2IO 



Journal of A gricultural Research 



Vol. XIX, No. 5 



applied is seen lO be fairly uniform for the more critical lower ranges in 

 both tests and lies between i to loo and i to 200. 



On the mature grapefruit leaves used for the tests there was practically 

 no infection except at the freshly made punctures, and the counts are 

 therefore free from errors that might have arisen from secondary spread 

 if the unwounded tissue had been highly susceptible. 



Two things are involved in the causation of infection by very dilute 

 inoculum: (i) the chance for the organisms to reach the punctures and 

 (2) the average number of organisms required to initiate infection 

 successfully at a given point. In using cotton swabs, a considerable 

 proportion of the organisms would be held at a distance from the leaf 

 surface, and of those actually in the surface moisture film many would 

 be at relatively great distances from punctures. On the other hand, 

 motility of the organism and the possibility of rapid numerical increase 

 by division would increase the chance for infection. As to the minimum 

 number of P. citri organisms necessary to set up infection on reaching 

 a given puncture, further careful experiments must be conducted before 

 an opinion can be ventured. 



PERSISTENCE IN VARIOUS TYPES OF SOIL 



To secure as great diversity as possible with types of soil conven- 

 iently at hand the following kinds Vv^ere selected: (i) stiff clay subsoil 

 thrown out from an excavation several months previously; (2) leaf mold 

 screened from ground surface in forest; (3) rotting compost of sod and 

 manure thoroughly decayed; (4) garden soil, a clay loam of moderate 

 fertility. Four-inch pots were used in duplicate for each soil type. 

 The inoculum for each pot amounted to 10 cc. of a 2 -day beef bouillon 

 culture of P. citri mixed with about one-fifth of the washing from a 

 6-day potato cylinder culture, the whole being diluted to 100 cc. and 

 evenly mixed with the upper 3 inches of soil in the pot. Inoculations 

 were made on August 14, 191 8. The pots were held in the greenhouse 

 shaded from direct sunlight, and were given ordinary watering. Each 

 percentage in Table II is based on the number of infections developed 

 in 17 days in a total of 600 leaf punctures and represents the average of 

 duplicate pots of each soil type. 



Table II- — Percentages of infection on grapefruit leaves inoculated with graded dilu- 

 tions of solutions from four types of soil -made at various ititervals after the soil had been 

 inoculated with P. citri 



