GEXERAL PRACTICE. DISEASES. 229 



Burrill, Illinois University, U.S.A. ; and the following subjects are founded upon the 

 recorded investigations of Professor Plowright for canker and Dr. Beijerinck for gum. 

 Various sources have been consulted with regard to apple scab (including Nicholson's 

 "Dictionary of Gardening," L. Upcott Gill, 170, Strand, London), both European and 

 American ; and Mr. Worthingtou G. Smith in respect of mildew ; to all of whom we 

 desire to express acknowledgments. 



Canker. This malady affects the apple and pear, also ash, beech, elm, oak, and 

 other trees. Its attacks are not confined to any particular period of the tree's age. 

 Young trees and lately-introduced varieties do not escape its ravages. Yet it is most 

 mischievous in aged trees, and varieties which are increased from old specimens are 

 more subject to it than others are. This attests constitutional peculiarity rather than 

 heredital susceptibility. Sorts resisting cankerous affections do so by the density of 

 their cuticles (their external covering of the bark). Yet there are no varieties of apple 

 and pear that in all conditions of soil and climate are canker-proof. Some conditions, 

 however, favour it more than others, for it luxuriates in trees crowded in neglected 

 gardens and orchards, and is most prevalent in those grown in stubborn clay; in 

 land stagnated with water ; in dry, gravelly brash ; in a shallow layer of surface mould 

 overlying chalk, and particularly in soils restricted in cultivation to scratching the 

 surface, beneath which, at less than a spade or subsoil-plough depth, is a hard 

 impenetrable pan impregnated excessively with iron oxides and injurious acids. Yery 

 rich loose soils assist trees to canker ; also the moisture, mist, fog, and hoarfrost inci- 

 dental to sites near rivers, streams, lakes, and bogs. Exuberant growth, severe and 

 careless pruning, the removal of large branches late in spring, late summer pruning 

 which induces soft shoots certain to be injured by severe frost all invite the disease. 



The conditions mentioned, being inimical to the health of the trees, must be remedied, 

 as follows : crowded trees in neglected gardens and orchards should be opened out and 

 cleansed ; heavy soil lightened, shallow deepened and enriched, wet efficiently drained, 

 dry rendered more retentive, pan be broken up, loose ground firmed, pruning be 

 conducted intelligently, sites chosen insuring disease-resisting vigour, and every 

 cultural essential provided to maintain a normal condition of all the organs, to enable 

 the tree to profit by the influences of external circumstances which are beneficial, whilst 

 its constitution is not materially impaired by temporary unfavouring conditions. 



Conducements to canker over which the cultivator has little control are : chills in 

 spring, which injuriously affect the sap- vessels ; a late season, inducing excessive growth 



