INSECTS, DISEASES AND SPRAYING 55 



condition or conditions favouring the appear- 

 ance of the mildew. It comes from spores, 

 which falling upon the surface of the plant 

 will quickly germinate and produce a cob- 

 webby coating to the foliage. This fungus 

 is so different from the others that have been 

 mentioned, that a word is needed in way of 

 explaining the habit of the parasite, as this lies 

 at the foundation of the rational treatment of 

 the pest. In the first place the rose mildew 

 is a surface feeder and only attaches itself to 

 the epidermis of the host, through which by 

 means of small suckers it derives its nourish- 

 ment from the deeper-seated substance. Being, 

 therefore, entirely upon the surface, the fine 

 threads of the fungus may grow rapidly and 

 make a mildew patch in a very short time 

 that is easily seen. The exact conditions that 

 favour this development of the spores are 

 pretty well known to rose growers. Some 

 of them claim that they can develop the fun- 

 gus at will, on plants under glass, by omitting 

 some essential point in heating, watering or 

 ventilation, particularly the last. If a certain 

 current of air is sure to produce the mildew, 

 it follows that the spores of the mildew were 

 widely scattered through the house, and that 



