224 MANUAL OF FRUIT DISEASES 



perithecia. The spores come to rest on the young leaves or 

 fruits. Following germination of the ascospores a weft of 

 mycelium develops which grows superficially. At certain 

 places, however, small sucker-like bodies, called haustoria, 

 are sent into the host cells; these are the organs of feeding. 

 Nourishment is derived from the host and the mycelium grows. 

 In the early part of the growing-season summer spores are 

 formed. They are produced in chains on erect branches sent 

 up from the weft of surface mycelium. The conidia (spores) 

 are produced in great numbers, and soon fall away, giving the 

 lesion a dusty or powdery aspect, whence the name powdery- 

 mildew. The conidia are scattered by the wind, and they give 

 rise to new infections similar to those just described. Toward 

 autumn the perithecia are formed, and these serve to carry the 

 fungus through the winter. 



Control. 



This is one of the most difficult of powdery-mildews to con- 

 trol, particularly in wet seasons. However, if remedial measures 

 are thoroughly applied, less difficulty will be experienced. The 

 work may be done at a slight expense. 



The standard remedy has been to spray with potassium-sul- 

 fide (liver-of-sulfur), one ounce to two or three gallons of water. 

 In New York State and elsewhere lime-sulfur solution, at the 

 rate of one to forty, has proven effective. In Oregon it has 

 been found that an application of lime-sulfur at dormant 

 strength on dormant branches, followed by a solution diluted 

 one to thirty, gives excellent satisfaction. It is suggested that 

 it may be desirable to use potassium-sulfide in later applica- 

 tions, since lime-sulfur leaves a deposit on the sprayed parts. 

 The applications of a fungicide should be made as follows : (1) 

 when buds break open; (2) at intervals of ten days until at 

 least five applications have been made. The more susceptible 

 varieties may demand more sprayings. 



Pruning in the fall may reduce the amount of the inoculum. 



