444 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 4 



elm is pH 6.0 to 7.5, according to Spur- 

 way (1941). 



Elm Seeds. — Seeds were collected 

 from 10 wetwood-affected elms in Hins- 

 dale and planted in the spring of 1941 to 

 ascertain if the disease was transmitted 

 through the seed. Nine of these trees had 

 wilted previous to 1941, three in 1939, one 

 in 1940 and five in both 1939 and 1940. 

 Verticillium had been isolated from the 

 three trees which wilted in 1939 and from 

 the one tree which had not wilted. The 

 seedlings that were obtained grew nor- 

 mally. 



Growth Associations. — Growth of 

 the wetwood organism in association with 

 four fungi — Verticillium albo-atrum R. & 

 B., Dothiorella ulmi V. & M., Coniothy- 

 rium sp. and Alternaria sp. — was studied 

 because these fungi were found occasional- 

 ly in trees affected with wetwood. Each 

 fungus to be tested was planted at the 

 center of a Petri dish of potato dextrose 

 agar, and each test was run in duplicate. 

 Four days later, the wetwood organism 

 was planted in streaks along two sides and 

 about 20 mm. beyond each growing fun- 

 gus colony. The plates were held 30 days 

 for observation. 



The wetwood organism was inhibitory 

 to growth of Verticillium and Dothiorella 

 upon contact. However, this inhibition 

 was overcome in 12 days, and thereafter 

 both Verticillium and Dothiorella grew 

 slowly over the bacterial colonies and 

 spread over the agar beyond. The wet- 

 wood organism produced only a slight, 

 temporary inhibition of growth of Conio- 

 thyrium and Alternaria. This inhibition 

 occurred when the fungus and bacterial 

 colonies were in contact with each other. 

 Both Coniothyrium and Alternaria grew 

 through and over the bacterial colonies, 

 and the bacteria spread along the hyphae 

 of both fungi. In each test, the bacterial 

 colonies spread more rapidly from the 

 fungus colony than toward it. 



Conclusions 



From wetwood-affected elms in Illinois 

 a bacterium was isolated, and with it the 

 wetwood disease was reproduced experi- 

 mentally. The bacterium is quite similar 

 to Erwinia salicis Day, the watermark 

 disease bacterium, and to Pseudomonas 

 lignicola Westerdijk & Buisman (1929), 



but because of certain differences it is 

 regarded as a separate species and has been 

 named Erwinia nimipressuralis, new 

 species. 



Erwinia nimipressuralis inhabits mainly 

 the heartwood and older sapwood of the 

 trunk. Fermentation by this organism in 

 the infected wood liberates gas which, if 

 it does not escape, produces high pressure. 

 Sap accumulates in the infected wood and 

 produces the water-soaked condition which 

 gives rise to the name wetwood. Much of 

 the abnormal gas and sap can be drained 

 out of the affected wood by the installation 

 of drains. Radial cracks may occur, prob- 

 ably during the winter months, in wet- 

 wood-affected trees. These cracks may or 

 may not reach the cambial region. If they 

 extend to the cambial region, they serve 

 as avenues of escape for the sap and the 

 gas generated in the infected wood. Sap 

 and gas flow out through the trunk cracks, 

 killing the surrounding cambium and 

 forming pockets between the bark and 

 wood. The bark external to these cracks 

 may split ; then the sap and gas seep out, 

 and the sap flows down the trunk. In many 

 trees, cracks form in branch crotches, 

 and sap and gas escape through these 

 cracks. Likewise, wounds caused by the 

 removal of branches may allow the sap 

 and gas to escape. The flowing out of the 

 sap and gas through these vents is com- 

 monly called fluxing, and the escaping sap 

 and gas are known as flux. 



Wilt develops wherever sufficient quan- 

 tities of the wetwood sap are taken up in 

 the current-season wood and carried to 

 the branches. The severity and extent of 

 wilt appear to depend largely upon the 

 amount of toxic sap that is carried into 

 the branches. Some trees show yellowing 

 of leaves and premature leaf drop in July 

 and August without the development of 

 wilt. Trees that wilt may show symptoms 

 any time during July and August; oc- 

 casionally they show symptoms in late 

 June and early September also. Although 

 wilting branches may lose all of their 

 foliage, if no twig or branch dieback oc- 

 curs they may produce a new crop of 

 foliage within 3 weeks. If, however, severe 

 wilt occurs, not only do the leaves wilt 

 and fall but the twigs and branches die,, 

 back to larger laterals. I 



There is very little spread of wetwoou, 

 into the roots of elms, and usually it does 



I 



