APPLE DISEASES 



99 



The first external evidence of Armillaria root-rot is that of a 

 poor growth of the affected tree, accompanied by a yellowing or a 

 wilting of the leaves in midsummer or later. In this stage the 

 tree is beyond recovery, for infection in such a case occurred 

 one to three years previously. Old and young trees are alike 

 attacked by this root-rot pathogene. 



Cause. 



This root-rot is caused by a mushroom known as Armillaria 

 mellea. Sometimes the common names honey mushroom, or 

 honey Agaric, are 

 applied to it, on 

 account of its light 

 yellowish brown 

 color. It occurs 

 widely in the 

 woods, orchards, 

 berry patches and 

 in newly cleared 

 lands, and is ex- 

 tremely variable 

 in form and ap- 

 pearance. The 



long list of plants which it may attack affords opportunities 

 for an almost certain perpetuation. It may live as a sapro- 

 phyte on buried wood, spreading through the soil by means 

 of its rhizomorphs. In some fashion or other, the roots of 

 the doomed tree are injured, and through such wounds 

 the rhizomorphs (Fig. 27) of the pathogene enter the bark. 

 Some contend that they enter roots through uninjured bark. 

 The roots may be penetrated by way of the trunk. The 

 rhizomorphs upon entering the root tissues spread out into the 

 separate hyphse which make up the strand. The hyphae, or 

 mycelial threads, grow between and into the root-cells, killing 

 them, and finally the root decays. Destruction of the tree 



FIG. 27. Armillaria root-rot fungus ; rhizomorphs 

 and young sporophores. 



