Bacterial wetwood

The pathogens

Wetwood is a wilt disease that is thought to be caused by the gram-negative bacterium Erwinia nimipressuralis De Bary. However, such bacteria as Bacillus megaterium De Bary, Pseudomonas fluorescens Migula, and Clostridium sp. also appear to be involved in the induction of the wetwood syndrome. Probably, different bacterial species that simultaneously colonize the infected xylem tissue and ray cells act in concert to produce disease symptoms {[896],[901]}.

Although bacterial wetwood is known to affect many plant species, it is most prevalent in the elm. Observations of wetwood-infected elms come from America and Asia. The reported infections occur in U. alata, U.americana, U.minor (including U. minor var. vulgaris), U. parvifolia, U. pumila, and U. rubra {[901]}. Bacterial wetwood is found to be the main cause of death of old U. pumila trees in the deserts of Mongolia {[900]}.

Most wetwood bacteria are common soil and water inhabitants. Wounds in the outer sapwood, like those made by injection of systemic fungicides, may result in wetwood infection. Bleeding wounds release a large number of bacteria that may subsequently infect other stem and branch injuries. Insect-mediated transmission of the pathogens to neighboring trees is another possibility. The native American elm bark beetle Hylurgopinus rufipes, one of the vectors of DED, is also found to carry E. nimipressuralis {[901]}.

Symptoms

Bacterial wetwood infection is characterized by a dark brown discoloration of the affected wood (Photo 66). The disease can be found in the trunk, branches, and roots. Generally, in grafted roots, wetwood is not observed. Especially in the springwood, grayish brown streaks can be found that may extend from the bark and spread into the branches. A cross section of an infected trunk usually shows a solid brown ring in the wood of one or more seasons. Wilted branches show streaking in the wood of the current season. In xylem tissue less then 10 years old, discolored tissue is often not easy to detect. Freshly cut wood has a noisome odor and a water-soaked appearance.

In 15- to 30-year-old stems, the central core of wetwood-infected tissue shows its tapered end at the juncture between stem and roots. Although in large stems wetwood may extend to the cambium, it usually appears to be restricted to a central discoloration zone and to the innermost sapwood free of discoloration. Discoloration of the outer sapwood is found in trees with wilted twigs and branches.

Photo 66:  Cross-section of an elm showing wetwood infection (Courtesy of W.A. Sinclair, Cornell University, Ithaca, NY, USA).

Carbohydrate fermentation and further metabolic activity of the bacteria within the infected tree result in an abnormally high gas pressure in the trunk (4.22 kg/cm²). The gas pressure shows a seasonal fluctuation, and reaches its highest levels in summer. On average, the gas is composed of 46% methane, 34 % nitrogen, 9% carbon dioxide, and 7% oxygen. The high internal gas pressure often opens old wounds at stem crotches or in bark and frost ribs. A difference in water potential between healthy and infected tissue may be the cause of the high moisture content of the diseased wood. However, the excess liquid may also result from bacterial metabolic activity. A characteristic symptom of wetwood infection is the bleeding of sap, induced by high gas pressure, from wounds and torn tissue in high-angled branch crotches (Photo 67). After air exposure, the transparent to yellow sap turns dark brown. Particularly in younger elms, disturbance of the water balance induces foliar wilting, leading to premature leaf abscission and further dieback of scattered branches. Older trees are likely to showa gradual decline of the entire crown over a number of years {[901]}.

Photo 67:  Bleeding of sap, a characteristic symptom of elm wetwood infection (Courtesy of W.A. Sinclair, Cornell University, Ithaca, NY, USA).

Control

Although as yet there is no method for effectively controlling bacterial wetwood, there are strategies to limit spread and damage of the pathogens in the elm. To prevent the rapid dispersal of wetwood bacteria, only shallow injection holes for DED control agents should be used. Further tree injury and infection spread may be avoided by the installation of drain tubes in the stem, in order to remove the excess liquid and lower the internal pressure from the infected sap {[901]}.