530 SPECIAL PLANT PATHOLOGY 



Root-rot (Armillaria mellea, Vahl).' — The "hallimasch" of the 

 Germans, or the so-called honey mushroom, is a fungus of considerable 

 interest to the forester (Fig. 15). The spores, ifblown to an exposed 

 branch stub, may germinate and produce a mycelium which works up and 

 down the tree. Infection may be also by the mycelium growing across 

 from the roots of a diseased tree to a healthy one through the soil of 

 the forest. In either case, the young mycelium grows into the cambial 

 layer, attacks the living cells, and finally completely encircles the trunk 

 of an infected tree. Later the hyphae are converted into strands, which 

 show a characteristic apical growth, thus providing for the elongation 

 of the strands through the host. The strands of hyphae turn a deep 

 chocolate-brown color and are known as rhizomorphs (Fig. 15), which 

 may anastomose under the bark of the tree. Ultimately, as the tree 

 dies, the bark splits off and the rhizomorphs are found flattened against 

 the woody cylinder of the tree. If such trees are used as mine props, 

 the strands may keep on growing under the moist even temperature of 

 the mine and there they may hang down in long streamers into the mine 

 galleries, as specimens of such in the botanic museum of the Univer- 

 sity of Pennsylvania indicate. The effect of the mycelium in the tree 

 is to kill its top with the ultimate death of the entire tree. The 

 rhizomorphs formerly known as Rhizomor pha subterranea grow out 

 into the root system of the tree, which they kill, and here they may 

 live for a number of years, endangering the nearby healthy trees, 

 because they extend out into the soil toward other tree roots. It is 

 this subterranean growth, which makes the honey mushroom an ex- 

 tremely dangerous oife to the hardwood forests, where it is found. 

 The fruiting bodies of this fungus usually occur grouped in considerable 

 numbers about the base of the affected tree arising from the dark-brown 

 rhizomorphs, which thus serve to connect together isolated groups of 

 the sporophores. The sporophores produced most commonly from 

 September to November are honey-colored, i.e., yellow to orange- 

 brown, and their umbonate tops have a more or less viscid character 

 with small black spicules scattered over the surface. The stipes are 

 slightly swollen at the base and a short distance below the pileus is 

 found the ring, or annulus. The lamellae are dirty-white and from 

 each pyriform basidium four white basidiospores fall until surround- 



'LoNG, W. H.: The Death of Chestnuts and Oaks due to Armillaria mellea. 

 Bull. U. S. Dept. Agric. No. 89, 1914. 



