410 



Diseases of Economic Plants 



In the seed bed the same disease may occur as damping- 

 off. See p. 19. It seems that original infection always oc- 

 curs in the seed bed, never in the open. 



Plants started in the open are not susceptible. If it is 

 necessary to start them under glass, too thick sowing should 

 be avoided, and they should be ventilated thoroughly. 



Soil that is known to be in- 

 fested should not be used. 

 The disease may be pre- 

 vented by avoidance of the 

 conditions that favor damp- 

 ing-off. 



Yellows. ''^^ — Growth is 

 spindling and the shoots and 

 leaves are yellow, but no 

 death of affected parts 

 occurs. In badly diseased 

 plants no flowers, or only 

 rudiments of such, develop 

 upon the sick shoots. In 

 other cases the flowers turn 

 greenish-yellow, wholly or 

 in part, with malformed 

 floral parts producing little 

 or no seed.. No parasites 

 are present and the cause 

 of the condition is unknown, 

 but the striking similarity 

 to peach yellows is obvious. 

 The same disease affects the marguerite. 



Rust (Coleosporium soUdaginis (Schw.) Thuem.). — ■ 

 Orange-red sori, waxy in consistency, are produced in great 

 numbers upon the leaves, which when badly affected curl, 

 die, and thus bring about the death of the plant. Stem- 

 rot (Corticium). — • See p. 21. 



Fig. 216. — Rust on barberry. 

 After Stakman. 



