724 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



PLANT DISEASES. 



[A Resume of Six Lectures delivered to the Students in the Society's 



Gardens at Chisicick.] 



By George Massee, F.L.S. 



I. — General Considerations respecting Plant Diseases. 



Most, perhaps all, gardeners and farmers recognise the existence of 

 plant diseases in some form or another. This is more especially true 

 of the injm'ies caused by insects, due partly to the fact that the work of 

 destruction can in many instances be observed in actual progress ; and, 

 furthermore, the existence of insects in their various pliases of develop- 



FiG. 303. — Pear-leaf Cluster-cup {Gymnosporangium sabince). 



A fungus growing on two different kinds of plant at different periods of its life- 

 cycle. 1. The spring stage of the fungus on a living Juniper branch, reduced in 

 size. 2. Spore of same, x 300. 3-4. 'Cluster-cup,' or summer form of fungus fruit 

 on living Pear-leaves, reduced in size. 5. Two ' cluster-cups,' one cut open, 

 slightly X . G. Spores of cluster- cup condition, x 300. 



ment is perfectly familiar to most people li^■ing in the country, from 

 practical experience in more or less successfully combating the persistent 

 attacks of "blow-flies," Cabbage and Gooseberry caterpillars, Turnip 

 flies, &c. 



Now with the majority of destructive fungi the case is very different. 

 Attempting to convince people, who accept as an inviolable rule the old 

 adage that "seeing is believing," that a fungus so minute as to be quite 

 invisible to the naked eye is capable of doing as much injury to plants as 

 the codlin moth or the Cabbage butterfly, creatures of quite respectable 

 dimensions, is asking too much. 



Again, the entire life-history or mode of life of most fungi is so very 

 different from that of any of the plants with which the gardener or the 



