By Carolyn King
By Carolyn King
Bacterial ring rot is one of the most difficult infectious potato diseases to combat. Growers rely on sanitation of potato storages and equipment for prevention. With advances in microbial science, new sanitation products and better testing technologies, now is an opportune time to update and improve Canadian sanitation strategies and tools for ring rot control. So a leading-edge project is doing that, and more. “Bacterial ring rot is probably the most feared potato disease, especially for seed growers. There is a zero tolerance for it in seed potato farms. As little as one infected tuber on a farm will result in complete decertification of the farm unit. So finding bacterial ring rot on a seed potato farm would have devastating consequences, putting the operation out of seed production for two or more years until the producer is able to clean up and demonstrate that the infection has been eradicated,” says Dr. Ron Howard, a plant pathologist with Alberta Agriculture and Rural Development (AARD), who is leading the project.
The disease can also be a very serious problem in commercial production of table stock and processing potatoes. It causes tuber rot and reduced yields, and can spread easily from field to field, from the farm to the potato processing plant, and from there to other farms on contaminated potato equipment and vehicles such as trucks.
A major component of Howard’s project is to test the effectiveness of existing and newly identified disinfectants on the ring rot pathogen. In Canada at present, there are two licensed disinfectants that can be used for potato storages and equipment: General Storage Disinfectant and 1-Stroke Environ. “The General Storage Disinfectant has been used for many years, and the 1-Stroke Environ is a more recently introduced product. Growers have asked whether these products are still effective on the ring rot organism, given reports that you hear about plant disease organisms building up resistances, or tolerances to products. And naturally, growers are always on the look-out for products that might be more effective than the older existing products,” says Howard.
He adds, “I’m hoping we’ll end up with more products being registered for use against ring rot as disinfectants. I think there are other products as effective or maybe more effective than the two we have now. The data that we are amassing as part of this project could help support registration of those products, and we would encourage the manufacturers to register them if their efficacy is superior.
In the past, disinfectant effectiveness was tested only on the single-cell forms of microbes. Howard’s project involves a significant step forward because it is testing the products on both the single-cell and “biofilm” forms of the ring rot bacterium, Clavibacter michiganensis subspecies (sp.) sepedonicus.
He explains, “Biofilms are communities of microbes that grow attached to surfaces and are bound together by a slimy matrix. This matrix protects the microbial cells from exposure to disinfectants and antibiotics. As well, the microbial cells in the biofilm have differences in gene expression and physiology, as compared to the non-attached, single-cell, or “planktonic” form of the microbe. Consequently, biofilms can be up to 10,000 times more resistant to disinfectants than single-celled populations.”
Howard’s research team includes experts from Innovotech Inc., an Alberta-based company that has developed advanced technology to rapidly assess the effects of biocides on biofilms. Other team members include Dr. Tracy Shinners-Carnelley, a potato specialist from Manitoba, and Dr. Solke De Boer, a bacterial ring rot specialist from the Canadian Food Inspection Agency in Charlottetown, Prince Edward Island. Federal and provincial agencies and a number of potato organizations are funding the national project.
Along with the disinfectant testing, the researchers are examining how the disinfectants fit into an overall sanitizing strategy for ring rot. A key aspect of this involves assessing disinfectant performance on different types of surface materials, because the surface material can influence both bacterial growth and disinfectant effectiveness.
Potato storages and handling equipment can involve many types of materials such as concrete, unpainted and painted steel, aluminum, galvanized steel, rubber, wood, spray-on foam insulation and sponge foam insulation. Howard says, “All of these surfaces could potentially become contaminated with bacterial ring rot if you had a serious outbreak and a lot of breakdown in storage. So we’ve taken those materials, contaminated them with ring rot biofilms, and exposed them to different disinfectants at various concentrations for various lengths of time.”
The researchers are also evaluating disinfectant corrosiveness on surfaces, safety issues like protective equipment for applicators and the value of adding products like foaming agents to disinfectant mixtures.
In addition to the lab testing, the researchers are using a mobile sanitation unit to conduct pilot-scale evaluations in commercial facilities to ensure the procedures work in real-life settings. Because bacterial ring rot is rare, the researchers have not come across a storage facility with a ring rot issue. So, the on-site tests are measuring the total microbial load – bacteria, yeast and fungi – in the storages before and after the treatments. In 2010, on-site testing was conducted at 10 commercial storages in Saskatchewan and Alberta, and that work will continue in 2011.
The project has already generated several important results, and Howard hopes to have improved national guidelines for growers within a year.
Three-step cleaning process confirmed as best approach
“Probably our most significant finding is that the sanitation process really does have to follow three key steps, and you can’t skip any step and hope to do a good job of disinfecting surfaces or equipment,” says Howard. He adds, “This three-step strategy is not new. Most growers are already doing that, but we now have the experience and the data to support that strategy through our studies.”
Step 1 is rough cleaning. This involves sweeping and scraping up debris, like tubers that have been squashed or rolled into corners, pieces of potato vines, and soil.
Step 2 is pressure washing. He says, “Pressure washing removes any residual organic material, such as soil and crop debris, and it also begins to blast away at the biofilms of pathogens like ring rot that may be left behind on the floors, walls, plenums, grates and drains. Pressure washing should be done with warm or hot water and a detergent. The detergent softens up the soil and the biofilms, and makes them more amenable to removal by pressure washing. We’re generally talking 2000 or 3000 pounds per square inch of pressure with warm or hot water to break up and flush away the remaining organic residues and soil.”
Step 3 is applying the disinfectant to the pressure-washed surface. It should kill any biofilms and planktonic cells remaining.
He emphasizes, “Steps 1 and 2 are the most critical because you can remove about 99 percent of the contamination by doing a good job with getting rid of the rough residue and biofilms by pressure washing. The disinfectant is just used to clean up any residual contamination.
“Many producers would like to skip Step 1 and Step 2 and just go in there with the disinfectant right off the bat. But we found that the disinfectants do not penetrate into the inside of contaminated stems or tubers or the slimy residues that may be left behind on surfaces. Also some of the disinfectants are neutralized by organic matter and lose their effectiveness.”
So, by leaving the disinfectant application to the last step, it minimizes the amount of disinfectant needed, which reduces disinfectant costs. Furthermore, some disinfectants are hazardous, so minimizing the amount used reduces the exposure hazard to applicators, potential corrosion damage to surfaces, and potential downstream impacts of excess disinfectant runoff on drains, lagoons and sewer systems.
Choose the right disinfectant for the type of surface
A second important finding is that is no single disinfectant does a 100-percent job on all of the different types of surface materials that can be found in potato storage facilities. For example, some disinfectants work very well on certain surfaces and are weak on others.
Howard notes, “The two most difficult surfaces to disinfect that we’ve found are wood and foam insulation because they are very porous and take a special effort to disinfect. You need to thoroughly clean them, of course, but you also have to make sure you choose the right disinfectant for the job.”
At the end of this project, Howard expects to be able to provide information about the risks and advantages of each disinfectant so growers can make informed choices.
Cutting rates or contact times cuts effectiveness
“Not surprisingly, pretty much all of the disinfectants kill the ring rot organism in the planktonic form, but it is much more difficult to kill as a biofilm,” notes Howard.
The project’s results show that lowering rates or contact times below those specified on the label can drastically reduce a disinfectant’s effectiveness. “In fact, against the biofilms we have often found that the label rates for general disinfection on many of these products will not eradicate the ring rot pathogen with short exposures of 10 or 20 minutes of contact time. You need to go either to a higher concentration or to a longer contact time,” he explains.
“As a general rule, with most of these products, we often have to go to twice the label rate or more to get a reasonable kill, and we need to extend the contact time sometimes up to 30 minutes. That means, after the surface has been cleaned and you’ve put your disinfectant on, you have to re-apply the disinfectant to make sure you get 30 continuous minutes of contact time between the disinfectant and the surface. Otherwise the disinfectant tends to run off or dry and there’s no further activity.”
The researchers are also developing recommendations for ensuring sufficient contact time. Howard says, “Those recommendations will include things such as applying disinfectants when air temperatures are cool versus warm or hot, and shutting down the doors and ventilation systems in storages to prevent the disinfectant from drying down too rapidly. If that’s not possible, then you may want to consider adding something like a foaming agent to your disinfectant to slow the dry-down.”
Beyond bacterial ring rot
The research methods and technologies used in this ring rot project have become a template for two similar projects. One project relates to controlling bacterial canker in greenhouse tomatoes. Howard says, “The bacterial canker organism is a close cousin of the bacterial ring rot pathogen, so we were able to do sort of a carbon-copy trial to look at methods to eradicate the tomato canker pathogen from the types of hard surfaces commonly found in commercial greenhouses.”
The other project involves clubroot, a serious canola disease that is mainly spread by contaminated equipment. “We’re looking at the efficacy of cleaning methods and disinfectants against the clubroot pathogen where spores contaminate surfaces of equipment and vehicles,” he explains.
As well, Howard is hoping to use the same research approach to evaluate the effects of the ring rot sanitizing procedures on other potato pathogens. The three-step sanitizing process is the best approach for any potato disease issue, but research is needed to determine the effects of the various disinfectants on the other pathogens.
It is quite possible that the ring rot sanitizing strategies will also control other potato pathogens at the same time. Many of the disinfectants used in the ring rot project are broad-spectrum biocides so they will likely kill various other bacterial and fungal pathogens. In addition, the ring rot procedures are aimed at virtually 100 percent eradication of Clavibacter michiganensis sp. sepedonicus, which is not necessary for most other potato pathogens.
Howard says, “We have set a very high standard for the cleaning and disinfection protocols being evaluated in this bacterial ring rot work. For other diseases, in most cases, growers would be satisfied if they were able to take the contamination level down to only one or two or five percent contamination left in the storage. But with bacterial ring rot, we need to take it down to zero because there is a zero tolerance for it in seed operations. And even in many commercial operations, growers treat it as though they want zero tolerance because of the stigma attached to the disease. So the disinfectants and the overall sanitation program have to be so robust as to totally eradicate the organism where at all possible.”