PLANT DISEASES 



OAK CANKER 



(^Diaporthe (Chorastate) tahola, Sacc. 

 Aglaospora tahola, Tul.) 



Oak-trees are susceptible to this disease up to the age of 

 about forty years. Brown dead patches of the cortex indi- 

 cate the presence of this disease ; and as these patches are 

 usually of large size, and on different sides of the trunk, 

 the cortex is killed and the tree dies. Numerous stromata 

 are formed in the dead cortex ; these first bear a crop of 

 new-moon-shaped, or sickle-shaped, colourless conidia, and 

 at a later stage perithecia are formed in the black stromata. 

 These are flask-shaped, and the necks of two or three peri- 

 thecia grow together and form a single common neck or 

 opening, which grows to the surface. Through this neck 

 the ascospores pass out into the air at maturity. 



The ascospores have a single median septum, from 

 which three spines project ; there is also a spine at each 

 end of the spore. Both conidia and spores are capable of 

 inoculating the tree, germinating first on a wounded portion 

 of the cortex. 



Preventive Means. — Hartig suggests that when the 

 disease appears in a wood, the younger diseased trees 

 should at once be felled. This gives the remaining trees a 

 better chance of recovery, and reduces the chances of 

 infection caused by the friction of branches of adjoining 

 trees, which injures the bark ; on such wounds the spores 

 or conidia frequently commence their attack. 



Hartig, Forstlich-naturwiss. Zeitschr., Jan. 1893. 

 Hartig and Somerville, Diseases of Trees, p. 99, figs. 



