274 



Minnesota Plant Diseases. 



Though a very common fungus, it does not very often occur 

 in sufficient amount or at the proper season to cause very ex- 

 tensive damage and usually no preventive methods are deemed 

 necessary. The ordinary sprays for the fungi with superficial 

 mycelia would probably prove effective against this disease. 

 (See also Fig. 52.) 



Powdery mildew of elm$(Uncinula macrospora Peck}. This 

 mildew is not uncommon, though it cannot be said to be abun- 

 dant throughout the state. It attacks only the young trees or 



saplings of elms. 

 On an affected 

 tree the fungus is 

 usually exceed- 

 ingly abundant, 

 covering many or 

 most of the leaves 

 with a very con- 

 spicuous mildew. 

 The latter attacks 

 the leaves and oc- 

 curs on the upper 

 surface. White, 

 circular patches 

 of the superficial 

 mycelium are 

 formed and these 

 may combine 

 with neighboring 

 ones to cover the 

 entire leaf. These 

 patches are con- 

 spicuously white 

 and dense and 

 have a somewhat 

 starchy appear- 

 ance. The win- 

 ter-spore fruiting 



FIG. 135. Powdery mildew of elms (Uncinula macrospora), bodies are SOOre- 

 on an elm leaf. The exceedingly minute black spots are 



sac-capsules of the fungus. The fungus mycelial threads SaC-CaOStlleS and 

 form in spots which are very conspicuous. Original. 



