lEAF DISEASES AND INJURIES 37 



Cause. 



All the powdery mildew fungi belong to the same family and 

 have similar life histories. The black fruiting-bodies contain- 

 ing the spores over-winter on the fallen leaves. They crack 

 open the next spring, forcibly Uberating the ascospores which 

 are caught up by the wind and are carried away to infect the new 

 developing foliage. Infection is brought about by the asco- 

 spores which lodge on leaves of the right kind of tree, if conditions 

 of moisture and temperature are correct for germination. The 

 germ tube penetrates the outer wall of an epidermal cell of the 

 leaf and produces a thread of mycelium within the cell. Food 

 materials are thus obtained by the fungus and the mycelium 

 branches out from the external part of the germ tube, and forms 

 a richly branching growth over the surface of the leaf. Here 

 and there short lateral branches, called haustoria or suckers, pen- 

 etrate into other epidermal cells to obtain food. In this way the 

 fungus develops until its growth is visible to the eye and some- 

 times, from a single infection or by the intermingling of many 

 individual'infections, the mycelium may cover most of the leaf- 

 surface. Numerous short erect branches are formed and from 

 these are cut off large numbers of small spores which are so 

 abundant that the thready character of the mycelium is masked, 

 and the area appears powdery. These spores are disseminated 

 by the wind and may start new infections. The mycelium 

 and the spores, developed before the black fruiting-bodies ap- 

 pear, are so nearly alike for all powdery mildew fungi that they 

 cannot be distinguished one from another. The microscopic 

 characters of the black fruiting-bodies (perithecia) and the 

 asci and ascospores which they contain serve to classify the 

 mildews into several genera and species within genera. 



Control. 



Powdery mildews are easily controlled, since the mycelium is 

 external and can be killed by applying toxic substances in a 



