378 



Minnesota Plant Diseases. 



The powdery mildew or blight of the rose [Sphaerotheca 

 pannosa {Walk.) Lev.]. An enormous amount of damage is sus- 

 tained yearly by the ravages of this blight in gardens and green- 

 houses. The leaves of the attacked rose bushes become covered 

 with a fine white coat of the fungus mycelium and often become 

 distorted or stunted in 

 various ways. The 

 young leaves and buds are 

 especially damaged, and 

 many leaves are killed. 

 The mycelium sends suck- 

 er-like branches into the 

 interior of the epidermal 

 cells of the host and from 

 these draws its nourish- 

 ment. This of course re- 

 sults in a drain upon the 

 host plant. During the 

 summer erect threads are 

 produced on the surface 

 of the leaves and these , 

 form chains of spores, 

 which are carried about 

 by the wind and rapidly 

 spread the disease from 

 leaf to leaf and from plant 

 to plant. These summer 

 spores, therefore, act in a manner similar to those of the wheat 

 rust. Toward late summer and fall small black bodies about 

 the size of a pin-point are formed on the mycelium, and these 

 are the closed sac-capsules. They are yellowish-white, when im- 

 mature, becoming black when mature; they are attached to the 

 mycelium by special brownish appendages. TTiey have a more 

 or less membranous wall, which is divided into polygonal areas. 

 The sac-capsule, when broken, shows a single, spherical, colorless 

 sac, in which are found eight oval spores. The sac-capsule does 

 not open until spring, when the wall decays, setting the spores 

 free. These spores, therefore, function as winter spores. In the 

 spring they germinate, by sending out a fine tube, which again 



Fig. 203.— Powdery mildew of roses. A leaf of a 

 rose attacked by the disease. After Clinton. 



