CHAPTER XXXIII. 



SOME FUNGOUS DISEASES OF FRUIT TREES. 



By H. WoRMALD, D.Sc. (Lond.), A.R.C.Sc, Mycological Department, South Eastern 

 Agricultural College, Wye, Kent. 



I. — Introduction. 



Fungi are forms of plant life which 

 obtain their nutriment from substances 

 produced by other organisms. Some 

 fungi live on dead organic products only, 

 while others are either wholly parasitic 

 and unable to exist except on living 

 tissues, or they may adopt a parasitic life 

 when conditions are favourable to their 

 mode of attack. Many of the parasitic 

 forms cause great damage to fruit trees, 

 and the grower should be able to recognize 

 those which occur most frequently so that 

 he may take steps to prevent epidemic 

 outbreaks of fungous diseases, some of 

 which are now so prevalent that preven- 

 tive measures against serious loss are 

 adopted by the progressive fruit grower 

 as part of the general routine work on 

 fruit farms. 



The fungi are reproduced by spores 

 which are so minute that they are easily 

 blown in the wind, or carried about by 

 insects. This fact explains the appear- 

 ance of diseases in places that were pre- 

 viously free, and also shows how a neg- 

 lected orchard is not only uneconomic to 

 the owner, but may also be a source of 

 danger to other orchards in the neigh- 

 bourhood. The microscopic size of the 

 spores also renders possible their distribu- 

 tion from place to place on farm imple- 

 ments or on workmen's tools or clothes. 



The methods to be adopted in combat- 

 ing the parasitic fungi must necessarily 

 vary with the mode of parasitism of the 

 organisms. A treatment that would kill 

 or effectively keep in check such forms 

 as the powdery mildews (Erysiphace(ie) 

 which penetrate no further than the 

 epidermal cells, would be useless against 

 the apple canker fungus which extends 

 into the woody central cylinder of stems 



and branches. The characteristic features 

 (as seen by the naked eye) of the more 

 important diseases of fruit trees caused 

 by fungi are briefly described in the fol- 

 lowing article, to enable the grower to 

 identify them in the orchard or planta- 

 tion, and methods by means of which he 

 can keep them under control are given in 

 each case. 



II.— Control Measures. 



Apart from the principles involved in 

 breeding varieties resistant to disease 

 (which are outside the scope of this 

 article), the first general rule in disease 

 control in the plantation or orchard is to 

 render the conditions under which the 

 trees are growing favourable for strong 

 sturdy growth. They must be grown in 

 suitable soil, with adequate drainage, and 

 must not be over-crowded, for free access 

 of air and sunlight to all parts of the tree 

 is necessary for healthy growth. Weak, 

 " sappy " shoots often fall a prey to 

 disease when a stronger growth would be 

 resistant. In spite of such hygienic pre- 

 cautions, however, some diseases will 

 probably sooner or later appear, and steps 

 must be taken to stamp them out, or at 

 any rate to prevent their assuming 

 epidemic proportions. Neglect in dealing 

 with a disease on its first appearance has 

 often led to disastrous results. 



The measures which must be adopted to 

 keep fungous diseases under control come 

 under two main categories, viz., (1) 

 destroying the actively growing fungus 

 and its reproductive organs; (2) covering 

 those parts of the tree liable to infection, 

 by some protective covering which will 

 prevent the penetration of the germ tubes 

 of the spores. The first of these should 

 be adopted wherever practicable and in 

 certain cases it is the only effective means 



