This is the time of year that many orders go into propagators for next year’s lavender planting.  Over the past two years there have been many reports of Phytophthora root rot on lavender, and many of these reports can be traced back to contaminated plant material at planting. Phytophthora root rot of lavender can be a devastating disease because it can destroy numerous plants rapidly, it is nearly impossible to completely control, and it prevents replanting on the same ground.

The primary species that has been identified in Ontario on lavender is Phytophthora nicotianae, which is an oomycete or water mould. Contrary to some published reports, this pathogen is not very widespread in the environment and the vast majority of cases in lavender so far have been traced back to contaminated plant materials. The other cases have an unknown origin, but some could still be the result of contamination down the supply chain from nursery to nursery. There has been only one site in Ontario where this disease developed and the origin of the Phytophthora is unlikely to have been the transplants.

This year we confirmed that P. nicotianae does survive the winter in Ontario. However, it is unknown if this was due to a very mild winter, and is a relatively rare case, or can occur every year. Until we know more about survival, it is very important for growers to consider this a major threat to their entire operation.

There are a number of precautions that both propagators and growers should follow to prevent further spread of the disease.

Propagators:

1. Avoid bringing in contaminated plants. Propagators need to take care to avoid introducing Phytophthora into the greenhouse in the first place. The easiest way to introduce the disease to your operation is to purchase or dig up contaminated plants. Bringing in new plant materials only through cuttings is one way to minimize the threat. Any plant materials with roots or soil should be quarantined from the rest of the operation for a few weeks and monitored closely for disease development. This would also reduce contamination with other soil-borne pathogens such as Pythium and Fusarium.
2. Avoid using contaminated media. Most soilless mixes and ingredients (e.g. peat, perlite, vermiculite) are not necessarily sterile, but should not have major issues with pathogens because of where they come from. However, sand, gravel and organic matter sources (e.g. compost, mulch, etc.) can easily be contaminated with various soil-borne pathogens. Although the risk of getting Phytophthora nicotianae from these sources is low, all it takes is one contaminated source to cause major issues for your business. Avoid using these sources as much as possible or sterilize them through steaming before using them.
3. Practice good sanitation practices. Propagators should keep a clean operation that includes cleaning up debris from floors and benches, removal of any diseased or dying plant material from the operation, regular cleaning of tools, and using only sterilized trays and pots. At least once a year, the entire operation should be thoroughly cleaned (e.g. floors, walls, benches, heating pipes, vents, water tanks, water lines etc.) with soap and water and then sterilized with an approved disinfectant. When an outbreak occurs, remove and destroy all potentially contaminated plants and conduct a very thorough cleaning before conducting any new propagation activities. Avoid using benches, trays or pots made of materials that are difficult to sanitize such as wood and styrofoam. For more information on sanitation practices and disinfectants, consult OMAFRA Publication 370 Guide to Greenhouse Floriculture Production.
4. Avoid spreading disease through irrigation water. Phytophthora nicotianae is a water mould that has swimming spores. It can easily be spread in contaminated water or by splashing of spores from one pot to the next. There is no perfect irrigation system to minimize disease. Ebb and flow (flood) benches and floors that deliver water from the bottom up can easily spread disease from one contaminated plant to the entire operation if the same water source is used for the whole operation and the water is not sterilized. Overhead watering increases the risk of splashing. Be aware of the risks of these practices and minimize spread by applying overhead water as a finer mist and separating plants in distinct areas with separate water supply for flooded systems. If the water is being recycled or is pulled from surface water sources, sterilize the water with UV radiation or ozone systems. After sterilization, putting a solid sheet of plastic over the whole floor or bench below the new plant material can reduce the transfer of any spores that were missed during sanitation to the new plants.
5. Apply fungicides preventatively. There are several products registered for use on greenhouse ornamentals like lavender for both organic and conventional growers. Applying these preventatively can reduce both the incidence and spread of Phytophthora root rot and other root diseases. This should only be considered the last line of defence and should not replace the strategies listed above. For a list of registered fungicides and biofungicides in Canada consult Chapter 10 of OMAFRA Publication 370 Guide to Greenhouse Floriculture Production. Additionally, organic growers need to confirm with their certification body on whether the product is permitted for use.
6. Be up front with customers. While no operation wants to advertise the risk of disease, growers will find out eventually if the plant material is contaminated with Phytophthora. Be up front with your customers on the risks involved, and what you are doing to minimize the risk. This is also an opportunity to explain to customers on how they can minimize their risk of spreading any potential disease.

Growers:

1. Purchase plants from reputable propagators. A good propagator will have routine sanitation practices and have a history of providing vigorous and disease free plant material. Before ordering plant material, talk to other growers about their experiences with propagators and the quality of the plant material. Talk to the propagator about their practices and what they do to ensure the plant material is not contaminated. Ask specifically about their experiences with Phytophthora.
2. Ask about guarantees. It is difficult for any propagator to provide a 100% disease free guarantee, but they should be able to provide some assurances on the quality of their plants. Ask about any recourse if plants arrive dead after shipping or begin to die shortly after.
3. Quarantine newly purchased plants. It is a good idea for growers to practice routine quarantine of newly purchased plants. Keep the plants in their pots well away from the fields for a few weeks after purchase to monitor them for any disease issues that may develop. If a plant dies in the pots, consider sending the plant to a diagnostic clinic for identification. If Phytophthora nicotianae or another Phytophthora species is detected, it is best to destroy the whole batch of plants in a landfill site (not the compost pile). If destroying the whole batch is not possible, it is best to keep them in the pots as long as possible, removing plants as soon as they show signs of the disease. The disease may be confined to a cultivar or specific tray, and it is possible that only those need to be destroyed. Plant only healthy looking plants in an area that will have less impact on your operation if a Phytophthora outbreak occurs. This could include separate areas away from the display fields where loss of some plants will not have as much of an impact on the appearance of the operation. It is best to avoid planting these plants in wetter areas of the farm or where surface water may flow from the new planting into other lavender fields.
4. Consider doing your own propagating. Doing your own propagating greatly minimizes the risk from this disease. This can add to the cost of the operation but may be worthwhile if the disease becomes more widespread in the industry and you have yet to be affected. However, propagating lavender can be difficult without the right equipment and many growers who propagate for themselves still rely on some purchases from other operations to introduce new cultivars.

The potential impact of Phytophthora on the long-term survival of a lavender farm cannot be overstated. Growers need to consider this as a potentially major threat to their operation and follow as many precautions as possible to avoid introducing it to the farm. Over time we will learn more about the long term risks of the disease on a farm. Until then it is best to assume the worst and plan accordingly.