26 Diseases of Economic Plants 



Certain varieties of cowpeas (p. 335), tobacco, grapes, figs, 

 and watermelon show resistance. Calcium cyanamide, 

 1,000 to 5,000 pounds per acre applied several weeks be- 

 fore planting and followed by abundant water, reduces the 

 number of nematodes. 



Hairy-root, also due to nematodes, is discussed on p. 162. 



Certain fungi as Penicillium (Soft-rot or Dry-rot) p. 51, 

 Phytophthora omnivorum, Sclerotinia libertiana (Drop, Wilt) 

 p. 196, are so general in their habits as to be found on almost 

 any host. 



(i^ Disease due to Impure Air.'-^' -^ — ■ Plants are sensitive to 

 certain chemical impurities of the air and may either be killed 

 or greatly retarded in growth by them. These impurities are 

 chiefly smoke and gases or solids resulting from industrial 

 operations. The aerial emanations of smelters are of a 

 character peculiar to the processes involved and of a quantity 

 to make their effect apparent to considerable distances. It 

 has been clearly shown that even small quantities of illumi- 

 nating gas or sulfur dioxid are very deleterious in their 

 effect upon plants. So marked are the effects upon vegeta- 

 tion that definite zones of injury may be mapped in the 

 vicinity of large air-pollution centers, the outer zone extending 

 to a distance of from 75 to 100 miles. Illuminating gas es- 

 caping from mains into the air of the soil frequently causes 

 death of trees. There is considerable difference in resistance 

 between different varieties of plants and if the air pollution 

 cannot be abated the more resistant varieties should be 

 planted. 



Soil diseases.-*^ — A group of plant diseases of special de- 

 structiveness comprises those of which the causal organism 

 resides in the soil from year to year ready to infect any 

 susceptible crop planted thereon. 



Notorious among such are the wilts of the cotton, tobacco, 

 tomato, cucumber, muskmelon, eggplant, cowpea, water- 

 melon, cabbage, flax, sweet potato and potato; the club- 

 root of cabbage and other crucifers; black-rot of similar 

 plants; smut of onion. Other soil diseases are potato M'art 



