340 OHIO EXPERIMENT STATION: BULLETIN 214 
Fig. 20. This shows root portions of seedling lettuce plants with dark spots, lesions caused by 
attacks of the rosette fungus, /th/zoctonia, With the younger plants these attacks cause large mortality 
and in very small seedlings the stem of plantlet may early collapse after the manner shown in 
rotting specimens, (From Circular No. 57). 
THE AVOIDANCE AND PREVENTION OF SOIL INFESTING DISEASES 
We, perhaps, may assert that the law of nature is that of a diver- 
sified plant covering; at any rate the law of successful culture will 
permit of statement in termsofcrop rotation. And it is true that as 
culture ages the number and seriousness of plant diseases increase 
almost in geometric ratio. Itis further conspicuously true with 
respect to those areas devoted largely to continuous culture ina 
single crop or ina group of closely related crops such as the growing 
of wheat in Western United States and Canada, also in the growing 
of flax and other crops. Potato growing in San Joaquin county, 
California, illustrates this danger. Muck lands devoted to vegetable 
culture, tempt the grower to continue his crops of celery, onions, 
etc. Here we haveasatrue result the accumulation of diseases 
which attack these plants; thus for field culture we are sao ca 
