36 MANUAL OF TREE DISEASES 



Symptoms of powdery mildews. 



Of the fifteen species or varieties of these fungi on trees in 

 this country, all are confined to the upper or lower surface of 

 leaves, except Sphoerotheca lanestris, causing the brown mildew 

 rf leaves and twigs of oak, Sphoerotheca phytoptophila, on buds, 

 twigs and leaves of the witches'-brooms of hackberry, and, 

 Erysiphe aggregata on the fertile catkins of alders. In all of 

 these species, also, the mycelium is white and appears powdery 

 except that, with age, the mycelium of Sphwrotheca lanestris 

 becomes brown, and that of Erysiphe aggregata somewhat 

 yellowish. The mycelium of all the species is entirely super- 

 ficial and is visible in mass to the unaided eye. Under cer- 

 tain favorable conditions of growth, the mycelium becomes so 

 abundant that definite white (or in some cases brown or yellow, 

 as mentioned above) felts are formed. More often, however, 

 the growth is not so plainly visible and the affected areas 

 appear as blanched or slightly whitish and indefinite. In 

 the early stages of development, the mycelium produces enor- 

 mous numbers of white spores which cause the affected area 

 to appear powdery. This character is the main diagnostic 

 symptom for recognizing these mildews in the spring and 

 summer. 



As the season advances, the powdery white mycelium becomes 

 less apparent in most cases and only traces can be detected. 

 However, during late summer and autumn when these characters 

 become less distinct, many small yellowish, globose bodies just 

 visible to the eye begin to appear on the affected part of the 

 leaf. These bodies become dark brown or black and are the 

 fruiting-bodies which contain ascospores. Except when very 

 young succulent growth is attacked, no material damage is 

 done to the leaves. Young growth, however, may be stunted 

 in development, as when S. lanestris occurs on oak twigs and 

 leaves (see page 243). 



