Willow stands are typical of our agricultural landscape, but they are threatened by watermark disease. This wood disease is caused by the bacterium Brenneria salicis (Bs). This disease usually occurs irregularly and at specific locations.
Extensive observation and project research produced unexpected answers to questions that had remained unanswered for decades. New insight into the spread and expression of the disease has been generated, and this has inspired new approaches in studying other diseases of trees. Even untrained observers can see that tree diseases and decline have become more frequent. One hypothesis is that these diseases are not caused by new aggressive pathogens, but by organisms that are naturally associated with the trees, which are triggered for pathogenicity by environmental factors.
Watermark disease in willow. (a) July 2003, (b) the same stand May 2005
Our research has shown that an abiotic factor can indeed influence the occurrence of disease caused by Bs. Young willows were monitored during three consecutive years. The trees were planted on four different locations in an agricultural area; two locations had watermark disease in the old existing willow stands, and two locations had old willow stands without any watermark disease. Characteristics of soil and soil water were analyzed along with nutrient status of the trees. Compared with the healthy sites, the sites with disease had significant higher nitrogen concentrations in soil water, a doubled growth rate of the new plantation, and an imbalance in the N/P and N/Ca leaf elements. Taken together, these indicate the involvement of excessive fertilization in the development of the disease. The N/Ca ratio in leaves is also a marker for disease risk.
Many samples were taken, both from the test sites and from other random sites in the neighborhood. The presence of Bs in the wood was analyzed using several PCR methods. A three-primer PCR was developed to guarantee specificity for Bs, which excluded other related bacterial endophytes from trees. An important conclusion was that Bs is generally spread in willow, in healthy as well as in diseased, in young and old trees. We also detected Bs in other healthy tree species, such as in alder and poplar. Its endophytic and non-pathogenic lifestyle was also illustrated with a luminescent Bs transformant. Bs was spread throughout the whole plant without causing disease, and was exudated through the hydatodes on the leaf margins. Bs promoted growth and chlorophyll concentration in the leaves. Its role in the nitrogen management of the tree is further suggested by the presence of a structural gene for nitrogen fixation.
Despite being found throughout healthy trees, Bs has pathogenic properties. . We identified the enzyme cellulase as the main factor for pathogenicity: it attacks the cellulose embedded in the pectin layer of the wood vessel. When the Bs population reaches a high enough density to be under control of quorum sensing, cellulase starts to be produced. No other endophyte of willow wood had this feature. The chemical structure of the quorum sensing molecules was also identified.
Watermark symptoms in the wood. The affected wood colours brown upon contact with the air. The cut surface of a branch (a) that is healthy, (b) with recent symptoms, typically in the last-year wood rings, and (c) with end-stage disease
We found the autumn-winter period to be decisive for development of the disease. Our research revealed that willow sap in the wood vessels is rich in nutrients in that period. High Bs concentrations can be reached from the moment the sap immobilizes in the vessels at leaf shedding. . Dense Bs populations producing quorum sensing molecules andcellulase accumulate, which in turn degrades the wood. This is not the case for other tree species (alder and poplar), because Bs concentrations stay lower during the autumn-winter period and pathogenesis is not triggered. Depending on variable conditions of trees and climate in autumn and winter, Bs survival varies from year to year, which explains the irregular occurrence of the disease.
Based on our research results, we can formulate some practical guidelines to control and protect willows in our agricultural landscape.
- Pollard the willows as is traditionally done; this postpones the disease for a certain period. The preferential time to prune is February or even August, but not in late autumn or early winter.
- Avoid excessive nitrogen in soil and soil water; liming could be tested as a control measure.
- When planting new trees, choose more resistant cultivars, namely S. fragilis and S.x rubens , and not the sensitive S. alba cultivars.
Additionally, the sensitive S. alba could be useful as bio-indicator for high nitrogen concentrations in agricultural soils.
References
Bruno De Vos, Hanneke Huvenne, Eric Messens & Martine Maes (2007).Nutritional imbalance caused by nitrogen excess is correlated with the occurrence of watermark disease in white willow.
Plant Soil (2007) 301:215–232.
Martine Maes, Hanneke Huvenne & Eric Messens (2009). Brenneria salicis, the bacterium causing watermark disease in willow, resides as an endophyte in wood. Environmental Microbiology (2009) 11(6), 1453–1462.
Hanneke Huvenne, Eric Messens & Martine Maes (2009). Willow wood sap promotes the density-dependent pathogenesis of Brenneria salicis. Environmental Microbiology (2009) 11(6), 1463–1472.
Hanneke Huvenne, Geert Goeminne,Martine Maes & Eric Messens (2008). Identification of quorum sensing signal molecules and oligolignols associated with watermark disease in willow (Salix sp.). Journal of Chromatography B, 872 (2008) 83–89.
Contact person
Martine Maes