DISEASES OF UNCERTAIN ORIGIN 345 



mal cells contain the alga, which spreads over the leaf, 

 blackening and killing it. 



Karsten, A?m. Jard. Bot. Buiitn.^ vol. x., i pi. (1891). 



Cephaleurus minus, Karsten, destroys the leaves of 

 Zizyphiis jujiiba in Java. 



Karsten, Lc. 



DISEASES OF UNCERTAIN ORIGIN 



{Rhizoctinia violacea, Tul.) 



This remarkable fungous parasite, the fruit of which is 

 not yet known with certainty, is too well known as being 

 most destructive to saffron {Crocus sativtis). The roots 

 of many other plants are also attacked, including beet- 

 root, potato, lucerne, carrot, asparagus ; also many \\\\d 

 plants. The tubers or roots are invested by a more or 

 less dense violet or brownish mycelium, which gives off 

 violet rhizomorphs that anastomose irregularly, and traverse 

 the soil, attacking adjacent roots. By this method the dis- 

 ease extends rapidly from a diseased centre, unless very 

 prompt measures are adopted. Two forms of sclerotia 

 are also developed, one kind being often as large as a 

 hazel-nut, and minutely velvety, the other small, glabrous, 

 and often densely crowded, resembling the perithecia of 

 some Roselliiiia in general appearance. 



An ascigerous form of fruit — Leptosp/iaeria cirdna?is, 

 Sacc. — found on dead roots of lucerne {Medicago sativa) 

 killed by the Rhizoctinia, may prove to be the perfect form 

 of fruit, but this point is as yet undecided. 



Preventive Means. — If the disease appears, the area 

 should be isolated by a trench, throwing the removed soil 



