674 PROTECTION AGAINST DISEASES. 



annual zones or groups of annual zones of wood between the 

 heart and sapwood are attacked,* or the disease occurs in 

 patches, or attacks merely the central zones of the tree or 

 branch. The sapwood is never attacked by red rot. The 

 rotten wood may eventually be completely decomposed, when 

 it disappears, leaving a hollow cylinder, in place of the heart- 

 wood, and this frequently without involving the death of the 

 tree. Sometimes the innermost portion of the stem remains, 

 forming a thin columnal, hard strand of wood united with the 

 sapwood here and there by similar strands where branches 

 have been enclosed in the wood. 



The commencement of the disease may be recognised by a 

 light violet or reddish colour of the wood, and by the porous 

 spring zones being attacked before the harder autumn zones. 



2. Modifying Factors, 

 a. Species. 



Eed rot occurs in almost every species of forest tree. 

 Among broadleaved species oaks and elms suffer most, and 

 among conifers the spruce and Scots pine. The disease 

 usually commences at the roots of spruce trees. 



Eoot rot usually spreads upw^ards through the heartwood to 

 the branches. It may, however, on the contrary, gradually 

 descend from the branches through the stem to the roots. 



I). A(je of Tree. 



Eed rot is a normal condition of very old trees, but a disease 

 in the case of young trees. It has been observed in the spruce 

 from the age of 10 years and upwards. 



c. LocaUty. 



Wood may become rotten in all kinds of localities, but certain 

 conditions of the soil predispose trees to this disease. Such 

 are — soils very rich in humus, calcareous soils, soils very 

 compact or wet and cold, such as clays and peats which are not 



• Termed Mondring in German and lunure in Freneh. 



