1921.] 



Hop-" Mould m and its Control. 



151 



Humuli (D.C.) Burr.) — composed of a mass of interlaced fine 

 white branched threads (hyphce) — creeps over the surface and 

 at short intervals sends minute branches — the " suckers 

 (Jmusloria) — from the under surface of the threads into the cells 

 of the hop-leaf or " hop." These suckers extract from the sap 

 of the hop -plant the food the fungus requires — much as the 

 needle-like proboscis of the h.O'p-Aphis does when this insect is 

 feeding. From the spawn upright branches (conidiophores) arise, 

 each bearing a necklace-like chain of summer-spores (co)iidia)* 

 (Fuj. 2). 



As each summer-spore (conidium) at the top of the chain 

 becomes ripe, it separates and becomes free (Fig. 3). fresh spores 

 being formed below as the top ones become ripe and fall oft'. In 

 this way myriads of spores are produced in favourable weather 

 conditions and accumulate in white, " powdery " masses over 

 the surface of each spot of " mould " — a condition indicating 

 infallibly that " mould " is "on the run." These summer- 

 spores serve for the rapid propagation of the fungus, and being 

 exceedingly minute and light they are easily carried by the 

 wind for distances of a mile or more. This manner of distribu- 

 tion accounts for the fact sometimes to be noticed that an epi- 

 demic of " mould " appears suddenly in a hop-garden hitherto 

 entirely free. If our eyes were able to observe such small objects — 

 or if the summer-spores were much larger — we should frequently 

 see in the air during summer in hop-growing districts a white 

 dust-like cloud composed of myriads of these spores — much as we 

 often see masses of " thistle-down " blowing about. Prof. F. M. 

 Blodgett has computed that on a square inch of leaf-surface 

 covered with hop-mildew 2. 800. 000 summer-spores are produced, 

 and remarks! " from this it will be seen that, w 7 hile the large 

 majority of these spores may perish, there may still be enough 

 left on a single leaf to infect a whole hop-garden." 



Each summer-spore behaves like a seed in giving rise to a 

 new individual. On reaching a healthy leaf or " hop." it at 

 once proceeds to infect it; it puts out a root-like germ-tube, with 

 a terminal knob (apprcssorium) w T hich attaches itself firmly to 

 the surface of the hop-plant (Fi-f. 4^. Within 24 hours a sucker 

 is produced from the under surface of the knob. The sporeling, 

 or young fungus, is now both firmly anchored and supplied with 

 food: it rapidly develops the threads of the spawn ;ind. under 

 favourable conditions, a " powdery " patch of H mould " with 

 chairs of summer-spores is produced within ;i week or ten days. 



* These can easily be stvn with a |><>rket inaurnifvinLr glasP. 

 f Cornell University, Agric. Bxper. Stat., Bull. :*'JS (1913). 



