ROOT DISEASES 135 



extends below the surface of the soil, and can do this at any 

 depth provided there is aeration. It can also infest and extend 

 through the surface soil, independent of roots, where this is rich 

 in decaying matter, and through accumulations of leaves and 

 other decaying material on the surface, provided that in either 

 case the situation is sufficiently shaded and watered to remain 

 continuously moist. 



It does not appear that direct spore infections take place. 

 In all cases studied the mycelium has first been developed on 

 dead material before attacking living organs. 



Infection by contact or from the soil takes place on the 

 surface of the bark without the necessity for a previous injury. 

 When it has obtained a foothold on a root the fungus has three 

 lines of advance : (a) over the surface of the bark, (b) in the bark 

 and on the surface of the wood, and (c) in the wood itself. These 

 appear usually to follow each other in the order named. If, 

 as is commonly the case, the infection starts on an outlying root 

 the fungus spreads along it infecting other roots attached to it 

 or in contact with it until the crown is reached. Here the 

 mycelium can cross over on to adjacent main roots, which are 

 infested in turn. In nearly all cases there is sufficient moisture 

 and cover about the base of the stem to enable the mycelium to 

 attack the bark of the collar, and commonly to extend for some 

 distance up the stem. It is at this stage that the fungus becomes 

 visible on undisturbed trees. A smooth coating of mycelium, 

 which in the case of R. Pepo is black with a light grey margin 

 about half an inch wide, and in R. bunodes is at first white, then 

 black, is seen extending up the bark above soil level. 



In those trees of which a considerable proportion of the root 

 system has become affected before this stage is reached, a gradual 

 yellowing and thinning of the foliage will have been visible. 

 Where on the other hand the fungus has reached the collar 

 without much destruction of roots, as in the case of infection of 

 short surface roots or possibly the collar itself from the soil, 

 the envelopment of the stem may be seen taking place while the 

 tree is apparently in full health and vigour. The death of the 

 tree soon follows upon the complete infestation of the collar, 

 and in the latter type it takes place with startling suddenness. 

 In a few hours an apparently vigorous tree will stand naked save 

 for the fruit in various stages of development, and a few young 

 leaves on the tips of the shoots. Some weak shoots may after- 

 wards be put out, but they come to nothing. The only previous 

 indication of disease which is commonly present in such cases 

 is the production of an unusually large crop of flowers and fruit. 



The dependence of the fungus on continuous moisture usually 

 limits its development to the roots and to the base of the stem, 

 but living branches and the undersides of recumbent stems are 

 attacked if they come in contact with infested soil, while if dead 

 trees become thickly enclosed with weeds or are felled and left 

 to lie they become infested as far as they are kept moist. 



