CARTER: THE WETWCOD DISEASE OF ELM 



Cracks known as frost cracks sometimes develop in the 

 trunks of trees during the winter months when the temperature 

 falls to very low^ points. Toxic sap in w-etwood-affected heart- 

 wood, under pressure during the growing season, is forced out 



Fig. 9. — Fluxing sap, .4, of wetwood-affetted trees, is suHieiently toxic 

 tard or prevent formation of callus and lo kill bark at the base of a 



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Fi 

 to retard or pre\^... ..,. .....w^.. v,. ^„..„.^ ....^^ .„ .x... .,....> „i i,,c ,j<inc 



pruning wound, B, as shown where the outer bark has been cut away. 



through such cracks. It kills the cambium for some distance 

 around the cracks (Fig. 10). Killing of the cambium results in 

 the separation of the bark from the underlying w'ood. Cracks in 

 the bark with flux oozing through them become apparent in June 

 or July. Many such cracks callus over during the growing season 

 in which they are formed. The callused-over cracks will not flux 

 in succeeding years if the callus tissues produced are strong 

 enough to withstand the pressures that develop in the underlying 

 diseased wood. 



Foliage Yellowing. — Leaves on some wetwood-affected trees 

 droop and turn yellow but do not wilt. Many leaves that turn 

 yellow fall prematurely in late July and in August. This yellow- 

 ing of foliage is not easily differentiated from yellowing that re- 

 sults from adverse climatic conditions. It does not result in any 

 noticeable efl'ect on the growth of trees. 



Foliage Scorch. — Foliage scorch resulting fiom wetwood ap- 

 pears as browning of tissues, usually l)etween the veins but occa- 



