236 Diseases of Economic Plants 



toes frequently show cavities at the centers, lens-shaped 

 clefts with browned linings, due apparently to tension result- 

 ing from unequal growth. Though this is not a true disease 

 such tubers are discriminated against in grading. Varieties 

 subject to this trouble should be planted closely since crowd- 

 ing helps to overcome the tendency. 



Internal brown-spot, scattered, rusty brown spots, 2-10 

 cm. in diameter, occur irregularly throughout the potato. 

 No parasite is present and the diseased condition is not trans- 

 mitted through the seed. However seed showing this chai- 

 acter should be avoided. 



Net-necrosis. — Netted, brown or black areas, beginning at 

 the stem end, extend through the tuber. The disease is not 

 due to parasites and probably affects the yield but little. 

 Seed potatoes showing this disease should be avoided. An 

 Anthracnose (Colldotrichum) has been described on stems. 



Disease Prevention 



In general to prevent potato diseases: 



1. Plant, if possible, in uninfested soil. 



2. Destroy disease-bearing refuse. 



3. Rotate crops judiciously. 



4. Use only seed free from seed-borne diseases, i. e., scabs, 

 black-scurf, wilts. 



5. Disinfect seed in case such diseases as scab and black- 

 scurf are present, p. 240. 



6. Spray with 4-4-40 Bordeaux mixture. 



Potato Spraying -•"• -^^ 



As with other crops, spraying should not be directed or 

 planned to meet merely one of the diseases, but rather to 

 give the plant the best possible protection against all of its 

 enemies. The chief enemies to be met by spraying are 

 early-blight, late-blight, and the Colorado beetle or "potato 

 bug." 



