20 FOREST TREE DISEASES. 



Disturbances of the root functions caused by drought 

 or by an excess of water in the soil, total or partial 

 destruction of the roots by parasitic fungi, insect-. 

 rodents, etc., or starvation of the root tips caused by 

 diseases or destruction of the living bark above by 

 fungi, insects, porcupines, etc., all tend to cut off the 

 water supply of the tree, the results of which are felt 

 first in the needles or leaves. Certain winter conditions 

 have the same effect (see p. 16). Kesin flow at the butt 

 of the tree frequently indicates root disease. 



When twigs appear to be diseased examination should 

 be made of the part of the twig where the sound and 

 diseased tissues meet. In the case of fungus attack 

 small black spots will often appear on the diseased or 

 dead twig. Sometimes twigs are killed by rodents 

 eating the bark or by deer brushing it off from the 

 underside. If insects have attacked twigs, their pres- 

 ence is indicated by borings in the bark or wood. All 

 this applies, of course, not only to twigs, but also to 

 leaders; in fact, to all the younger shoots of mature 

 and immature trees. The youngest twig endings of firs 

 may be killed in spring by frost, by a parasitic fungus, 

 or by insects. Galls on the twigs or branches are caused 

 by insects or by a fungus (Peridermium) . 



Dead branches and limbs in the otherwise healthy 

 crown are always suspicious. The various causes of 

 death have been mentioned under "Premature dying 

 of the foliage, tops, limbs, branches, or twigs." 



Any alteration of the normal branching system of a 

 tree suggests the presence of some injurious agent. 

 The mycelium of certain fungi spreads into the twigs 



