10 MASS. EXPERIMENT STATION BULLETIN 213. 



Dissemination. 



No experiments directly dealing with dissemination were undertaken 

 during the season of 1922, but observations throughout the year confirm 

 the conclusions of 1921 in most respects. There is one notable exception, 

 — the experiments and observations in 1921 led us to believe that all 

 field infection originated from plants which were diseased when taken 

 from the beds. The majority of the field infections, and the worst ones 

 which we have seen in 1922, did come from that source and could be 

 traced without any question to the seed-bed. On the other hand, a num- 

 ber of cases have come to the ^vriters' attention where the beds were free 

 from disease (if it is possible at all to tell when they are free), but disease 

 developed in the fields set from these same beds. A few cases may be 

 mentioned : — 



1. Anderson inspected the beds of a certain Sunderland grower at intervals of 

 three or four days throughout the season and is positive that they were free from 

 disease. Yet parts of the fields set from these beds were very badly diseased. 



2. Tobacco fields owned by a grower in South Deerfield, but located near 

 Brattleboro, Vt., became badly diseased, and were visited by A. V. Osmun and 

 Anderson in June. Most of these fields were set from beds near the fields, but 

 some plants were brought from the beds in South Deerfield. A most searching 

 examination of the beds at both places failed to reveal a single diseased plant. 



3. A field of tobacco on a farm in Whately was isolated from all other tobaccO' 

 fields and surrounded on all sides by woods. Plants were taken from the beds on 

 the same farm. During the spring these beds were repeatedly inspected by C. M. 

 Slagg, a wildfire expert, but he failed to find any infection. Yet wildfire became- 

 fairly prevalent in the isolated field. 



4. The seed-beds of a grower in North Hadley were frequently inspected by- 

 Anderson during the spring, and not a trace of wildfire could be found at any time. 

 During August some diseased plants were found in the middle of the grower's field. 



5. Wildfire occurred in a field of the Massachusetts Experiment Station farm 

 which was not being used for wildfire work, but not a trace of it had been seen in the- 

 beds at the experiment station where the plants were raised. 



6. A certain Windsor grower kept his seed-beds covered at all times with copper- 

 lime dust, and frequent inspections by Chapman and Slagg showed no infection. 

 He planted two fields, about 3 miles apart, from these beds. One of the fields de- 

 veloped a hea-vy infection diuing the gro-wing season; on the other, only a trace 

 of wildfire was found. 



Many similar cases were reported by growers, but were not checked 

 by the personal observations of the writers. The evidence is conclusive 

 that not all field infection comes from the seed-bed. We are now con- 

 fronted with the problem of determining how such infections did start. 

 Rain could not have brought them from other fields because they were 

 too far removed. There is some probability that in the Sunderland field 

 the bacteria were in the soil over winter, since the worst infection occurred 

 in the same place as last year. In the other cases, however, either no 

 tobacco had been planted during the previous year on these fields or no 

 wildfire had been observed there during 1921. Apparently there is some- 

 long distance disseminator which we have not yet found. Those that 



