March 1896.] 



INJURIOUS I>^SECTS AND FUxNGI. 



437 



A Disease of Potatoes. 



In October last, the Board received reports of a peculiar 

 affection of stored potatoes in Surrey and Kent, which was 

 afterwards found to be present in other counties. Upon exami- 

 nation of affected potatoes, it was seen that they were all super- 

 tuberated, and that at the elongated end decay was beginning 

 at the tip, and extending downwards. In some cases, the rotting 

 part was as much as an inch long, in others, it had only 

 commenced. Many ends of supertuberated potatoes, which were 

 apparently sound, on being sliced oft showed a faint mark all 

 round the tuber, just inside it and close to the skin. On cutting 

 off slices, the mark was visible, in some tubers throughout the 

 supertuberated part, in others only for a short distance. Upon> 

 examination of this mark with a microscope, the mycelium of a 

 fungus and its branching filaments could be discerned. In the 

 potatoes where disease had further advanced, the fungus was 

 more plainly apparent, with its peculiar septate mycelium, varying 

 curiously in its diameter, which is characteristic of the fungus 

 known as Rhizoctonia violacea, Tulasne. There is no doubt that 

 the fungus was present in all the diseased tubers, but whether 

 it was the sole cause of this disorder is not clear. It was sup-^ 

 posed at first that the disorder was occasioned by a bacillus. 

 Bacteriological investigations of the infected tubers have 

 shown that there is a bacillus present, but it has not yet been 

 ascertained whether it is the cause of any injury to the potatoes. 

 These bacteriological inquiries are being continued, and the 

 results will be published later. 



It is, however, quite clear that Rhizoctonia was present in the 

 diseased tubers, and it has been clearly demonstrated first by 

 Tulasne,* and since by Berkeley, f Prillieux, and others, that a 

 fungus of this species destroys the roots and bulbs of plants, 

 e.g., lucerne, clover, asparagus, beetroot, the crocus, and saffron. 

 Klihn describes another species of Rhizoctonia as R. solani, which 

 infects potatoes and injures them in the manner described above, 

 and another specie?, Rhizoctonia allii, is noted in " Grevillia " as 

 attacking onions. { 



The skin of the end of the super-tuber affected is dark in 

 colour, much wrinkled, and soft to the touch. On taking a 

 section of a diseased part it is found to be a brown, mottled, , 

 pulpy mass, which gradually extends as the disorder advances, . 

 and when dry looks like cork. 



* Fungi Hypogcei. 



f Berkeley remarks in a communication to the Gardeners' Chronicle in 1879, " I; 

 suspect the roots of the asparagus are attacked by Rhizoctonia. It attacks also 

 mint, potatoes, and is known by gardeners under the name of ' Copper web.' " 



X The species known as Bhizoctonia medicaginis causes serious injury to lucerne 

 in France, especially in the south. It is called mort de la luc me. 



