PESTS OF THE HOP CROP. 149 



soon increase in size, generally regularly from a center, so 

 that the spots are approximately circular. As the patches 

 increase to about one-eighth of an inch across, they become 

 whiter in color (Fig. 76), and have a dusty or floury appear- 

 ance. 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 are 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 develop- 

 ment is stopped. Often the white patches of mold 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. More frequently, however, if the 

 mold is allowed to remain unchecked, and the weather is 

 unfavorable to the growth of the hop plant, the patches, 

 especially on the lower surface of the leaves and on the 

 young hops, become covered with extremely small, dark, 

 rusty-brown specks, and the white, dusty character of the 

 spot gradually disappears. 



The time at which mold is first observed varies with 

 the season. Gardens once seriously attacked and neglected 

 are always specially liable to an annual recurrence of the 

 disease, unless measures are taken to get rid of the trouble. 



Cause The ordinary symptoms can readily be seen and 

 followed by the naked eye, but the exact cause and process 

 of development can only be appreciated fully after making 

 observations with a good microscope. A mold spot in its 

 early stages is then seen to be made up of a tangled mass 

 of branching threads (hyphae). The threads are clear, 

 transparent, hollow tubes, filled with living substance 

 (protoplasm), and constitute the body or spawn (mycelium) 

 of a fungus, known as SpJiacrotJieca castagnei, which is 

 one of a large class known as "true," or "surface" mildews. 

 Careful observations show that the threads are not merely 

 resting on the surface of the leaf, like a tangled skein of 

 cotton upon a table; they cannot be blown away or washed 

 off, as at various points they are attached by short suckers 

 (haustoria) which just penetrate into the substance of the 

 leaf and serve the double purpose of holding the fungus in 

 its place and acting as roots to suck up and convey the sap 

 from the hop plant into its own body. The spawn of the 

 fungus or mold thus lives upon the substances manufactured 

 in the hop leaf, and is enabled to grow and spread. Not long 

 after the fungus threads are established, they send up into 

 the air short branches which give rise in a little time to 

 rows or chains of minute oval-shaped bodies (Fig. 76a). 

 known as spores (conidia). These spores, which for ordinary 



