212 DISEASES OF CULTIVATED PLANTS 



{Plowrightia ribesia, Sacc. ) attacks the stem and larger branches 

 of gooseberry, red and black currant, and is not uncommon 

 in neglected gardens,, more especially where aphides or 

 currant scale are present. The fungus is a wound-parasite, 

 and in all probability aphides or scale enable the parasite to 

 gain an entrance into the living tissues of the host, as is known 

 to be the case with apple-tree canker, larch canker, etc. 



fl * , ft 



,CJ, 

 P 



^ ' 



,-*'" "\ t 



"V 



FIG. 60. Plowrightla ribesia. i, branch of 

 gooseberry with black knot, nat. size ; 2, black 

 currant branch with black knot, nat. size ; 3, sec- 

 tion through a stroma, slightly mag ; 4, asci 

 containing spores, highly mag. 



The first indication of disease is the wilting and yellowing 

 of the foliage, which falls early in the season. As a rule a 

 branch is not killed during the first year after infection ; during 

 the second season the leaf-buds remain only partially expanded, 

 and the branch dies, owing to the ascent of water being cut 

 off by the copious development of mycelium in the conduct- 

 ing vessels. 



The fungus does not appear on the surface until the branch 



