88 



STATE HORTICULTURAL SOCIETY. 

 POWDERY MILDEWS ( PerisporidcecB.) FIG. 4. 



While the downy mildews flourish, as noted, in cold, wet seasons, the 

 the powdery forms luxuriate in seasons of drouth, so that, whatever the 

 summer, the conditions will favor one or the other of these diseases. 



In their structure, also, 

 the powdery mildews are 

 quite unlike the other 

 form. The mycelium in 

 the form of white, jointed 

 filaments, spreads like a 

 cobweb over the leaves 

 and stems of the host 

 j)lants, but the threads 

 do not enter the tissues, 

 the food being taken up 

 from the underlying 

 cells by minute 

 haustoria(Fig. VV','^) 

 which p e n e t rate the 

 epidermis. This fungus 

 is generally found on the 

 upper surface of leaves. 

 Under favorable con- 

 ditions the fungi wax 

 and grow strong, and 

 at length send up fruit- 

 ing hyphfe (Fig. 4, ') 

 on which the summer 

 spores are produced by 

 constriction. The one at 

 the tip soon falls off and 

 the others follow. If 



Fig. 4.— Powdeky Mildews of Rose, Cheery, and Geape. they tall on gOOCl SOll 

 1, summer spore of rose mildew; 2-4, development of carpogonium; they at OnCC germinate 

 5, winter spore (perithecinm) with ascus, containing ascospores a n d produce a uew 

 escaping; 6, summer spores of cherry powdery mildews; 7, winter i .■*■ 



spore of same; 10, mycelium with haustoria in cells; 11, 12, 13, theinyceiium. 

 same, of the powdery mildew of the grape. —After De Bary. The willter SDOreS are 



formed in much the same manner as those of the downy mildews, but their 

 structure is far more complex. After the fertilization of the carpogonium, 

 finger-like processes are developed around it (Fig. 4,^), and becoming seg- 

 mented (Fig. 4,*) they have a cellular or raspberry appearance. From the 

 exterior, appendages of various forms are developed. In some cases they 

 are straight, in others forked at the extremities, and in yet others are 

 coiled as in the Uncinula of the grape (Fig. 4,"). Within these 

 sporocarps or perithecia, one or more oval bodies (asci) are developed, 

 and inside these are the ascospores, varying in number from two to eight. 

 These winter spores remain in their cases on the fallen leaves until spring, 

 when tha asci burst the walls of the perithecinm and the ascospofes escape 

 through orifices at the extremities of the asci. They soon germinate and 

 the cycle is completed. 



Living, as it does, entirely outside its host, this fungus is readily amen- 

 able to fungicides; sulphur or sulphur fumes are used with success, where 

 the air is confiiied, and all of the copper mixtures are effectual. 



