Minnesota Plant Diseases. 



379 



infects a rose plant. The mycelium, thus produced, soon com- 

 mences the formation of summer spores. It is the abundance of 

 the latter spores, and the rapid infection by their means, that 

 makes the rose mildew dangerous. 



Flowers of sulphur dusted on the leaves of the plant are chief- 

 ly employed to prevent the germination of the summer spores. 

 The mycelium is also killed by the sulphur treatment. "For 

 greenhouse treatment paint hot water pipes with mixture of sul- 



FIG. 204. Powdery mildew of roses, showing the superficial mycelium and summer spores 

 on the leaves. A germinating spore is seen in the foreground. (On a peach leaf.) 

 After Tulasne. 



phur and oil. Potassium sulphide or an ammoniacal solution of 

 copper carbonate can be sprayed on the foliage. Spraying out of 

 doors can be done with bordeaux, if there is no objection to the 

 sediment on the leaves." (Conn. Ag. Ex. Sta. Bull. 142 1903.) 

 Powdery mildew of chrysanthemums (Oidium chrysanthemi 

 Rabh.). The powdery mildew is an occasional destroyer of 

 chrysanthemum plants in homes and greenhouses. As in the 

 other powdery mildews, the mycelium is superficial and forms a 

 cobwebby or mold-like growth on the surfaces of the leaves. 

 From this mycelium arise necklace-like strings of spores in a 



