Bacterial Leaf Scorch (Xylella fastidiosa) also known a BLS, is considered one of the most threatening plant pathogens affecting more than 600 plant species worldwide. The main distribution area of BLS is on the American continent, from Canada to Argentina, but the bacteria is also raging in many countries in Europe.
Impacts – In the Americas, the pathogen is widespread and has caused severe losses to Citrus, coffee and grapevine production. Significant damage to the urban tree landscape has occurred in some regions of the US, which have incurred economic costs and reduced the quality of the visual environment and street scene. In the New Jersey region, up to 35% of urban plantings were affected by the pathogen. These chronic infections have been very difficult to control. In Europe, the pathogen has been devastating to the olive production in the
Puglia region of southern Italy.
Disease Transmission – There are four known subspecies of Xylella and within each subspecies multiple strains exist. The disease is transmitted by xylem (wood) feeding insects, primarily spittlebugs, sharpshooters, and leafhoppers. The flight range of insects is around 100m, but they can travel longer distances aided by wind. However, the main pathway over long distances is the trade of contaminated plants.
Symptoms – The bacterial pathogen colonises xylem vessels; and when the vessels become blocked causing symptoms that resemble those of water deficiency, nutrient deprivation, including wilts, diebacks, stunts and leaf scorches. The most prominent symptom is the scorching of leaf margins. Starting at the edges and moving inward, leaves appear burned or scorched ranging from yellow to brown in colour.
Disease Progression – As the disease progresses, leaves may curl and drop earlier than healthy ones in late summer and early fall. BLS diseased plants often show branch die-back and overall reduction in vigour and health. BLS gradually worsens over several years, whereas Oak Wilt and Dutch Elm disease for instance kill susceptible trees in just a few months.
Note: It is important to understand that the symptoms of Bacterial Leaf Scorch can resemble other diseases or environmental stresses. Just to be sure, accurate diagnosis requires laboratory testing.
Host Plants – Worldwide, hundreds of host plants have been found to be infected with the various subspecies and strains of Xylella fastidiosa, although in many hosts, infections do not become systemic and so no symptoms are produced. Woody perennial plants (e.g. grapevine, olive, oleander, Citrus, coffee, Prunus spp. and hardwood trees) can be vulnerable to damage if disease carrying insects are present to spread the pathogen. Herbaceous plants growing in the environment may become disease reservoirs even though infections may be symptomless.
| High Risk Plants Almond – Ash – Blueberry Cherry – Citrus – Coffee – Grapevine Lavender – Maple – Oak – Oleander Olive – Plum – Rosemary |
Disease Mitigation – Evidence found in 2021 confirmed zinc, copper, and citric acid treatments as effective in mitigating the impacts of Xylella fastidiosa in Italy, particularly in the olive-producing region of Puglia.
A recent project development by European Union-funded Biovexo, three new compounds have been found effective to target the Xylella fastidiosa bacterium, which could help combat the epidemic, although they cannot cure infected plants. Once available, the products fine-tuned by the Biovexo consortium would represent a crucial step towards curtailing the Xylella fastidiosa epidemic.
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