ORNAMENTAL PLANTS 453 



extending rapidly in all directions. Eventually the dis- 

 coloration extends into the bark, which rots away. 



In the seed bed the same disease may occur as damping 

 off. See p. 60. It seems that the original infection 

 always occurs in the seed bed, not 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 infected should not be used. The 

 disease may be prevented by avoidance of the conditions 

 that favor damping off. 



Rhizoctoniose. See carnation. 



Rust (Coloeosporium sonchi (Pers.) Lev.). — Orange-red 

 sori, waxy in consistency, are produced in great niunbers 

 upon the leaves, which when badly affected curl and die and 

 thus bring about the death of the plant. 



BALM, SILVER 



Rust (Puccinia Menthce Pers.). — Some thirty species of 

 members of the mint family, including peppermint, thyme, 

 catnip, monarda, are affected by this rust. It is recogriized 

 by its sori, which are at first cinnamon-colored, later chest- 

 nut-brown. Badly diseased leaves curl and die. 



Infected refuse should be burned. 



BLETU 



CoUetotrichose (Colletotrichum Bletiae Halst.). — The 

 leaves of this orchid are frequently disfigured by very dark, 

 almost black spots. In late stages the dead tissue falls 

 away, leaving merely shreds of veins traversing the place 

 of disease. 



