474 



FUNGOUS DISEASES OF PLANTS 



a considerably advanced stage of decay in the region of the cam- 

 bium, including both wood and bark. There is present an abundant 

 white mycelium and very characteristic mycelial strands, as subse- 

 quently described. 



The abundant, white mycelium is particularly rich in stored 

 nutrients. It commonly extends several feet above the crown, 

 mostly between the wood and bark. The characteristic mycelial 

 cords, by which this fungus is best known, are shining, gray-black 



Fig. 235. Armillaria mellea on a Stump of White Oak 

 (Photograph by Geo. F. Atkinson) 



strands which may measure from 1 to 2^ mm. in diameter. They 

 are typical rhizomorphs. These begin as complex hyphal masses 

 which become readily sclerotial in character. These strands attain 

 enormous lengths. They may course upward and downward in the 

 affected tree, generally under the bark, or merely in close contact 

 with the outer surface of the bark. They also grow through the 

 soil to considerable distances, thus serving to spread the disease 

 to neighboring trees. According to Hartig this strand is differ- 

 entiated near the apex into several layers. The outer, more gelati- 

 nous layer becomes somewhat horny ; some loose hyphse, however, 



