238 Diseases of Greenhouse Crops 



the chr}^santhemum rust attacks this plant and no 

 other host. Uredospores from dandelions, burdock, 

 ox-eye daisy, when sown on the chrysanthemum 

 failed to produce the rust. On the other hand, ure- 

 dospores taken from the chr^-santhemum and sown 

 on chrv^santhemum hosts reproduce the disease. The 

 disease no doubt is brought in with infected plants, 

 or cuttings made from a rusted plant. 



The Organism. It is very strange that the uredo- 

 spore stage (fig. 50, d) is the only stage of the 

 chrysanthemum rust that is found in the United 

 States. This makes the fungus short lived unless 

 it is continually transmitted from living chrj'santhe- 

 mum leaves to others. The uredospores are spheri- 

 cal to pyriform, possessing a spiny membrane and 

 three germ pores. The teleutospores were mentioned 

 and figured by Massee ^ and by Roze.f However, 

 without making cultures it is doubtful whether these 

 claims can be accepted as final. 



Control. It is claimed that the variety Queen is 

 very susceptible to rust. It is also believed that pot- 

 grown plants are less resistant to rust than are plants 

 growing in benches. Hand picking, selecting of 

 clean, strong stock, and inside culture are recom- 

 mended to keep the rust in check. The chrysanthe- 

 mum rust, although serious, need not be feared by 

 the careful grower who selects his stock and who is 

 careful about the watering and the ventilation of 

 the house. 



*Massee, G., Gard. Chron. 24: 269, 1898. 

 t Roze. Bui. Soc. Myc. de France 17: 88, 1900. 



