1921.] 



Hop-" Mould " and its Control 



then germinates at once, infecting the leaf in exactly the same 

 maimer as the summer-spore does (Fig. 4), and in a few jays a 

 " powdery " patch of white " mould " is produced, bearing 

 hundreds of summer-spores. Thus is brought about the first or 

 primary outbreak of " mould " for the season. 



As the result of infection by a single winter-spore there may 

 arise, within three weeks, many hundreds of little patches of 

 " mould " scattered through the hop-garden, unless by the use 

 of sulphur during May and June the primary outbreak is 

 smothered and killed before its summer-spores have time to 

 spread. The hop-grower must realise that when in May and 

 June he sees the " fly " arriving on his hops, at the same 

 period the winter-spores of the mildew (unfortunately invisible) 

 are being shot out of the soil and the first infections are 

 occurring. 



From the point of view of the practical farmer, the most 

 important facts in the life-history of the hop mildew are (1) 

 the fungus is confined to the surface of the above-ground parts 

 of the hop-plant, never entering the " hill " or occurring on 

 the roots; (2) during the winter months it exists only in the 

 form of winter-spores within closed conceptacles which become 

 dispersed over the surface of the soil of the hop-garden. The 

 following year, about May, the winter-spores are liberated 

 and produce at once the highly infectious white " mould." 

 There is no living spawn existing through the winter. 



Damage caused by " Mould — 



(a) To the Leaf. — Although serious injury to the hop-leaf is seldom 

 caused by "mould/' it is essential that it is destroyed on the leaves, as 

 otherwise the summer-spores produced there in immense numbers spread 

 the disease to the ''burr" and hop-cones where irreparable damage may 

 be done. Further, it is comparatively easy to kill the mildew on the 

 leaves, since the mildew is fully exposed and hop-leaves arc easily 

 sulphured, whereas " mould " on the hop-cones is difficult to deal with. 



(b) To the "Burr." — The greatest damage is caused when " mould" 

 attacks the ''burr," i.e., the female inflorescence which develops later 

 into the hop. Each inflorescence when ready for fertilisation by pollen 

 from a male hop shows a bunch of spreading branches {stigmas) — a stage 

 called the "brush" by farmers. If summer-spores are carried by the 

 air to the "brush.'' they readily infect it, it becomes white with " mould.** 

 and is soon converted into a hard white knob, with the result that no hop 

 is formed. {Fig. S.) The infection of the "burr" is usually brought 

 about by summer-spores set free from powdery patches of ''mould" on 

 the hop-leaves in the same garden : more rarely the spores are carried on 

 the air from a neighbouring hop-garden infested with " mould." If the 

 "burr" becomes attacked by "mould'* it is impossible to save it. 



