8 MASS. EXPERIMENT STATION BULLETIN 343 



successive years, but normal rings may come between those showing dis- 

 coloration. 



Peeling the bark will reveal the brown discoloration if such has been formed 

 immediately under the bark in the last annual ring. However, peeling the 

 bark of a tree that has formed considerable summer wood may show no dis- 

 coloration whatever even though it has developed in the spring wood. Usually 

 streaks appear in roots and large branches of trees as well as in the twigs, 

 but discoloration may be somewhat restricted or even confined to a single 

 branch. It appears that discoloration develops to a considerable extent in the 

 sapwood of some trees before any external disease symptoms are noticed. 



Figure 3. Young Culture of the Fungus which Causes the Dutch Elm Disease. 



(Irown from a tissue planting of discolored wood on potato-dextrose-agar medium. Actual size 



A microscopic examination of a thin section of diseased wood shows the 

 discoloration in the wood to be caused by the presence of brown substances in 

 the water-conducting vessels, which are thereby plugged up, or in the im- 

 mediately surrounding wood. However, a brown discoloration in wood is not 

 limited to wood affected with the Dutch elm disease. Fungus organisms 

 known to be associated with other diseases have been isolated from brown 

 streaks and there may be other causes. Only a laboratory study can determine 



