PLANT DISEASES. 253 



in the control of the disease, especially if the first ap- 

 plication is made when the seedlings emerge from the 

 ground; the second when the first true leaves have ex- 

 panded; and the third when the pods begin to form. 



Cabbage Club Root. This disease affects cabbage, 

 cauliflower and turnip, as well as a number of other 

 plants belonging to the mustard family. It is caused 

 by the growth within the roots of a low type of fungus. 

 The roots, large as well as small, become swollen and 

 distorted in the peculiar way that suggests such com- 

 mon names as " finger and toe " and " big root " for 

 the disease. Affected plants may wilt and die during 

 a period of drought, because of the insufficiency of their 

 root systems, and in any case the heads will be stunted. 

 The organism causing the disease lives over for sev- 

 eral years in the soil. The most important means of 

 control is to grow only non-susceptible crops on in- 

 fected soil for about four years. A heavy dressing of 

 lime thoroughly worked into the soil the preceding fall 

 sometimes satisfactorily controls the disease. Ex- 

 treme care must be taken not to infect clean soil by set- 

 ting out plants that have become infected in the seed 

 bed, or by transferring infected soil from any source 

 on tools or on the roots of transplants. 



Potato Scab. This disease is caused by a thread- 

 like fungus which grows in the skin of the potato 

 tuber, producing irregular, rough, corky areas. The 

 fungus lives over from year to year in the soil, and in- 

 fected soil should not be planted to potatoes oftener 

 than once in three or four years. The fungus flour- 

 ishes in soil that is somewhat alkaline, and for this rea- 

 son lime, wood ashes and stable manure favor the de- 

 velopment of but do not directly cause scab. Most 

 commercial fertilizers and green crops when plowed 

 under reduce scab. Clean soils become contaminated 

 by planting infected seed potatoes; these should be dis- 

 infected by soaking the uncut tubers for 2 hours in 

 a solution of formalin, i pint in 30 gallons of water; 

 or of corrosive sublimate, 4 ounces in 30 gallons of 

 water. The only way to secure freedom from scab 



