128 DISEASES OF CROP-PLANTS 



times found smooth. Apparently this is due to weathering, 

 though possibly (cf. R. hunodes) there is some variation in the 

 amount of roughness developed. 



The most striking characteristic of this species is the produc- 

 tion of fans or stars of white mycelium in the region of the cam- 

 bium (between bark and wood) of the roots. The presence of 

 these distinguishes it at once from R. bunodes. A somewhat 

 similar appearance may be produced by R. paraguayensis, 

 but, in the examples of the latter fungus seen, the growth has 

 been very much less vigorous. 



On the roots the mycelium forms an irregular coating over the 

 surface. In its early stages it is smoky grey in colour, but soon 

 becomes black. It is gathered at first into rather loose branching 

 strands with spreading hyphae between them. Later the whole 

 is combined into a mass which is more or less woolly on the sur- 

 face, and tending to be carbonaceous below. It forms dense 

 layers and pockets in the outer bark, and is connected with 

 whitish strands more or less vertical to the surface which every- 

 where penetrate the cortex. On reaching the surface of the wood 

 these repeatedly branch and spread in all directions over it, 

 so that, when " bark " and wood are separated, a conspicuous 

 white pattern of branching lines, stars and fans is seen on the 

 " bark." From this layer strands penetrate the wood in radial 

 lines along the medullary rays, and send out hyphae which invade 

 and fill the large vascular elements. In this species the mycelium 

 in the wood, owing to its lack of colour, is not apparent to the 

 naked eye unless a cut surface is exposed for a day or two, when 

 the hyphae grow out and turn black. In long-infested wood, 

 thin plates, seen as black lines in a section, mark off certain 

 areas, and are believed to belong to this fungus. Such plates, 

 however, are very common in dead wood, and so have little 

 diagnostic value. 



The external mycelium is most fully developed and conspic- 

 uous when the fungus reaches the base of the stem and appears 

 above the surface of the soil. It is then seen in broad spreading 

 fans, or an advancing sheet, which from the beginning or at 

 an early stage encircles the stem (Fig. 34). The margin while 

 the fungus is actively advancing is light grey in colour for a width 

 of about half an inch, behind which the colour shades off to brown 

 or black with a greenish tint. On smooth bark the sheet is even 

 and glossy, rather like wet fur. 



The height to which the fungus reaches is determined by the 

 moisture conditions. When the stem is well exposed, it reaches 

 no further than the few inches for which the moisture of the 

 soil can affect it ; if the stem is enclosed by weeds or low branches, 

 or sheltered by a log, it commonly goes up for a foot or more. 

 The limit of the external mycelium is also the limit of the ultimate 

 infestation of bark and wood. If a diseased tree is uprooted, 

 or is cut below the limit of infestation, and thrown aside where 



