152 Transactions British Mycological Society. 



A RHIZOCTONIA CAUSING ROOT DISEASE 

 IN UGANDA. 



(With Plates V and VI.) 



By W. Small, M.B.E., Ph.D., F.L.S., Biological Laboratory, 

 Department of Agriculture, Kampala, Uganda. 



Introduction. 



In Uganda the silky oak (Grevillea robusta A. Gunn) has been 

 planted to a large extent as an ornamental tree in gardens and 

 elsewhere and as a shade-tree and wind-break on coffee estates. 

 There is, therefore, a certain amount of interest in its diseases 

 and pests, and the death of a silky oak is sure of notice. In 

 recent years, many trees have died in gardens, on roadsides, 

 and, more lately, among coffee, and it has been customary for 

 the layman to assume the losses to be due to the work of white- 

 ants {Termes bellicosus L.). Clear evidence of the presence of 

 the insects is seldom lacking; in fact, they may demolish the 

 underground parts of a silky oak so rapidly that the first sign 

 of trouble is the breaking of the tree at ground-level while the 

 foliage is, to a casual glance, normal in appearance, and so 

 thoroughly that they obhterate almost entirely traces of other 

 harmful agents that may be present and leave only evidence 

 of their own work. The attacks of the termites, however, can 

 be shown to be confined to trees that have already been weakened 

 by, or are dying of, root disease, and the insects are therefore 

 to be regarded as secondary agents in the loss incurred. On 

 occasion, diseased trees escape the attention of the insects, and 

 there is then no difficulty in allocating to a parasitic fungus the 

 real responsibihty for deterioration and loss. 



Only one root disease of Grevillea, the subject of part of the 

 present paper, is known in Uganda. It occurs on trees up to 

 five, six or more years of age, and it has been shown to be 

 caused by a species of Rhizoctonia. The writer is not aware of 

 any record of a similar fungus on silky oak, and the fungi 

 associated with root disease of the tree in other parts of the 

 world, particularly Ceylon and the East, — Ustulina zonata Lev., 

 Rosellinia arcuata Fetch, Ganoderma applanatum (Pers.) Pat., 

 and Ganoderma australe (Fr.) Pat., — have not been encountered 

 in Uganda. Up to the present, the Rhizoctonia has not spread 

 from its host to adjoining trees of Coffea arabica shaded or 

 protected by Grevillea, although the respective root-systems are 

 in close association with each other. It will be unfortunate if 

 the Rhizoctonia attacks Arabian coffee, an important crop that 

 so far has been remarkably free from root diseases. Single 



