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 voung 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 Fie. 27. — Armillaria root-rot fungus; rhizomorphs 
in form and ap- and young sporophores. 
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 hyphe which make up the strand. The hyphe, or 
mycelial threads, grow between and into the root-cells, killing 
them, and finally the root decays. Destruction of the tree 
