78 DISEASES OF TREES 



ROSELLINIA QUERCINA l 



The oak-root fungus, Rosellinia quercina y is one of the most 

 interesting of parasites, and especially so because its mycelium 

 displays the same diversity of form as that of Agaricus melleus. 

 The mycelium is one of those parasitic mycelial forms which 

 were formerly referred to a special genus, Rhizoctonia. 



The disease produced by Rosellinia qiiercina appears only to 

 attack the roots of oaks from one to three years old, but it is 

 very prevalent, especially in north-west Germany. In oak seed- 

 beds it gives indications of its presence by the young plants 

 becoming pale and withered, especially during rainy seasons. 

 The leaves near the apex of the shoot are the first to wither, but 

 later on the lower ones go too. If a plant showing the first 

 symptoms of the disease be lifted out of the ground, we per- 

 ceive a few black spheroid bodies of the size of pin-heads situated 

 on the tap-root, especially at the points where the delicate 

 lateral rootlets are met with (Fig. 26). It is also observed that, 

 at certain points, the roots are closely embraced in a web-like 

 fashion by delicate ramifying strands which resemble so many 

 threads. These are the Rhizoctonia, which penetrate also into 

 the adjacent soil, and, as we shall see, spread the disease 

 underground from root to root. In the neighbourhood of 

 these black tubers, and wherever the Rhizoctonice have been 

 closely in contact with the surface of the roots, the cortex 

 turns brown. The apex of the tap-root is often quite 

 rotten, but even plants whose roots remain alive to the tip 

 display the pathological symptoms already described. 



Upon older plants that are already dead the Rhizoctonice are 

 no longer white, but brown, and there the black spheroid bodies 

 are often to be recognized in large numbers. Sometimes the 

 latter are also to be found on the lower part of the stem that is 

 to say, on the epicotyledonary axis : they may be most easily 

 discovered after the plant has been very carefully washed, be- 

 cause then the lustre of the black tubercles readily betrays their 

 presence. During damp warm weather all the plants on patches 

 a yard or so in diameter may become withered and die. 



1 R. Hartig, Untersuchungen aus dem Forstbot. Institut, I. pp. i 32. 



