Substrate blocks invaded by hairy root disease
The bacterial disease known as hairy root disease has been found in greenhouse crops in Brittany, prompting regional organisations to conduct tests.
A bacterial disease called hairy root disease, which has already been detected in the UK, has spread to tomato growers in Brittany. “The consequences of this disease are currently limited, with plants becoming more vegetative and, in some cases, a particular reduction in the size of fruits. “In extreme cases, root asphyxiation has been observed,” says Sonia Hallier from Vegenov-BBV. The Breton tomato sector has united to tackle this disease (see chart). The new working group’s objectives are to gain a better understanding of the scope of the disease and identify effective solutions aimed at combating the problem. The initial results were the fruit of a partnership between the various tomato producer groups (Savéol, Sica and UCPT), together with the CATE and SECL experimental stations and Vegenov-BBV research laboratory.
In 2007, an initial study was conducted, involving growers, with the aim of determining the impact of the disease and identifying the source and crop management systems that encourage the disease. The results did not make it possible to demonstrate the prevalence of a particular source of infection. In addition, no cultivation technique seems to be solely responsible for the disease. It seems to be rather a combination of several factors that increase the risk of developing the disease.
A second investigation confirmed the previous analysis. During the 2008-2009 season, infectious bacteria were detected by means of specific molecular markers for Ri Plasmid developed at Vegenov-BBV. “This study also enabled us to demonstrate the precocity of the infection, without being able to detect pathogenic bacteria in the boreholes. The spread of the disease to adjacent plants could also be observed in some cases, which was apparently linked to contamination of the irrigation system,” explains the Vegenov-BBV team . The bacteria isolated during these analyses were then characterised biochemically. The results made it possible to confirm that, with one exception, the Ri Plasmid-carrying bacteria were Agrobacterium rhizogenes. At the same time as the analyses conducted on these bacteria, more general water analyses made it possible to determine the heterogeneity of microorganism concentrations in the irrigation systems used in the different greenhouses analysed. During the 2009-2010 season, two greenhouses used for cultivation were again monitored, in addition to two experimental greenhouses, focusing on the first few months of the season.
At the same time, samples were taken from two plant growers. The results made it possible to confirm the hypothesis that early infection may have come from young plants and/or poorly disinfected irrigation systems. These factors will be studied in greater detail during the 2010-2011 season.
Symptoms of hairy root disease may reduce fruit size and lead to root asphyxiation
REDUCED CONTAMINATION ON STONE WOOL
The SECL experimental station in Plemeur Gautier also conducted tests on hairy root disease, in order to compare different cultivation techniques. Agrobacterium rhizogenes had been receding at the SECL for at least three years. The origin of its presence was not clearly identified. “During the 2008 season, large-scale development of the bacteria led to major consequences for Plaisance crops in the second half of the season, including major root proliferation, exuberant vegetation, twisted stems, reduced size and quality,” says Hervé Floury from SECL. In order to attempt to limit proliferation at the site and understand the factors that encourage the disease to develop, an experimental programme was set up in 2009, in partnership with the OP Bretonnes, Vegenov-BBV and CATE. Its aim was to study the crop parameters that may influence the development of Agrobacterium rhizogenes (nature of the substrate: mineral or organic, continuous purification of irrigation water by injecting bleach, type of plant and/or rootstock type). The results show that Agrobacterium rhizogenes and Ri Plasmid were certainly present at root level in all the samples from the end of January (monitored by Vegenov-BBV). The origin of the bacteria has not been clearly identified. It is anticipated that it may have been present on the plants when they were delivered.
Between the compared treatments, the following points were observed:
• Effective water purification using bleach (sodium hypochlorite). Even without eradication, this process makes it possible to delay and limit (if the injection functions properly) the development of bacteria. It should be noted that the storage of purified water, which allows sufficient time to react and stabilise the water using bleach, is preferable to direct injection in the irrigation line.
• More limited contamination on the stone wool compared to coconut fibre.
The organic nature of the substrate could encourage the bacteria to develop. However, on the same substrate, the irrigation ducts may also affect the contamination.
The objectives of the tests conducted in 2010 were to confirm the substrate effect (using the same type of block) and compare the different irrigation methods, by looking at the dosage and frequency of irrigation. The objective was to see whether a drier system might limit contamination. The tests will continue in 2011.
Contamination ratings were obtained at the end of the 2009 season from a test conducted at the CATE on several rootstocks with the Plaisance variety. “The rootstocks that promote highly vigorous growth (Optifort, Emperador, Maxifort) appear to be more sensitive to contamination by Agrobacterium rhizogenes. In contrast, rootstocks with less or no vigour (Beaufort, Unifort, Arnold) show less sensitivity,” explains Alain Guillou from the CATE. No Ri Plasmid was identified in this test but the symptoms (proliferation of small roots, particularly close to emitters) were visible. Sometimes, development is limited to the water feed points (Agrobacterium hardly developed) or has spread to the whole substrate (fully developed Agrobacterium).
The intra-species and medium vigour Efialto rootstock also appears highly sensitive (Agrobacterium fully developed on 70% of substrates). In this test, however, the presence of Agrobacterium does not affect the behaviour and general balance of plants. The best yields at the end of the season are obtained on Beaufort and the three most vigorous rootstocks (Maxifort, Emperador and Optifort). In contrast, Unifort (very low vigour) appears significantly less productive, with a lower fruit size. For the Arnold rootstock, comparisons with other greenhouse crops cultivated on stone wool shows less sensitivity with this substrate, which confirms the observations made at the SECL.
HORMONAL DYSREGULATION IN THE ROOTS
The bacteria are able to transfer a DNA fragment to the root cells of the tomato plant, in which they cause hormonal dysregulation.
Hairy root disease is a disease caused by flagellated soil bacteria, known as Agrobacterium rhizogenes, which carries a circular DNA fragment, known as Ri Plasmid (pRi). This is the disease vector. The bacteria are able to transfer this plasmid DNA fragment to the root cells of the host plant. The plant then includes this DNA, which contains bacteria genes, in its genome. This phenomenon causes hormonal dysregulation in the roots of the host plant. This leads to major root proliferation, with the plants becoming more vegetative to the detriment of fruit development. In some cases, it is possible to observe root hypertrophy, which renders the plant sensitive to weakness pathogens.
PARAMETERS FAVOURABLE TO DEVELOPMENT OF THE DISEASE
- Use of coconut fibre-based substrates rather than stone wool-based substrates;
- Use of thin substrates, which increases the risk of moisture building up on the substrate surface;
- Frequent irrigation;
- Maintaining damp areas in the greenhouse;
- The chosen variety;
- Poor disinfection between growing seasons;
- Irrigation water composition favourable to the maintenance or even development, of root pathogens.
Pathogenic tests are in progress at Vegenov-BBV, which aim to support the CATE and SECL22 experimental stations with the identification of solutions to this disease. These tests will also make it possible to compare different cultivation parameters: variety/rootstock, substrate, irrigation frequency, treatment product, etc. By means of these tests, Vegenov-BBV will also study the virulence of the various isolated strains.
Réussir Fruits & Légumes (“Success with Fruit and Vegetables”) – January 2011 – No.302