A Rhizoctonia causing root disease in Uganda. W. Small. i6i 



The Rhizoctonia on Tea and Arnatto. 



Until the discover}' of the Rhizoctonia on tea, the only root 

 disease of that host recorded in Uganda had been a rot caused 

 by Ar miliaria mellea Fr. Since it happens that the single affected 

 tea plant has been growing in close proximity to a diseased 

 Grevillea robusfa tree, it is concluded that infection has been 

 brought about bv contact between the hosts of the fungus. In 

 any case, the fact remains that tea is susceptible to the Rhizoctonia 

 root disease, though on a smaller scale than Grccillea, and this 

 remark apphes also to the imdermentioned third host of the 

 fungus, the amatto (Bixa Orellana L.). 



No external black plates of tissue have been found on the 

 tea roots, and the sclerotia are few in number and concealed 

 in the bark and wood. The wood of affected roots is dr\' and 

 hard and so permeated in parts by the h\-phae of the Rhizoctonia 

 that it is darkened in streaks, while the black lines or sclerotial 

 plates are of finer texture and of sparser occurrence than in 

 Grevillea. Their structure, however, is similar to that already 

 found in Grevillea, and the same spong\' extensions filling ad- 

 jacent cell ca\'ities are apparent. The internal mycehum, again, 

 is not of so consistently dark a coloiu: as that in Grevillea. The 

 h\-phae ramify in all directions, the larger and older passing 

 longitudinally down the vessels and being at times sufficiently 

 numerous to form a tangled mass, while the yoimger and more 

 hyaline threads, branching from the older invariably at right angles 

 to them, pass freely through obstructing cell walls. The h\-phae 

 vary- from 2 to lo ju in breadth, and their largest cells measure up 

 to 35 ^ in length. The sclerotia differ from those of Grrcillea in 

 being less niunerous and smaller in size, and, owing to their being 

 shut up in the tissues, in their ill-defined and less distinct outline. 



The fungus has been grown in pure culture in the media 

 pre\-iously used and found to behave in the same way as that 

 from Grevillea. No fruiting stage has been encountered either 

 in nature or in culture. 



At a later period and in a different locality, a tree of Bixa 

 Orellana growing beside a d^ing Grevillea tree was observed 

 to wilt, and both specimens were dug up for examination. 

 The Grevillea being a t\-pical case of Rhizoctania root disease, 

 it is not surprising that the Bixa has proved to be a second 

 example of a host infected by contact. The wood of its roots is 

 dry and brittle and stained in the manner of the tea roots. 

 The external indications of the Rhizoctonia consist in the 

 presence of sclerotia on the bark of the smaller roots. Being 

 rounded on their free sides, the sclerotia protrude from their 

 substratum. They correspond in size to those of the Grevillea 

 fungus and they contain the same comparatively large quantities 



