382 
SOME GENERAL PLANT PROBLEMS 
seed and seed beds. These failed because the fungus was 
in the soil of the field and not on the seeds or in the seed 
beds. (6) Using new land for seed beds. This failed for 
the same reason, (c) Crop rotation, to give the fungus a 
chance to die out. A period of fourteen years was found to 
be too short, so that also was impracticable. ( d ) Fertiliza¬ 
tion of the soil with a view to obtaining such vigorous plants 
that they could resist the disease. No success was attained, 
(e) Soil disinfection, for the purpose of ridding the soil of the 
fungus. Nothing was found that was cheap enough or 
that would kill the fungus without being detrimental to the 
growth of the crop. (/) Finding plants able to resist the 
disease, based on the experience of finding now and then a 
head in a whole field that had been able to live when the 
rest were killed. 
The method used was to take such heads and raise seed 
from them for next year’s crop. These plants were found to 
produce a higher percentage of resistant plants, the best of 
which were then selected to produce the seeds for another 
crop. In this way strains have been developed that are 
practically immune to the attacks of the fungus. Much 
still remains to be done, but enough has been accomplished 
to make sure that success in combating the disease lies 
in producing disease-resisting plants. No one has yet dis¬ 
covered what the differences are that make immunity pos¬ 
sible in one plant and not in another. 
318. Potato Wart. — This is a disease dangerous to the 
common potato which has been known in Europe for many 
years, but which has been found in the United States only 
since 1918, when it was discovered in Pennsylvania. It was 
introduced on an importation of several millions of bushels 
in 1911, which were distributed over the eastern part of the 
United States. Much attention is now being given to 
locating centers of infection, in order that quarantines may 
be established. 
