A Rhizoctonia causing root disease in Uganda. W. Small. 163 



and the Casuarina trees form, in one case, part of a wind-break 

 and, in another, part of an ornamental garden. 



In the Coffea robust a disease, death had apparently been 

 sudden, for each affected tree found in the field was standing 

 with all its leaves in position. The smallest rootlets are drv and 

 shrunken, and the bark of the smaller roots cracks and "peels 

 off readily to expose numerous black, flattened or rounded 

 sclerotia embedded in the cortical tissues. The wood of the 

 roots is hard and brittle, as in the pre\-ious cases of the disease, 

 and often discoloured by the presence of mvcehum, and its 

 surface is dotted in certain parts by sclerotia.' Black sclerotial 

 plates are present. The sclerotia occur also throughout the wood, 

 but they are most frequent in the cortex. In the latter position, 

 they are often so irregularly-shaped and numerous that they 

 form an almost continuous black sheet underhing the bark, 

 which recalls a similar appearance in Grtvillea rohusta roots. 

 The average size of the sclerotia is -45 by -3 mm., and thev are, 

 in general, and apart from their smaller size, indistinguishable 

 from those of the Grevillea, tea or amatto fungus. In the collar 

 of an affected tree, sclerotia can be found in the cortex of the 

 first few inches of stem above soil level; the accompanying 

 mycehum is confined to the cortical and outer tissues of' the 

 wood. The h^-phae of the fungus are similar in colour, shape and 

 measurements to those of tea and amatto. The larger pass do%%Ti 

 the main tracheids in the wood, and give off branches at the 

 usual right angle. The hyphae are most plentiful, however, in 

 the cortical tissues where they frequently give rise to extended 

 plates of flattened, t\-pically Rhizoctonia-Yik.Q mvcehum re- 

 sembling that of the sclerotial plates of Grevillea. In this 

 myceUimi, the swollen lateral hyphae are connected at times 

 to the parent h\-phae by extremely narrow necks, and there is 

 enough globular sweUing among the ceUs to recall the curious 

 cell-formations found in Grevillea cultures. Some of the cell- 

 walls of these h\-phae possess spiny processes similar to those 

 described from the Grevillea h\Taenophore, but no further like- 

 ness to the CorticiumAike stage of the fungus can be noted. 

 Sclerotia in all stages of development are common in the cortex 

 of some of the roots, and one of them, in which the outhne 

 of the young sclerotium is conditioned by the surroimding 

 tissues, is sho%\'n in Plate \l, fig. 21. 



With regard to the fungus on Casuarina, there is nothing to 

 add to the details given for the other hosts. Mvcehum and 

 sclerotia, the latter measuring up to -7 by -4 mm. and, on the 

 whole, more irregular in shape than those of Grevillea, are found 

 in abundance on the stem of the tree above soil level. Black 

 plates are also present in the wood of the roots. 



