July 23. I9I7 Black Rootrot of Apple 169 



tained with a layer of sterilized sand. Roots of apple, peach, hawthorn, 

 and walnut remained alive and in good condition in these moist chambers 

 for more than three months, often sending out shoots several inches long. 

 The oak roots did not keep so well and showed no adventitious budding. 

 One series included inoculations with isolations of Xylaria hypoxylon 

 from seven sources and with Xylaria sp. Each moist chamber contained 

 one inoculated apple root, one control root wounded but not inoculated, 

 and one inoculated apple limb. All the strains of X. hypoxylon proved 

 pathogenic on apple roots and limbs. On roots the progress of infection 

 was seen in the production of surface mycelium on the bark (PI. 16, F) 

 radiating outward from the point of inoculation accompanied by drops 

 of brown exudate and some splitting of the bark. The mycelium, at first 

 white, later developed pigment and formed a black stromatic crust over 

 the surface of the root, typical of that found on affected apple roots in 

 nature. The surface mycelium and crust developed most vigorously in 

 a saturated atmosphere. After six weeks the pieces were removed and 

 split longitudinally through the wound. The bark and wood were brown 

 and dead to some distance from the wound (PI. 16, A). The smallest 

 lesion produced extended i cm. and the largest 9 cm. proximally and dis- 

 tally from the point of inoculation. The depth of the lesions varied from 

 3 mm. to 1.5 cm., sometimes extending to the center of the root. Abun- 

 dant mycelium was found in the vascular bundles, while but little was 

 present in the medullary rays. All the control roots remained healthy. 

 Additional inoculations have confirmed these results. The inoculum has 

 been recovered with ease whenever attempted. 



On limbs the mycelium of Xylaria hypoxylon produced more conspicu- 

 ous surface mats than on the roots (PI. 16, B) and from which the black 

 encrustation developed (PI. 16, C). Six-inch lengths of limbs were com- 

 pletely covered with the mycelium in less than six weeks; the wood, 

 however, was only penetrated to a depth of a few millimeters. Xylaria 

 sp. proved almost as vigorous a parasite as X. hypoxylon. In 10 weeks 

 the root infection had spread 2 cm. proximally and distally and 5 mm. 

 inward, while that on the limb had advanced 3 cm. in the bark and had 

 penetrated the wood 2 mm. 



Xylaria hypoxylon proved only slightly pathogenic on roots of red oak, 

 white oak, and walnut. A heavy black crust was formed over the 

 wound, but the wood was penetrated but slightly after three months. 

 The hawthorn root was attacked somewhat more vigorously, the wood 

 having been penetrated to a distance of i cm. in three months. 



Mycelium of Xylaria polymorpha produced only slight penetration of 

 the bark of apple roots after three months, and similar results were 

 obtained with peach roots. Ascospores sown on the wound surface of 

 an apple root produced a small amount of mycelium which did not pene- 

 trate the sound wood. 



