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Text by Hans Neefjes
Photo by Wageningen UR, Glastuinbouw Nederland
All Ralstonia pseudosolanacearum strains that are known in the floricultural industry can spread in cut-rose plants. This was the outcome of a recent study. Disease development does however strongly depend on temperature, bacterium strain and rose cultivar. Just like in previous studies, the bacterium was also found in plants that showed no symptoms of disease. The bacterium has a big impact.
When Ralstonia solanacearum (Rsol) was first found in rose, this was unique. Unique, because the bacterium had never been found in rose before 2015. And unique also in terms of damage, because the finding of this Q-bank listed organism means complete clearing of the nursery, which cost the affected 15 rose companies (growers and propagators) in the Netherlands many millions of euros.
The total damage is estimated at €15 million, around €1 million per hectare. The costs for lab research, additional labour, disinfection, controlled clearing of the crops and lost earnings, can not be claimed on any liability insurance or crop insurance. The Dutch government doesn’t provide any financial support, either. And any lawsuits that were held against the government so far, were all lost by the growers.
Business risk
Responsible minister Schouten described the damage as significant, but foreseeable, and she considers it a normal business risk. Growers on the other hand, feel that they should be able to claim some sort of compensation, because the bacterium had never been detected in rose before 2015, and it took four months, even for specialised labs, to identify the troublemaker that was attacking their roses. And the Netherlands Food and Consumer Product Safety Authority (NVWA) didn’t even have a protocol for the Q-bank listed organism at the time.
But minister Schouten pointed out that the reporting obligation with regards to (the suspicion of) harmful organisms such as Rsol, is a professional responsibility of growers and propagators. That’s because a harmful organism at one company can affect other companies in the immediate vicinity, or even have an impact on the entire sector.
Source unknown
Despite all the NVWA investigations, the source of the infection still hasn’t been found. The latest, relatively small outbreaks at Dutch rose nurseries were in the spring of 2018. They all concerned one specific cultivar.
In March 2017, several organisations, including LTO Glaskracht Nederland, various crop cooperatives, Plantum, Naktuinbouw (the Netherlands Inspection Service for Horticulture), NVWA, Wageningen University & Research and Utrecht University started a joint, three-year project. One of their three goals is to develop a reliable, cheap and fast lab test. They also want to collect knowledge about the bacterium, which will help reduce the risk of introducing and spreading it in rose as well as other crops. Finally, they conduct research into increasing resistance (through the use of probiotics for example) of plants.
The damage is significant, but foreseeable, and she considers it a normal business risk
About Carola Schouten, Dutch minister of agriculture
Prevention seems to be the magic word. For plant materials, the ‘Select Plant Propagation Materials Rose’, is going to be a voluntary addition to the legally required, compulsory, standard inspections. Various rose propagators say that within their company, they take even stricter measures than the ones outlined in this system. The propagators have already increased their hygiene measures and interim inspections. Rose branches can only be used for propagation after extensive checks in the lab these days. Similar measures are in place for other crops sensitive to Ralstonia.
Statistical probability
Inspections don’t provide a 100% guarantee, though. There’s still a statistical probability of finding the organism in your crop. But all testing methods, including those of the NVWA, are getting more refined. Since drain water is an important source for the survival and spread of bacteria, Wageningen University & Research conducted a study into the survival of rose Rsol in drain water.
The bacterium lived the shortest (60 days) in drain water of 4°C, while it stayed alive the entire, 110-day trial period at 20°C, and only a few days less at 28°C. So, it turns out that the so-called tropical variant of the bacterium can endure fairly low temperatures in drain water for quite a while. If it manages to infect a host plant, its survival rate is even bigger. The moment of freezing/thawing is most crucial. Crystallisation kills bacteria.
Heat
No Rsol was found in drain water that was disinfected using UV or heat. According to the NVWA, there’s no reason to doubt the effectiveness of UV-dosages and heating protocols against most pathogens in drain water.
Especially around 20°C, most infections don’t cause any visible symptoms, but the bacterium can spread to the branches. This increases the risk of undetected spread of the pathogen. Plants can also be infected with the bacterium via a heavily infested rockwool slab. It’s therefore important to constantly check both the crop and the rockwool water for the pathogen and to take the appropriate sanitary measures to prevent any spreading via feed and drain water.
Various harmful clones
The various Rsol clones can infect different crops. Tomatoes for example, are sensitive to many Rsol variants. That’s exactly why this crop is used a lot for sample testing. If you spray tomato with high doses of Rsol, you can see an effect within 8 days.
Researchers at Wageningen University discovered that high air temperatures (28°C) and mechanical infection cause the fastest development of Rsol in rose, but not every cultivar has the same sensitivity to the bacterium.
A study involving three different strains of Rsol, one that was found in cut rose, one in anthurium and one in curcuma, demonstrated that all three can cause damage, especially at 28°C, but the strain that was found in rose was by far the most aggressive.
European audit
All outbreaks of Rsol in rose in the Netherlands concerned young plants. Plant material from the infected propagation companies also went across the border. The bacteria was found in Belgium, Germany, Austria, Poland, Portugal and Switzerland.
The European Commission conducted an audit in September 2017 to determine whether the Netherlands had taken all appropriate measures against the Q-bank listed organism in rose. The answer was yes, and that’s why no emergency measures were introduced for Rsol in rose.
The NVWA emphasises that when an infection is found, the most up-to-date measures are always taken. The organisation pointed out that destruction of the infected crops is an EU requirement. This is regardless whether the infected plants show any symptoms, as the disease might be latent.
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