I30 DISEASES OF CROP-PLANTS 



R. Pepo described above, and no character has been detected 

 by which they may be distinguished in the two species. 



In respect of the general course of its development and in its 

 main characters R. bunodes closely resembles R. Pepo. There 

 are, however, certain definite specific differences in the appearance 

 of the mycelium which enable the two to be readily separated 

 in the absence of perithecia. 



At the point of infection on a root a dead patch extending to 

 the cambium is formed in the cortex. From this, the fungus 

 spreads over the bark and through the cortex, the infestation of 

 the wood following later. The mycelium on the surface consists 

 mostly of closely appHed, firm-textured, branching black strands 

 which thicken into irregular knots along their course. On 

 examples of Castilloa roots examined such knots (the sclerotia 

 of some writers ?) had formed over and infiltrated the 

 iarge transverse lenticels, and on hibiscus similar bodies in the 

 form of dome-shaped papillae have been seen. 



At a later stage the space between the strands may fill in so 

 that a dense layer is formed, which on its lower surface grows 

 between and encloses the outer layers of the bark, and on its 

 upper surface bears in relief a closely ramified system of rhizo- 

 morphic strands. From the lower surface abundant cylindrical 

 strands, with a black periphery and a white core, penetrate the 

 cortex in a direction more or less vertical to the plane of the 

 cambium. On reaching the wood they run for a short distance 

 over its surface, and distribute themselves into it by vertical 

 branches which penetrate by way of the medullary rays. From 

 these the adjacent large tracheids are invaded and filled with 

 bundles of parallel hyphae, hyaline at first, black later. 



The result of this distribution is that the strands are seen 

 in the wood of a transversely cut root as black dots and radial 

 lines ; in a median longitudinal section as vertical and horizontal 

 lines ; in the bark as black threads of irregular length and course. 



As in the case of R. Pepo, the surface mycelium ascends the 

 stem for some distance above ground as an encircling sheet. 

 In this species the whole sheet when quite fresh may be creamy- 

 white in colour, but eventually turns black. The " bark " 

 beneath commonly remains healthy for some time after it has 

 become covered, but it is eventually penetrated and killed. 

 By the further development of the mycelium a sort of crust is 

 formed on which the conidial fructifications and perithecia 

 successively arise. 



The characters by means of which R. bunodes may be readily 

 separated from R. Pepo at any stage may be gathered from a 

 comparison of Figs. 35 and 38, and by noting that in contrast with 

 the condition shown in Figs. 39 and 40, the mycelium of R. Pepo 

 does not show in the wood until very late, and then only in long 

 zigzag lines formed by cutting through a continuous black 

 film or plate. There is only one caution to be observed : where 



