102 DISEASES OF TREES 



appendages spring from the middle of the spore, and one from 

 each end* It is the gonidia and spores that infect the cortex. 

 It would appear that before infection can occur the periderm 

 must be slightly injured, and this happens with great frequency 

 in thick oak woods owing to the rubbing 

 of the branches of adjoining trees against 

 each other. Soon after the cortex has been 

 killed the alburnum becomes brown, and it 

 frequently happens that absolute decom- 

 position sets in, in the course of the year. 

 The sound portions of the tree begin to 

 FIG. 49. a, gonidia, f orm a callus along the edge of the dead 



and b, asci of A. 



Taieoia. part, so that sooner or later the latter is 



again covered over. The dead fibrous cortex 

 maintains its position for some years, but in the end it is entirely 

 cast off, so that the dead wood becomes visible (Fig. 46, 2). Of 

 course the same tree may in the course of years be repeatedly 

 infected at various points, as the accompanying cross section 

 (Fig. 46, 3) shows. 



This disease is accountable for the death of a great number 

 of oaks, and renders frequent felling and strong thinning 

 necessary. The dominant fast-growing trees recover from their 

 injuries more rapidly than those of slower growth, the con- 

 sequence being that one is left with a thin wood, consisting for 

 the most part of the largest trees. 



When the disease appears in a young oak wood, it is 

 advisable at once to fell the infected trees, except where 

 they belong to the dominant classes. The latter are thus 

 stimulated to increased growth and enabled to recover from 

 their wounds, and this result will follow with the greater 

 certainty if the wood is underplanted with beeches or some 

 other soil-improving species. The chances of infection are 

 also thereby reduced, partly owing to the removal of the 

 diseased trees, and partly because there are afterwards 

 fewer opportunities for the occurrence of wounds induced 

 by friction. 



Plowrightia morbosa 1 (Cucurbitaria morbosa). The 



1 W. H. Farlow, "The Black-knot." Bull, of the Bussev Institution. 

 Bot. Articles, 1876, p. 440. 



