504 Journal of Agricultural Research voi.xx, no.6 



active in the tissues so long as the host cells are active, and when the 

 plant is forced into dormancy the organism becomes inactive and the 

 disease is then quiescent. 



From the present extent of our knowledge of this disease, it can be 

 concluded that environmental conditions play an exceedingly important 

 rdle in the susceptibility and resistance of Citrus plants. Thus, environ- 

 mental conditions determine to some extent the anatomical structure of 

 the plant parts attached by canker, by influencing the size and rapidity 

 of maturation of the new growth and the leaf texture. Apparently, each 

 species studied has a definite reaction to its environment and differs 

 from other species in its behavior under a given set of conditions. There- 

 fore, one should be able to forecast the susceptibility and resistance of a 

 given plant under certain environmental conditions. Lastly, the in- 

 fluence of humidity and temperature on the host favors to some extent 

 the increased or decreased virulency of the organism toward a definite 

 species. It appears that it will be necessary to study the behavior of 

 the host plant in its environment before any scientific selection or breed- 

 ing for disease resistance can be made. 



SUMMARY 



(1) The temperature relations of Pseudomonas cilri Hasse in culture 

 are similar to those of the plant-disease bacteria of the Pseudomonas 

 group. With the time factor included, the minimum temperature for 

 growth in culture is about 5 C, the optimum between 20 and 30 , the 

 maximum about 35 ° for a period of 24 hours, and a thermal death point 

 between 49 and 52 . 



(2) The influence of humidity on the viability of the organism is very 

 distinct and is closely associated with temperature. At low temperatures, 

 humidity appears to have little or no influence, while at high temperatures 

 and high humidities it is the limiting factor. At medium humidities at 

 all temperatures the organism is viable for the period of the experiment. 

 Some factor or factors other than the rapidity of drying are responsible 

 for these results. 



(3) The Citrus plants used in the greenhouse experiments vary mark- 

 edly in their reaction to temperatures and humidity, especially at low 

 and high temperatures. However, with the time factor included, the 

 optimum temperature for all the plants used lies between 20 and 30 C. 

 With some slight variations, the same temperature relations hold in the 

 field. 



(4) Three conditions are essential for infection — the presence of free 

 moisture on the plant, a suitable temperature, and an actively growing 

 plant. 



(5) The life of the organism in culture and outside the host plant 

 is ruled by an entirely different set of conditions from those which 



