156 DISEASES OF CULTIVATED PLANTS 



exotic origin, but how or from where it was introduced into 

 Europe is by no means certain, as it occurs on native plants 

 both in the United States and in Japan. The mycelium of 

 the fungus forms white or greyish patches on the upper 

 surface of the leaves, young shoots, inflorescence, and fruit. 

 These patches, which are small at first, gradually extend until 

 the entire surface of the organ attacked becomes covered 

 with a white mildew. After a time numerous upright 

 chains of white conidia are formed, and as these chains 

 collapse, the surface of the mycelium becomes covered with 

 a powdery mass of conidia, as if it had been dusted with 

 flour, hence the name powdery mildew. These conidia are 

 dispersed by wind, rain, etc., and consequently the epidemic 

 continues to spread throughout the season unless stringent 

 preventive measures are resorted to. The injury caused by 

 the fungus is due to its haustoria or food-absorbing organs 

 entering and killing the epidermal cells of the host. The 

 effect produced depends on the organ attacked. When 

 young leaves are attacked their further growth becomes 

 irregular, and they usually shrivel and die ; in the case of 

 older leaves the mildew may cause but little injury, merely 

 causing brown spots to appear on the portions attacked. 

 When young shoots are infected, the upper portion usually 

 blackens and dies, whereas when older branches become 

 diseased, the presence of brownish stains afterwards indi- 

 cates the portions occupied by the parasite. When the 

 flowers are attacked no fruit is formed, and when the fruit is 

 attacked it usually cracks and becomes useless. When the 

 seasonal vigour of the host is on the wane the ascigerous 

 form of fruit appears on the mycelium, under the form of 

 minute, rounded bodies, at first yellow, -gradually changing to 

 dark brown. These fruits or perithecia contain the winter 

 spores, and are furnished with several radiating appendages, 

 each one hooked at the tip. 



Ascigerous form. Mycelium occurring on both sides of 

 the leaf, sometimes almost disappearing; perithecia minute, 

 scattered, globose, depressed, blackish, furnished with a variable 

 number of straight, or slightly-wavy appendages, curled at 

 the tip, the lower half brownish. Asci varying in number from 

 4-9, spores 4-7 in an ascus, elliptical, ends rounded, 18- 



25 X 10-12 fJL. 



Conidial form. Chains of hyaline elliptic-oblong conidia. 

 Professor Galloway states : * It succumbs readily to sulphur, 



