72 MASS. EXPERIMENT STATION BULLETIN 203. 



between times and kept under favorable conditions for infection. Since 

 no infection resulted, even after several weeks, it seemed apparent that 

 ■wind alone cannot spread the disease. 



Leaj Contact. — Since in the seed-bed, and to some extent in the field, 

 the leaves of adjacent plants come into contact, it seemed possible that 

 the bacteria might thus pass from a diseased to a healthy plant. In order 

 to determine whether this is possible, two diseased plants in pots were 

 placed under a closed bell jar with two healthy plants in such a position 

 that healthy leaves were in contact with diseased leaves in a natural way 

 without any device for keeping them together. They were not watered, 

 but moisture soon accumulated inside the bell jar. In a second bell jar 

 the same experiment was repeated, but the jar was open at the top except 

 for a thin piece of cheesecloth used to exclude msects. Infections resulted 

 in both cases. There can be no question then but that wildfire may spread 

 by contact. This factor is probably of more importance in seed-bed dis- 

 semination than in the field. When plants have been pulled and piled 

 together in boxes or baskets until ready to be set in the field the moisture 

 conditions are very favorable for the spread of the disease to healthy 

 plants. Occasionally such plants are kept thus for days before planting, 

 and, if diseased plants are present, no better opportunity for spreading 

 wildfire bj^ contact could be found. 



Handling by Workjnen. — Do workmen, in weeding, transplanting, hoe- 

 ing, plowing and topping help to spread the disease? In order to answer 

 this question diseased leaves were crushed between the fingers and then 

 leaves of a healthy plant drawn through the fingers without rupturing 

 the leaves. The plants were then kept protected under a bell jar. A 

 few of the leaves thus treated developed typical lesions of the disease. 

 Check plants treated in the same way after healthy leaves had been 

 crushed in the hand remained free from disease. There is no doubt, then, 

 that wildfire may be spread by workmen during the ordinary manipula- 

 tions of the crop. The danger of spread is much greater while the plants 

 are wet, and if there is any disease at all present, all operations while 

 water is on the leaves should be avoided. 



Insects. — It would seem almost impossible for insects to work alter- 

 nately on diseased and healthy leaves without spreading the bacteria. 

 Wolf and Moss (6) state that "flea beetles are to be regarded as carriers 

 of infection, since the wildfire organism has been isolated from indi\dduals 

 which had been feeding on diseased plants." Flea beetles were the most 

 common insects found on tobacco both in seed-bed and field in this State 

 during the present year, and it was suspected that they carried the bac- 

 tei'ia. To determine whether such was the case, large numbers of flea 

 beetles were caged with diseased plants; then, after they had fed on these 

 plants for several days, they were transferred to healthy plants, where 

 they riddled the leaves. No wildfire lesions ever developed on these plants. 

 Thinking that possibly the beetles had not eaten from the diseased spots 

 in the infected plants the writers caged another lot in tubes in which only 



