July 23, I9I7 Black Rootrot of Apple 165 



age, and the percentage of death at seven years may equal or exceed that 

 of the original trees at 1 5 years. All varieties of the apple probably suffer 

 equally from the disease. We have found it equally prevalent on York 

 Imperial, Winesap, Ben Davis, Black Twig, Grimes Golden, Albemarle 

 Pippin (Yellow Newtown) , and other varieties under equal conditions of 

 exposure. 



The appearance of roots of affected trees is quite characteristic. Young 

 trees in advanced stages of infection may readily be uprooted by hand. 

 In uprooting, the larger roots break oflf near the trunk, and often only 

 a few small superficial roots support the tree. The bark, when not too 

 far disintegrated, is covered with a thin black encrustation which is easily 

 peeled off. The wood is brown and is marked with dark-brown zonations. 

 On recently killed roots the wood is firm, and the zonations may penetrate 

 it to some depth. The margin between sound and diseased bark and 

 wood is sharply marked by a change in color, and thin brown strands may 

 extend into the sound tissue for some distance. Sunken, discolored 

 areas are usually found on the surface of the sound root beyond the main 

 lesion (PI. 17, C). They have the appearance of surface infections and 

 may be superficial, or may penetrate to some depth. Usually but little 

 evident mycelium is present on the surface of roots or within them. In 

 advanced stages of decay roots are punky and brittle and vary in appear- 

 ance ; various fungi may be present and acting on them. Rhizomorphoid 

 strands have been found associated with this rootrot in a few cases only, 

 and then on roots in advanced stages of decay. The crown of the tree 

 almost invariably shows the discoloration of the wood and bark found on 

 the roots. The discoloration may extend up the trunk a foot or more 

 above the surface of the ground and is often restricted to one side. Trees 

 showing recognizable foliar symptoms of the disease are found to have 

 the greater part of their root system affected. The initial infection of 

 the root system may take place at any point, and the infection apparently 

 progresses with equal rapidity toward and away from the crown. Infec- 

 tion may be communicated laterally to healthy roots in contact with 

 diseased roots through uninjiired rootlets or through wounds in large 

 roots. Infected roots die back to the crown establishing the infection 

 there, and the sound roots become infected subsequently through their 

 attachment with the crown. Death of the tree probably results within 

 a comparatively short time after the infection becomes established in 

 the crown. Figure i shows the condition of the root system of a tree 

 which showed pronounced rootrot symptoms. 



FUNGI ASSOCIATED WITH ROOTROT 



The absence of fruiting stages of fungi on affected roots has been 

 noteworthy. The early removal of diseased trees as practiced in most 

 orchards partly explains this. Perithecial stromata of Xylaria poly- 

 morpha (Pers.) Grev. have been found on the roots of several affected 

 apple trees in two orchards at Barber (PI. 15, C; 17, D), and numerous 



