Seven days later P. citri was seen growing in triangular areas 

 between the limbs of the crosses, in isolated patches with concave borders 

 between the spots, and not at all on the plates with numerous colonies of 

 the inhibitor. It appeared only where the distance was at least 15 to 

 18 mm. from the nearest border of an inhibiting colony. A hand lens and 

 the low power of the microscope brought within range of vision two suc- 

 cessive graded zones each about 3 mm. wide of smaller P. citri colonies 

 edging the clearly visible areas. The average distance from the edge 

 of the inhibiting colony to the P. citri colonies of microscopic size was 

 about 10 mm. Repeated attempts to cultivate P. citri from bits of 

 agar from this clear lo-mm. zone failed, although the abundance of the 

 original inoculation would have made it easy to recover the organism at 

 any point, if it were still alive. It was recovered in culture from the 

 microscopic and the clearly visible zones, and no extension of the killing 

 effect could be determined after a further lapse of seven days, during 

 which time there was no apparent growth of the inhibiting colonies. 



The testing of some 40 miscellaneous soil bacteria and fungi on beef 

 agar plates by the streak or the spot method showed about one-fourth 

 of the number to have some degree of inhibiting effect, while three 

 seemed to stimulate or accelerate the development of P. citri colonies, 

 at least at the beginning of their development. 



On other media the effects of certain of these inhibitors differed from 

 those exhibited on beef agar, the inhibiting effect being reduced or en- 

 tirely lost on certain media. 



Tests in the soil itself must be conducted before definite statements 

 can be made as to the part such potential inhibitors or destroyers actually 

 play in the decline of P. citri under soil conditions. However, the hypo- 

 thesis that the deleterious effects on P. citri are brought about by certain 

 organisms in the soil is in harmony with the experimental evidence thus 

 far obtained and seems to be a reasonable explanation of the phenomenon. 



It seems reasonable to suppose that P. citri can persist in dry soil 

 partly at least because of suspended activity of deleterious organisms, 

 and that the addition of water makes possible a renewal of their unfa- 

 vorable activity. 



INFECTION OF GRAPEFRUIT ROOTS BY P. CITRI 



The question naturally arises as to whether roots of citrus species are 

 highly susceptible to citrus-canker infection. The following tests bdar 

 on this point. 



On May 20, 191 8, eight pots of soil were inoculated heavily with P. 

 citri culture and planted with grapefruit seed, about 50 per pot. Eight 

 other pots were similarly prepared without inoculation. In another set 

 both seed and soil were inoculated, and in still another only the seeds were 



