May 13, 1918 Some Bacterial Diseases of Lettuce 369 



the leaves in the compost. Those lettuce growers who had farms entirely 

 free from disease did not use this compost under two years' aging (PI. 30, 

 B). Those who had had only a small amount of the disease had used it 

 when about i year old or when they found that the grass had disintegrated. 

 These last farms were protected by windbreaks and were not so exposed 

 to the extreme cold; consequently little damage by the disease followed. 

 The grower who lost 98 per cent on one plot had used compost only 7 or 

 8 months old and not thoroughly decomposed; even at the time of the 

 lettuce harvest in April much of the marsh grass incorporated into the 

 compost was sticking from the soil as stubble, and the plot, which suf- 

 fered severely, was unprotected by windbreaks. This piece of land had 

 never been planted to lettuce, and the year before a crop of cowpeas 

 (Vigna sinensis) had been grown on it. 



Cross-sections of stems in the blue-green stage and also in the brown 

 stage were examined microscopically and bacteria were found swarming 

 in the tissues. No fungi were present. Both the pith and the vascular 

 region were involved. Moderately diseased plants were darkened only 

 in patches in the vascular region of the stem. Bacteria were found in 

 the brown spots of the leaves on plants where the stem was not diseased. 

 The same organism was isolated both from the stem and from the leaves, 

 and with it the disease was reproduced repeatedly by inoculations. 



Some of the soil from two different farms in Beaufort County was 

 obtained for tests in Washington, D. C. One of these farms was the one 

 which suffered the 98 per cent loss. Lettuce plants in seedling stage, 

 half-grown plants, and nearly mature plants were transplanted to pots 

 containing this supposedly diseased soil. The plants were watched 

 carefully for nearly a month, but no trace of the disease appeared. 



Samples of soils from diseased and healthy fields were examined by 

 Dr. Oswald Schreiner, Biochemist in Charge of Soil Fertility Investigations, 

 Bureau of Plant Industry, but he could find no significant differences 

 between the analyses of the diseased and healthy samples. It seems 

 reasonable to suppose that the weakened state of the plants, owing 

 to the extreme cold, put them in a condition in which bacterial organisms 

 could readily gain access; and the continued weakened state of the 

 plants after the cold spell passed allowed these organisms to use the 

 plants as a good medium for their own growth and multiplication. There 

 must have been considerable expansion and contraction of cells during 

 the freeze and afterwards. This was shown by the frequent occurrence of 

 splits in the stems at the surface and just below ground. This splitting 

 or absence of splitting might account for the presence of bacteria in some 

 stems and not in others. And the presence of the bacterial spots on the 

 leaves where there was no stem infection might be the result of practically 

 the same conditions following the expansion and contraction of cells of 

 the leaves. The lower leaves and those nearest the soil were always the 

 most spotted. 



