The Bulletin. 21 



Symptoms. — Individual leaves become pale and misshapen, fre- 

 quently on one side of the mid-rib, and droop. In time the whole plant 

 may be involved. The stem may show on external view elongated 

 streaks, dark and somewhat sunken. On cutting across the stems very 

 dark areas are seen in inner bark, wood ring or central pith. On 

 squeezing a cut stem a milky fluid exudes from the darkened areas, 

 which is quite different from the normal clear watery sap. The root 

 system shows a number of dead and blackened roots. 



On tomato and Irish potato this disease may be confused with the 

 Fusarium fungous wilts that attack these plants. The exuding of the 

 milky fluid is a distinctive mark of the bacterial disease. 



Cause. — It has been proven beyond question by several workers that 

 this disease is caused by bacteria of the species Bacterium solanacearum. 

 They make up the milky fluid referred to above. Some of this fluid 

 from a diseased plant introduced by a needle puncture into young tissue 

 of a healthy plant will produce symptoms of the disease in one to sev- 

 eral weeks, depending on the rapidity of increase of the bacteria. The 

 organisms may persist in the soil for a number of years, but do not in- 

 crease markedly unless susceptible plants are allowed to grow. These 

 bacteria are themselves plants, and their ability to maintain themselves 

 in the soil is influenced by the physical structure, chemical composition, 

 water content, aeration, temperature and doubtless other conditions of 

 the soil, just as crop plants are influenced by such factors. When more 

 exact knowledge is obtained about the special requirements of these bac- 

 teria it may be possible to handle or treat the soil in such a way as to 

 hinder their development without interfering with tobacco production. 



Dissemination. — These bacteria may be spread through any transfer 

 of infected dirt, commonly by water washing the soil, by farm imple- 

 ments carrying it up and down the rows in cultivation, or from one field 

 to another, by animals carrying it on their feet, and by adherence of 

 dirt to the roots of plants of any sort transplanted from infected spots. 

 It is possible that the blowing about of dust may convey it, but the 

 vitality of the organism is affected very unfavorably by drying. The 

 disease may start in the seed bed, and be carried to the field in plants 

 already slightly infected there. Insects that feed on affected plants and 

 afterwards on sound ones may infect the latter through the wounds by 

 means of bacteria smeared on their mouth parts. In topping and sucker- 

 ing the disease may be conveyed from sick to healthy plants. _ All of 

 these agencies naturally spread the infection over rather short distances, 

 and it is a characteristic of the disease that it travels slowly. It is 

 possible that manure, or tobacco seed, or cured leaves or dried stalks 

 that have been affected may play a part at times in its dissemination, 

 but direct evidence for this is lacking at the present, and the inability 

 of the organisms to withstand prolonged drying makes the matter ques- 

 tionable. 



Infection. — In most cases the organisms enter the plants from the 

 soil through some of the smaller roots and work upward in the plant. 

 This may occur in the seed bed, at transplanting, or later. Injury to 

 the roots at transplanting and nematode attack furnish ready means of 

 entrance. Insect injuries and the wounds made in topping and sucker- 



