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Halting Sudden Oak Death

April 24, 2008  By Michael Lascelle

Phytophora ramorum or sudden oak death (SOD) has recently been detected in Canada and was likely brought here through infected plants from the United States, and possibly the Netherlands.

Protecting Your Nursery from Contagious Diseases

Phytophora ramorum or sudden oak death (SOD) has recently been detected in Canada and was likely brought here through infected plants from the United States, and possibly the Netherlands. This fungus-like organism has the ability to infect a broad range of host plants, is very difficult to detect and has the potential to eventually change the way we handle plant material in this country, which in the end, may be a change for the better.


While there has been a lot of media coverage in regards to this crisis, I do not personally believe that we should be directing detailed information about SOD to the gardening public in general, until the trade itself is fully informed on the matter and all affected garden centres and nurseries across Canada are thoroughly tested for potential infestations.

However, I see nothing wrong with a frank discussion among horticultural professionals, in regards to our concerns about the SOD crisis and its potential impact on our livelihoods. And that’s exactly what you are getting here – one nursery manager’s informed opinion about a few potential problem areas that I believe all garden centres should examine very carefully, particularly in light of such a subversive pathogen.

On-Site Composting
For most of us, on-site composting is both an economic measure and a convenience. Usually all organic-matter refuse is disposed of here rather than paying to transport and dump off-site, or filling our garbage bins at an excessive rate. That refuse could be anything from dead or dying plants, weeds, lawn clippings, fallen leaves and even waste soil from those dead plants. Then there’s the question of what we are actually doing with the resulting compost, which is usually reincorporating it into a potting mix, using it as a soil amendment in our display gardens or even selling it as a component of a blended topsoil – all of which could potentially spread disease. But with the introduction of SOD-infected plant material, the potential for cross-contamination from our on-site compost areas has become very high, particularly with Phytophora ramorum, which can spread through soil, surface drainage, water droplets and even on the wind. So the last place you want to throw a shrub potentially infected with SOD is an on-site compost area.

Recent testing in California has proven that only the most stringent composting techniques are capable of negating this pathogen.  This includes clear separation of raw and mature compost, full control of water runoff, a minimum compost temperature of 131 F for a period of two weeks and wind barriers around raw compost to prevent the aerial infection of surrounding vegetation.

You would be hard-pressed to find a compost site in coastal B.C. without a potential host plant such as bigleaf maple (Acer macrophyllum), salmonberry (Rubus spectabilis) or Douglas fir (Pseudotsuga menziesii) nearby – all of which could provide a ruinous disease interface with our indigenous forests. With the high mortality rates evident among native trees (50 per cent for coastal live oak and 90 per cent for tanoaks) in the worst hit areas of California – we definitely do not want to see the same thing happening here or anywhere in Canada’s forests.

Plant Returns and Warrantees
In a market potentially infected with SOD – can you safely allow the return of host plant material under your existing warrantee? The honest answer is no and in the interim, we should be discouraging plant returns whenever possible. This may seem like we are sending out mixed messages but we have to remember that even with the recent infected camellia recall in British Columbia, homeowners were asked to report their concerns and leave the plants in the landscape for CFIA officials to assess. Unfortunately, garden centres often have to deal with plant samples brought to us on the spur of the moment by concerned customers. To avoid this, try to confer with the client before the plant is returned, and ask them to place a small leaf or stem sample in a clear, sealed sandwich bag so that you will be able to look at the problem without risking exposure.

SOD is an unpredictable disease, which may cause a substantial increase in unpredictable plant returns, and while we are all hoping that we will be able to eradicate its presence in Canada – are you prepared to replace host plants that your customers believe are infected with SOD?  Since this disease cannot be visually confirmed, the only way to verify their status is through a lab test, that would cost more than the original purchase price of the plant. We may just have to rethink the terms of our plant warrantees and how we plan to deal with plant returns in the near future.

Recycling Nursery Pots
The next time someone drops off a stack of used nursery pots for recycling take the time to separate them and find out what else you are getting.  You are probably going to discover a lot of loose soil, small pieces of pruned branches or broken roots, damaged leaves and all of the price tags jammed between the pot layers.

Now think about all those SOD host plant leaves sitting between the layers of your used pots, all of which can harbour the disease in one form or another for long periods of time – is this a safe practice? Tests at the University of California have shown that dried, infected rhododendron leaves were still infectious when rehydrated several weeks later. Even if you have the time to sterilize these used containers, what are you going to do with all the debris that comes with them? You can’t be sure that it is not infectious without further testing.
Recycling is good, but making sure that plant diseases don’t get a foothold in your garden centre is better!

Display Gardens
A well-designed and maintained display garden is an asset to any business. Should SOD prove to be more persistent or perhaps more widespread than originally anticipated, then we may have to drastically redesign our display gardens to protect ourselves and our customers.
One solution might be to use more annual flowers or non-host perennials in our display gardens, with the occasional use of larger trees and shrubs in temporary nursery pot, which can be easily accessed and sold. This would eliminate the use of permanent woody shrubs and trees in our display gardens – plants that could potentially become infected and spread any disease to the plants we sell.

Garden Centre Layout
While it might be a bit premature to make substantial layout changes to our garden centers, once SOD certification has been finalized and adopted, we will all have to adhere to these rules if we want to be certified. Nevertheless, I thought I would mention a few potential aspects of this restructuring.

•  Shipping and receiving areas should be located outside the nursery, to limit vehicle access and possible contamination.  The area should also be surfaced in such a manner that any debris can easily be swept and disposed of properly.

• Drip irrigation should be used whenever possible, and all overhead irrigation should be timed for early morning applications, to allow foliage to dry by nightfall.

• Irrigation water should be supplied from municipal sources. If this is not practical, the water source should be monitored on a regular basis to ensure that it is pathogen free.

• Phytosanitary mats should be placed and maintained at the entrance to propagation and production areas, to sterilize the footwear of visitors and staff.

• Customers must be effectively barred from entering production areas to prevent any chance of cross-contamination from other garden centres. ‘Off Limits’ signs and rope fencing barriers can also be utilized to protect these areas.

• If your nursery is terraced or built on a slope, surface drainage may have to be directed in such a manner as to prevent it from running from bed to bed.

• Avoid placing containerized host-plant shrubs below the canopy of SOD-host tree species particularly in the presence of overhead irrigation.

• Create an isolation bed at least 4 meters from any SOD host plants to hold any plant returns or quarantine suspect plant shipments.

A Final Word
The best course of action during this period of uncertainty is to stay informed about new host plants, improved phytosanitary practices and current regulations regarding the shipping and receiving of plant material. In British Columbia, our nursery trade association (BCLNA) has been instrumental in its role of informing all concerned parties and acting as a liaison with the CFIA (Canadian Food Inspection Agency) and USDA (United States Department of Agriculture) in regards to standardizing cross-border regulations and testing for SOD.

We must remember that SOD is not just British Columbia’s problem. This disease has spread extensively throughout the U.S. and testing is ongoing in Ontario, Quebec and the Maritimes. Some might believe that closing the provincial borders to B.C. plant stock might contain the outbreak, it’s my opinion that the North American and international horticulture market is far too integrated for this approach to be successful in the long-term.

I would also encourage you to participate in any federal or provincial certification programs, which may arise from this crisis. Such programs will help all wholesale growers and retail garden centres to harmonize our plant inventory tracking and improve our sanitary growing techniques and subsequent cultural practices, so that we will be in a much better position when the next disease comes along.

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