Survi\'al of Plant Pathogens in the Soil 291 



true of a nematode parasitie on the Japanese beetle, Popillia japonica, 

 which spends a large part of its life cycle in the soil. 



The mosaic disease of tobacco, caused by a filterable virus, may 

 persist in the soil. Tomato mosaic was also found to be able to live 

 for 4-6 weeks in field soils, but there was no evidence of overwinter- 

 ing of the virus. 



Different soil-borne parasites are affected very differently by en- 

 vironmental factors. High soil temperatiu'es stimulate the develop- 

 ment of the Fiisariiim disease of cabbage, and low temperatures 

 inhibit it. TJiielavia root rot of tobacco is checked in warm soils 

 and is seriously injurious only in cool soils. Jones emphasized this 

 seasonal contrast. Of two successive summers one was very cool, 

 with a midsummer soil temperature averaging about 5°C lower than 

 that of the succeeding summer; the root rot of tobacco was unusually 

 severe, whereas cabbage remained relatively free from disease. The 

 succeeding year with its warm midsummer period caused destruc- 

 tion of the cabbage crop because of the yellows disease, whereas the 

 tobacco was free from root rot. High soil temperatures favor vascu- 

 lar diseases, including flax wilt, tomato wilt, and cabbage yellows. 

 High temperature (25-27°C) also favors Sclerotium rolfsii and 

 certain other plant-pathogenic fungi. 



Soil moisture also exerts an important influence on many plant 

 parasites. Sanford found dry soils favorable and wet soils inhibitory 

 to potato scab. Soil fungi causing "damping off" are favored by 

 high moisture; wet soils, even to the saturation point, favor the club- 

 root parasite of cabbage. Powdery scab develops best in periods 

 of damp, rainy, and cloudy weather and is favored by poor drainage. 



The amount of organic matter present in the soil influences plant 

 infection, since it offers a source of energy for the saprophytic ex- 

 istence of the organisms. Certain pathogens cannot infect the host 

 plant in pure sand, but will do so in the presence of organic matter, 

 which allows the myceliimi to exist for some time. Clay soils are 

 more favorable to infestation than are sandy soils. The root rot of 

 peas is not disseminated by the seed, but spreads through the soil 

 and is especially favored by a high content of organic matter. The 

 cotton and alfalfa root rot spreads through the soil radially with a 

 growth similar to fairy rings; it is favored by heavy soils, humid 

 weather, and dense cover crops. Addition of organic matter to soil 

 also favors development of nematodes and rainworms. In some 

 instances, the application of organic matter has helped to check 

 the spread of a disease, for example, cotton rot. 



