The name Clitocybe Root Rot is used to distinguish this disease from other similar root troubles. Like the Armillaria Root Rot it affects plants other than the apple, although the host-range is less extensive. The peach, cherry and oak are more important among the other trees damaged by the Clitocybe Root Rot.
The disease is confined to the southwestern section of the United States. Affected trees have been reported from Oklahoma, Texas, Missouri, Illinois, Indiana, Georgia, Oregon and California. Greatest destruction has perhaps been wrought in the state of Oklahoma; whole apple and peach orchards are said to have been destroyed by this disease within the short period of two years.
It may be expected that Clitocybe Root Rot will be present most commonly in orchards which have been planted on recently cleared timber lands. It is not known on prairie soil. The most characteristic effect of the disease is the exudation of gum from the crown of the tree. The amount of exudate is at times so great as to unite with the soil, forming a gum-cemented soil about the base of the affected tree. This often becomes hardened, forming a sort of cast about the crown and the larger roots. From a distance affected trees may be recognized by either a yellowing or a wilting of the leaves. As in the case of 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.
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 encountered; entrance is gained through healthy or through injured bark. The flat-headed apple - borer is held responsible in many cases for producing wounds through which Clitocybe parasitica may enter.
This Root Rot disease is subject to the same remedial measures as those prescribed for the Armillaria Root Rot. (See page 100.)
Wilcox, E. M. A rhizomorphic Root Rot of fruit trees. Oklahoma Agr. Exp. Sta. Bul. 49: 3 - 32. 1901.