70 MASS. EXPERIMENT STATION BULLETIN 203. 



Life History of the Causal Organism. 



Although there is very little to learn about the structure of this very 

 simple organism, there are many things which it is important that we 

 should know about its life history. Control measures can be developed 

 only after determining important life-history facts, such as the manner 

 and place of overwintering, method of entrance to host, methods of spread- 

 ing in the seed-bed and in the field, and longevity of the organism in 

 various environments. About some of these life-history phases little or 

 nothing is known as yet; concerning others we now have more definite 

 information. It is hoped that experiments now in progress, but not com- 

 pleted at the date of issue of this bulletin, will clear up some of the places 

 in the life history about which we are now ignorant. At this time we can 

 only summarize the progress which has been made up to the present by 

 investigators in other States, and by the writers in Massachusetts. 



Infection. — Infection may occur at any stage in the development of 

 the tobacco plant. The bacteria thrive and cause injury by rapid propa- 

 gation inside the tissues of the leaf. The method by which they pass 

 from outside the leaf through the epidermis into the interior tissue is not 

 yet definitely known. The only openings through which they can pass 

 are the stomates (breathing pores), the hydathodes (openings on the 

 margins of the leaves for the exudation of water) , and accidental abrasions 

 or wounds. The writers have frequently demonstrated, however, that 

 visible wounds are not necessary. Healthy plants of various ages have 

 been inoculated by spraying with a suspension of the bacteria in water. 

 Almost without exception infection has resulted, although the most care- 

 ful examination has failed to reveal any wounds in the leaves. If the 

 stomates were the only avenues of entrance, one would expect a greater 

 percentage of infection when the lower surface of the leaves was inocu- 

 lated, but the amount of infection has been about the same, irrespective 

 of whether the upper or lower side was inoculated. The rather high per- 

 centage of marginal infection points toward the hydathodes as important 

 infection courts. Moisture has an important role to play in infection, 

 although perhaps more important in dissemination. Infection occurs in 

 the field principally during rainy periods. It is not essential that the water 

 should remain on the leaves for any long time. Successful infections are 

 secured by spraying water suspensions on the leaves even when they 

 become dry within a few hours. 



Incubation Period. — This period covers the time between the passage 

 of the bacteria into the interior tissues and the appearance of the first 

 symptom of disease. The length of this period as determined by care- 

 fully watched experiments at this station is three to eight days. In the 

 field growers usually begin to notice increased infection in five to seven 

 days after a rain, but since the first symptoms are inconspicuous, and the 

 casual observer does not notice the spots until they have been developing 



