1906.] 



Violet Root-Rot. 



66/ 



This disease {Rhizocionia violacea) was recognised and de- 

 scribed nearly two hundred years ago as the cause of serious 



disease attacked the roots of many other kinds of plants, both 

 wild and cultivated. From this period it has not only con- 

 tinued its ravages, but has attacked in turn almost every new 

 plant, excepting cereals, introduced to cultivation. 



In this country Rhizocionia has a special predilection for 

 lucerne ; clover, carrots, beet, mangold and potatoes sometimes 

 also suffer severely, and most frequently when they follow 

 lucerne, which appears to attract the stray mycelium of the 

 fungus present in the soil. The mycelium increases enormously 

 in quantity on the root of this plant, and a large stock remains 

 in the soil in a vigorous condition ready to attack any suitable 

 host. If the following crop happens to be a cereal, which the 

 fungus cannot feed upon, it attacks weeds of various kinds, and 

 thus tides over the period until a crop suitable to its require- 

 ments is planted, when a fresh stock of mycelium is again left 

 in the soil. 



The disease is readily known by the bright colour of the 

 mycelium of the fungus, which varies from rose with a tinge of 

 purple to a deep brownish purple when old. The mycelium at 

 first spreads as a delicate, much-branched network over the 

 surface of the root or tuber, and finally forms dense patches, or 

 covers the entire surface with a compact felt. The amount of 

 injury caused by the fungus varies to a great extent on different 

 plants. In the case of beet and carrots, the mycelium soon 

 enters the fleshy root and destroys it. In lucerne and clover 

 the active rootlets are killed, whereas in the case of potatoes 

 the entire surface of the tuber may become covered with the 

 fungus without sustaining injury, the mycelium being unable to 

 penetrate the skin except through a wound. When once it is 

 through the skin, however, the tuber is soon reduced to a pulp. 

 As a rule the fungus confines its attacks to underground parts 

 of the plant, but when the weather is continuously damp and 

 dull the mycelium sometimes extends up the stem, and even 

 passes on to the leaves and fruit. 



So far as at present known the fungus does not form fruit 



Violet Root Sot. 



injury to the saffron industry in France. 

 It was also noted at the time that the 



