August, 1945 



Carter: Wetwood of Elms 



429 



tissues affected with wetwood can cause 

 wilt and that, for the production of wilt, 

 Erwitiia nimipressuralis need not be carried 

 into the branches and leaves. 



The first evidence of leaf discoloration, 

 as shown in the greenhouse tests, is a faint 

 dull browning of the subepidermal cells 

 in the interveinal tissues. Visible only when 

 the leaf is held up to the light, the dis- 

 coloration is followed within a few hours 

 by more pronounced browning of the 

 upper and lower epidermal cells that 

 spreads toward the lateral veins, fig. 16. 

 This leaf browning, which is similar 

 in appearance to a type of midsummer 

 leaf scorch commonly seen in Illinois on 

 large fluxing elms, continues to spread 

 until the whole leaf margin is brown, and 

 it is followed by curling or wilting of the 

 leaf. Frequently the brown tissues are 

 bordered by a yellow band. Leaves usually 

 are brown and dead within 3 to 6 days 

 from the time the sap is introduced. Such 

 affected leaves abscise easily. 



The treated trees were examined for in- 

 ternal symptoms. A water-soaked appear- 

 ance in the current-season wood, together 

 with brown streaks typical of wetwood, de- 

 veloped only in the trees given wetwood 

 sap through incisions. However, not all of 

 the trees that received wetwood sap showed 

 both of these symptoms. In trees treat- 

 ed with acidified and alkalized water the 

 current-season wood was water-soaked 

 but had no typical brown discolorations. 

 The water-soaked appearance of the wood 

 depends upon the amount of moisture 

 present and can occur in healthy as well 

 as wetwood-affected trees. Wetwood sap, 

 however, is brown in color and it makes 

 the wood it affects appear darker than 

 healthy wood. Cut ends of stems or 

 branches affected with wetwood lose this 

 water-soaked appearance after drying. 



Grayish brown streaks appeared in 

 current-season wood of trees given un- 

 sterilized and autoclaved wetwood sap and 

 its filtrate. They did not appear in the 

 wood of the trees given the autoclaved 

 filtrate or the water suspension of living 

 bacteria. A brown flocculent precipitate 

 was formed when the filtrate was auto- 

 claved. The filtrate was much lighter 

 brown or tan after the precipitate was 

 removed. This indicates that streaking in 

 current-season wood is largely dependent 

 upon the amount or intensity of color in 



the wetwood sap that passes through it. 



Wilt did not occur in trees given acidi- 

 fied (pH 2.62) or alkalized (pH 9.5 + ) 

 water, indicating that the pH reaction 

 (pH 7.5 to 10.0) of wetwood sap has no 

 relation to the production of wilt. The 

 toxic material in wetwood sap was not 

 removed by precipitation, but it was 

 destroyed when the sap was reduced to 

 ash by burning. Trees treated with an ash 

 suspension did not wilt. 



From this experiment it appears that a 

 toxic agent is produced by Erwinia nimi- 

 pressuralis grown in nutrient broth and 

 in nutrient broth plus dextrose. However, 

 the cut shoot experiment described earlier 

 indicates that very little toxic material 

 was produced in nutrient broth plus dex- 

 trose. In the earlier experiment, cut shoots 

 of elm placed in wetwood sap wilted in 

 1.5 hours, while those placed in the fil- 

 trate of staled nutrient broth plus dextrose 

 did not wilt in 10.0 hours. 



Greenhouse Tests on 7- to 12- 

 Foot Elms. — An experiment was carried 

 out to determine whether wetwood sap 

 and other materials listed below would be 

 readily taken in through elm leaders one- 

 fourth to one-half inch in diameter and 

 whether any of these materials would 

 cause wilt. Seven American elms, 7 to 12 

 feet tall, growing in 14-inch flower pots 

 in the greenhouse were selected for this 

 experiment. Each tree was treated with 

 a different material. The materials tested 

 were ( 1 ) sterile distilled water, used as 

 a check, (2) wetwood sap, (3) filtrate of 

 wetwood sap, autoclaved, (4) fermenting 

 wetwood bacteria in nutrient broth cul- 

 ture, (5) filtrate of nutrient broth staled 

 with fermenting wetwood bacteria, auto- 

 claved, (6) fermenting wetwood bacteria 

 in sterile distilled water and (7) ferment- 

 ing wetwood bacteria in sterile distilled 

 water, autoclaved. 



A leader of each tree was bent over and 

 fastened in such a position that when a 

 bottle containing the material to be intro- 

 duced was slipped over the cut end of 

 the leader the liquid would not run out. 

 Before the leader was cut, bark on the 

 portion of the stem to be immersed in the 

 solution was disinfected with 70 per cent 

 alcohol and then rinsed with sterile dis- 

 tilled water. After the stem end was im- 

 mersed in the solution, sterile cotton was 

 packed in the bottle mouth around the 



