126 



Minnesota Plant Diseases. 



spores. Accessory spores are also found in abundance in these 

 fungi, many being enclosed in special capsules similar in ap- 

 pearance to the sac-capsules. The mycelium is, moreover, 

 often black. Many of these fungi live on leaves but not in a 

 typically parasitic fashion. They thrive well on the excretion 

 of certain insects and since such excretions are found abun- 



FIG. 52. The fruiting body of the powdery mildew of willow, showing the appendages and 

 spore-sacs. The latter have been forced out of splits in the sac-capsule. Highly mag- 

 nified. Microphotograph by E. W. D. Holway. 



dantly on the leaves of plants these fungi are also found on the 

 surface of the leaves. On account of the abundance of myce- 

 lium produced and on account of its dark color a vigorous 

 growth of mycelium may exclude sunlight from the leaves and 

 thus injure the leaf, although the fungus may not in itself be 

 harmful. These fungi are often known as the sooty molds on 

 account of the soot-like mycelium which is developed. 



