452 INSECTS AFFECTING VEGETATION 



noted even at the beginning of the formation. Immature tumors are 

 often formed on all portions of the tree, quite frequently occurring 

 at the base of twigs or springing from older limbs or from the trunk. 

 During the first period of their growth the swellings are covered with 

 a thick, fleshy layer of meristem and bark tissues. The growth for 

 the first year or two is often very rapid. At the end of a varying 

 period of time, usually one to three years, depending on the variety 

 of the tree and the conditions of growth in the locality where it is 

 found, the bark covering the tumors becomes ruptured by the rapid 

 formation in the interior of a very peculiar structure ; the bark tis- 

 sue ceases to grow, but the interior tissues develop a structure similar 

 to that found in root formation in the hairy-root disease. As the 

 bark breaks away small elevations, which resemble short thickened 

 root caps, appear on the surface of the tumors, giving them a very 

 warty appearance. The knots now resemble the origin of the forma- 

 tion of the woolly-knot form of hairy-root described by the writer in 

 a lecture to the National Nurserymen's Association last year. 



The tumors in their older form in some cases become hollowed 

 out by a slight decay of the tissue in the center, which results either 

 directly from the disease or from the entrance of wood-rotting or- 

 ganisms which are foreign to it. (U. S. B. P. I. Cir. 3.) 



With our present knowledge, therefore, and as a result of the ob- 

 servations and experiments previously mentioned, these tumors on 

 the limbs and trunks of apple trees may be considered the same dis- 

 ease as the woolly-knot form of hairy-root ; in other words, they are 

 the aerial form of hairy-root. The simple form of hairy-root, which 

 was described first by Stewart, Rolfs, and Hall and mentioned later 

 by the writer as a form of disease distinctly different from crown-gall, 

 is in the opinion of the writer only a form of the same disease as 

 woolly-knot and these aerial tumors. The simple form of hairy-root 

 occurs most frequently upon seedling trees ; it is present both on the 

 stem and root portions of the trees. It consists, when it occurs on the 

 roots, of numerous fine roots growing at right angles from the main 

 roots or the taproot. These side roots are very soft and succulent at 

 first, but later harden and have a wiry appearance and texture. On 

 the roots they are usually quite uniformly distributed in definite 

 belts. Where this form occurs on shoots it springs more frequently 

 from the vicinity of a bud often just beneath it. These fine roots are 

 formed in groups, or bunches, a great many often springing from a 

 single origin. The origin of each of these clumps of roots is similar 

 to that of the rudimentary roots in the aerial tumors. It follows, 

 then, that the hairy-root disease may have at least three forms, which 

 are more or less distinct : The simple, the woolly-knot, and the aerial 

 forms. (Cir. 3, U. S. B. P. I.) 



Root Rot. This is a reddish brown toadstool, measuring from 

 two to three inches in width. It grows in dense clusters from the 

 crown of living trees and dead stumps. The sphorophores form from 

 September to October. It also produces black subterranean rhizo- 

 morphic strands which aid materially in spreading the disease. Wil- 

 cox states that this fungus invades the apple, peach and cherry trees. 



