APPLE DISEASES 103 



Armillaria root-rot, black rhizomorphs are found adhering to the 

 roots ; accompanying these is a peculiar mushroom odor. The 

 sporophores, or mushrooms, or the causal pathogene, appear in 

 groups at or near the base of the diseased -tree. Their presence, 

 while not constant, affords the most positive evidence of this 

 root-rot. 



Cause. 



The fungus, Clitocybe parasitica, a mushroom, is responsible 

 for this trouble, whence the name of the disease. It was first 

 found and described in Oklahoma about 1900. The rhizomor- 

 phic strands mentioned above are found on the surface of the 

 roots. These can be traced to an organic connection with 

 mycelial bands between the cortex and cambium, that is, just 

 beneath the bark. From these latter, subcortical mats of 

 hyphse, mycelial threads enter the woody tissue by way of the 

 medullary rays. They grow vertically in the wood-ducts ; side 

 branches are given off and these enter adjoining cells through 

 pits in walls, destroying the contents of the invaded cell. A gum 

 is formed ; this fills the sap-tubes, thus interfering with the 

 ascent of sap. 



The parasite propagates itself in two ways : (1) by forming 

 sporophores, or mushrooms, from the rhizomorphs, and (2) by 

 the growth of the rhizomorphs from one tree to another. In 

 the cases where mushrooms are developed, countless spores are 

 formed on the gills on the lower side of the cap of each. These 

 spores at maturity are scattered by the wind. In suitable 

 infection-courts they give rise to mycelium, and subsequently 

 to rhizomorphs and sporophores. The rhizomorphic strands 

 spread from one point to another by growing through the soil. 

 By this means the fungus may travel several feet to other trees. 

 Rhizomorphs have been found in the soil at a depth of about 

 eight inches. The roots or even the crowns of trees are en- 

 countered ; entrance is gained through healthy or through 

 injured bark. The flat-headed apple-borer is held responsible 



