152 DISEASES OF CULTIVATED PLANTS 



bered that Sphaerotheca humuli> the notorious ' hop mildew, 

 occurs abundantly also on about twenty British weeds, it is 

 not difficult to understand the infection of strawberry 

 plants by winter-fruit produced on one or other of these 

 hosts. 



The conidia are, as usual in the genus, produced in chains, 

 and when mature are elliptical, 30-40X17-24 /x. Peri- 

 thecia minute, dark coloured, with numerous short, brownish 

 appendages mixed up with the mycelium. Asci from sub- 

 globose to broadly ovate, containing up to eight spores, 

 broadly elliptic, hyaline, i5~2ox 12-15 I 1 ' 



It is important that constant watch for the first appearance 

 of the fungus on the leaves should be kept, and prompt 

 spraying with sulphide of potassium should follow. By such 

 means the disease can be arrested before the fungus attacks 

 the fruit. When the disease is general on the foliage, the 

 wisest course to pursue in the late autumn, when the leaves 

 are beginning to fade, is to strew the flat with loose straw or 

 litter of any kind that will burn readily, and fire it. By so 

 doing, all the leaves are destroyed, and although winter-fruit 

 has not actually been found, this does not prove that such do 

 not exist, and should such be the case, they would be 

 destroyed. 



The ground should be kept clear of weeds that harbour 

 the fungus ; among such may be enumerated, wild hops, 

 willow-herb, meadowsweet, etc. 



Hop mildew (Sphaerotheca humuli^ Burr. = Sphaerotheca 

 castagnei, Lev.) is the cause of serious losses to cultivators 

 of hops, both in this and other countries. The mildew 

 commences as small, delicate white patches on the leaf, most 

 abundant on the under surface perhaps. These patches 

 gradually increase in size, until, under favourable climatic 

 conditions, the entire leaf is covered with mildew, which 

 soon presents a mealy appearance, due to the formation of 

 immense numbers of conidia. So long as the mildew is 

 confined to the foliage the injury is not so grave, but when 

 it passes on to the female cones the damage becomes serious. 



Mycelium often disappearing, sometimes persistent, peri- 

 thecia variable in size, blackish, appendages very variable in 

 size and length, ^usually brown, often eight or nine times as 

 long as diameter^of perithecium, the solitary ascus contains 

 eight spores of an elliptical form, averaging 22X15 p. 



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