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Fire blight monitoring generates new knowledge

Plant diseases such as fire blight, a common infectious disease that predominantly affects pomaceous fruit trees, can reach epidemic proportions. Weather-based prediction models, which are routinely used to calculate fire blight infection risk, can now be replaced by a molecular test that is far more accurate. This new test is used by Bio-Protect GmbH and Büro für Biologische-Ökologische Beratung, both located in Konstanz and members of BioLAGO, in cooperation with the University of Hohenheim with the aim of exploring the spread of this tree disease in the Lake Constance region using a transnational monitoring approach.

Fruit mummy, a small, unripe, dried-up fruit of the previous year, on an apple tree. © Kunz

The three project partners, Dr. Stefan Kunz, Prof. Dr. Ralf T. Vögele and Dr. Michael Ernst, met for the first time at the University of Konstanz. Dr. Stefan Kunz, now employed at Bio-Protect GmbH, and Prof. Dr. Ralf T. Vögele, now at the Institute of Phytomedicine at the University of Hohenheim, then worked together on the development of a PCR-based method for the identification of fire blight. This method provides the researchers with information on the presence or absence of Erwinia amylovora, the cause of fire blight, long before the affected fruit tree exhibits actual fire blight symptoms. “Specific probes that bind to a specific DNA sequence are generated; and in the case of fire blight, probes that are specific to an E. amylovora plasmid are produced,” Dr. Michael Ernst says. The target sequence is amplified, a process during which fluorescent dyes are incorporated into the DNA. This real-time PCR method (also known as quantitative PCR, qPCR) not only provides information on the presence of a certain pathogen, but also on the quantity of the pathogen in the sample under investigation,” said the plant disease expert.

The three research partners use the PCR-based identification of the pathogens for exploring the infestation of fruit trees and the spread of fire blight. They have divided the work amongst them: the Institute of Phytomedicine at the University of Hohenheim is tasked with the scientific coordination and planning of the projects, while Bio-Protect GmbH is mainly tasked with the collection of field samples and with carrying out laboratory analyses. Dr. Michael Ernst from Büro für Biologische-Ökologische Beratung analyses the data and prepares papers for publication in international scientific journals. The partners expect their cooperative studies to increase our scientific knowledge of fire blight as well as lead to more effective approaches in preventing and abating fire blight infestations. 

Lake Constance-wide network is involved in the monitoring

Fire blight-infected apple rootstock before (left) and after (right) qPCR sampling © Kunz

The molecular test is being used to monitor the occurrence of fire blight around Lake Constance and many fruit grower counselling bodies are extremely interested in the test. Bio-Protect therefore offers the identification of the pathogens as one of its services. Numerous institutions from Germany, Austria and Switzerland are involved in the collection of test samples. In 2013, samples were collected at 102 different locations around Lake Constance. The monitoring procedure starts in February with the collection and analysis of fruit mummies, i.e. small, unripe, dried-up fruits of the previous year, and continues with the collection of blossoms in spring. Weather forecasts are used to calculate the days that fire blight infection is most likely to occur.

“Two days prior to a potential fire blight infection, samples are taken and analysed overnight. One day before the expected fire blight infection, we therefore have information available on the potential spread of the pathogens and can come up with recommendations on how to combat them,” said Dr. Kunz of Bio-Protect. Counselling bodies and fruit marketing organisations that have taken the samples are provided with test results as quickly as possible after analysis. These organisations also cover any analysis costs and issue recommendations on how to combat the pathogens based on the acquired data. “It is highly important to calculate the overall risk for the entire Lake Constance area. These calculations are based on the percentage of positive samples and quantity of pathogens found,” explains fire blight expert Dr. Kunz. If less than 5% of all samples are tested positive, treatment is generally not considered necessary when the number of pathogens is below 1000 bacteria per blossom. 

“The systematic investigations show that the quantity of pathogens seems to play a crucial role in the outbreak of the disease. Below a certain number of pathogens, the occurrence of fire blight symptoms is considerably reduced,” said Kunz. The owner of a tree plantation is informed whenever one of his or her samples is tested positive. The fruit grower then knows that the plantation has a high risk of infection and that he or she is well advised to apply pesticides at times when the risk of infection is particularly high during the flowering season. 

Pathogens are also found on non-host plants

Flowering pear trees in the flowering season during which the researchers from Konstanz are collecting the monitoring samples. © Kunz

The studies have shown that Erwinia amylovora bacteria not only hibernate in small areas of dead tissue (cankers), but also in shoots that display no symptoms despite being infected. “Fruit mummies also enable the fire blight-causing bacteria to survive and are therefore sources of infection, i.e. sources from where the bacteria can reach out and infect blossoms in spring,” explains Dr. Ernst. The studies have also shown that a high number of bacteria is associated with a higher frequency of infected blossoms and as a result with obvious signs of disease. Moreover, the researchers have detected fire blight pathogens in the blossoms of other plant species in infected orchards. “These plants are not the hosts of fire blight pathogens; they are not damaged and do not show any disease symptoms whatsoever. However, their blossoms enable the pathogens to survive harsh times and propagate – even long after bloom,” Ernst explains. As a result, such plants might contribute to the re-infection of the fruit trees as well as to the inconspicuous spread of the pathogens through insects. 

The future: combined prognosis models

The application of the molecular test enables the fire blight risk to be assessed long before weather forecast models are able to come up with information on the conditions that favour fire blight infections. In future, the assessment of fire blight risk conditions will be based on models that also involve biological components such as the actual number of pathogens present. As mentioned above, the absence or low number of pathogens does not lead to infections. Fruit growers will be provided with more accurate recommendations as to if and when to initiate preventive measures. In addition, the information can be provided at a much earlier stage than before. “The monitoring measures and studies are aimed at making the application of Erwinia amylovora pesticides more specific at the same time as preventing their unnecessary application,” Dr. Kunz explains.

Further information:
Dr. Stefan Kunz
Bio-Protect GmbH
Lohnerhofstrasse 7
D-78467 Konstanz
Tel.: +49 (0) 7531 690 661
Fax: +49 (0) 7531 690 660
E-mail: kunz(at)bio-protect.de

Dr. Michael Ernst
Wissenschaftliche Dienstleistungen
Postfach 10 05 62
D-78405 Konstanz
Tel.: +49 (0)1578 820 2301
E-mail: bio-beratung(at)gmx-topmail.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/fire-blight-monitoring-generates-new-knowledge