' HOP-MOULD ■ 749 



whicn usually remain dormant during winter and germinate in 

 the following spring, when they are set free by the bursting of 

 the enclosing ascus and perithecium. 



The conidial-forms of these fungi were formerly regarded as 

 species of the genus Oidium ; and in cases where the ascocarps 

 are unknown, this generic name is still adopted. 



Many of the Erysiphacese are common injurious parasites of 

 farm and garden crops. The chief genera worthy of mention 

 are Sphcerotheca, Erysiphe, and Uncinula. 



(i) In Sphzrotheca the perithecium is spherical and contains 

 one ascus only ; the hyphal appendages to the perithecium are 

 simple (h, Fig. 255). To this genus belorigs the destructive 

 parasite known in hop-growing districts as ' hop-mould.' 

 ' Hop-Mould.' 



Symptoms. — In the earliest stages the mould is seen as small, 

 light-coloured patches, chiefly upon the upper surface of the 

 leaves. If the nights are cold and damp and the hop plants 

 in a backward or weakened condition the patches soon increase 

 in size, generally regularly from a centre, so that the spots 

 are approximately circular. As the patches increase to about 

 one-eighth of an inch across they become whiter in colour 

 and have a dusty or floury appearance. Fresh spots show 

 themselves on the younger leaves, and in bad cases the malady 

 spreads from the lower leaves, where it is generally first seen, 

 to those higher on the plant and even to the tender shoots and 

 young hops. 



In all cases the plants suffer in health, but it is only when the 

 tender shoots and young growth is attacked that serious damage 

 is done. The young hops and tips of the laterals on the bine 

 then lose their soft, succulent character and become deformed ; 

 the parts attacked dry up and development is stopped. 



Often the white patches of mould do not spread ; the spots 

 lose their dusty appearance and vanish, leaving behind always 

 a small yellow or brown dead place upon the leaf attacked. 



