DISEASES OF CULTIVATED PLANTS 673 



SOIL INFESTING PARASITES IN FIELD AND FORCING HOUSE. 



The cultivated soil is a medium in which many species of bac- 

 teria arid fungi survive from year to year. The public is familiar 

 with the doctrine of bacterial infection or inoculation of the soil 

 in its relation to the nodules or tubercles of clover, alfalfa, soy beans, 

 cowpeas and other cultivated plants of the Family of Leguminosae. 

 One form of bacterium is not sufficient for both clover arid alfalfa. 

 This flora of the soil both in relation to bacteria and fungi of con- 

 siderable range of species, is enriched by the applications of manure 

 and by the practices of culture; by this is meant that the growing 

 of a given crop a second time or a third time consecutively in the 

 soil increases the probability that the plant roots remaining in the 

 soil are carried over from one crop to the next together with root 

 parasites which cause disease in the plants of this crop. Manifestly, 

 likewise, if in preparation for a given crop to be grown for the first 

 time upon the land, rather liberal applications are made of fresh 

 stable manure containing spores or mycelium, more especially the 

 resting forms of mycelium called sclerotia, the soil will become 

 infected by this manurial application. While this source of infec- 

 tion is rather rare in field culture we have specific examples as in 

 the scab disease of potatoes transmitted in this way; the scab of 

 sugar beets may be carried in like manner. But in forcinghouse 

 culture where heavy applications of manure are made, the chances 

 are greatly increased that soil infection will be produced from the 

 manure. 



It is of value to remember that seed infesting or seed infecting 

 organisms are also very largely capable of survival in the soil nidus 

 of cultivated soils, thus our troubles multiply adequately if our 

 care be inadequate to avoid them. (Ohio E. S. B. 214.) 



The Avoidance and Prevention of Soil Infesting Diseases. We, 

 perhaps, may assert that the law of nature is that of a diversified 

 plant covering; at any rate the law of successful culture will per- 

 mit of statement in terms of crop rotation. And it is true that as 

 culture ages the number and seriousness of plant diseases increase 

 almost in geometric ratio. It is further conspicuously true with 

 respect to those areas devoted largely to continuous culture in a 

 single crop or in a group of closely related crops such as the grow- 

 ing of wheat in Western United States and Canada, also in the grow- 

 ing of flax and other crops. Potato growing in San Joaquin county, 

 California, illustrates this danger. Muck lands devoted to vege- 

 table culture, tempt the grower to continue his crops of celery, 

 onions, etc. Here we have as a true result the accumulation of dis- 

 eases which attack these plants; thus for field culture we are con- 

 stantly facing these problems of soil infesting diseases and the 

 handling of the diseases is not an easy problem since change to 

 rotation may mean a serious decrease in the return from the crop 

 on the special type of soil. While for general field culture avoid- 

 ance of conditions may be successful, this is by no means a .simple 

 matter. Rotation is often absolutely necessary, but thLs same rota- 

 tion will not rid the soil of the onion smut fungus, nor of some 



