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CUjP. C 



The Wetwood Disease of Elm 



J. CEDRIC CARTER 



WETWOOD is a disease, common in elm. that is known to 

 occur also in many other kinds of trees, including apple, birch, 

 fir, hemlock, hickory, linden, maple, mesquite, mulberry, oak, 

 pine, poplar, redbud, sycamore, tulip tree, and willow. In elm, it 

 is caused by the bacterium Envinia nimipressu7'alis (Fig. 1). 

 Seldom is an elm found that does not show wetwood to some de- 

 gree. The Siberian elm, Ulums pumila (sometimes incorrectly 

 called Chinese elm), is unusually susceptible to the disease. 



As a chronic type of disease, wetwood may contribute to gen- 

 eral decline of trees, especially of old trees growing under ad- 

 verse conditions ; it does not cause rapid dying. Elms may show 

 some degree of wetwood annually throughout their life span and 

 still live for 100 or more years. 



Wetw^ood results when bacteria invade and grow in the 

 spring wood of one or more of the annual growth rings in trunks 

 and large branches. The invaded wood becomes dark brown and 

 water-soaked ; when the wood is cut, sap oozes out. In trunk cross 

 sections (Fig. 2 and front cover), discoloration appears as dark 

 brown streaks or broken bands in one or several annual rings; 

 or in limited portions of a single annual ring. Discoloration is 

 most extensive in heartwood and older sapwood. However, it may 

 occur in current-season wood. 



In wetwood-aflfected tissues, gas is produced in abundance 

 by the fermenting action of the bacteria on carbohydrates and 

 other materials in the sap. When this gas is confined in the trunk. 





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Fig. 1. — Erwinia nimipressuralis, 

 the bacterium that causes wetwood 

 in elm. is a small, motile, rod-shaped, 

 sinjile-celled orj^anism having up to 

 six peritrichous flaKella attached to 

 its surface. 





