Last year there were several reports of Phytophthora root rot on lavender across southern Ontario and much of the United States. Symptoms of Phytophthora root rot begin as sections of plants wilting down in the middle of the growing season, which usually rapidly progresses to complete plant death. The disease often begins to spread down the row and in rough patches in the field, especially under wet conditions. It is caused by the water mould Phytophthora nicotianae and several other Phytophthora species. In 2015 Phytophthora nicotianae was confirmed on all affected plants.

Up until now it was unknown if Phytophthora nicotianae would survive the winter in Ontario, because previous studies in the lab have shown that it cannot survive low temperatures close to freezing. This year, lavender on one of the affected farms in Ontario showed new symptoms of wilting in early summer despite the dry conditions this year. The plants were sent to the University of Guelph Pest Diagnostic Lab where they confirmed Phytophthora nicotianae was present again using DNA Multiscan. Since genetic tests can detect DNA even if the organism is dead, this did not provide enough proof that the Phytophthora was alive. The affected roots were plated out on agar and a Phytophthora species was found to grow out of the root tissue. The culture was re-tested with PCR techniques and it was confirmed to be Phytophthora nicotianae.

Unfortunately, this provides enough evidence to say that the disease will overwinter in Ontario, at least under the conditions experienced at the one farm. Since this summer has been very dry, most affected fields are not showing new symptoms of the disease. It is possible that the disease will continue to progress once wet weather returns. It is unknown how well the disease will survive in the long term. It is possible that the specific conditions necessary for the disease to progress in a field will not occur every year.

Now that we know the disease can overwinter, it is even more critical for growers to scout for and deal with disease outbreaks as soon as they are identified. Here are key steps for all growers to consider:

1. Scout fields regularly for the disease. Look for plants that show sudden wilt of a portion or all of the vegetative growth when neighbouring plants appear healthy. There are other causes of wilt in lavender, but assume it is Phytophthora to avoid delaying management. The only way to confirm it is Phytophthora is to dig up the whole plant and send it to a diagnostic lab.
2. Remove affected and neighbouring plants. Once the disease shows up in a field, it is essential to dig up and remove affected plants and at least one neighbouring plant on either side. This will reduce the amount of new spores produced by the water mould and limit the spread of the disease. Soil around the roots can also be removed from the field in an attempt to reduce the amount of inoculum left behind, but this can be labour intensive.
3. Prevent water movement through affected areas and reduce erosion. Movement of soil from the affected area through erosion, and surface water flowing through the area after heavy rains will spread the disease to new areas of the field. Prevent surface waters flowing through the area by creating berms to divert water flow. Cover the affected area in mulch, cover crops, or gravel to prevent splashing of infested soil to new areas and reduce movement of the soil on boots and machinery. Improving drainage in the field through tile drains could prevent soils from becoming saturated. Saturated soils are ideal for the spread of disease.
4. DO NOT PLANT NEW LAVENDER BACK INTO AFFECTED AREAS! Despite the hole you have now created in the middle of your lavender field, avoid the temptation to plant new lavender back into the empty spaces. This will just continue the disease issue. It is better to have to deal with a small hole in your planting than to have disease spread to the majority of your field.  Consider making this empty patch look like a planned feature of the field such as a bench, sculpture, or a flowering tree (making sure it is not a host).

Over time we will likely learn more about how this disease progresses from year to year and better ways to manage it. For now, we need to assume the worst case scenario and hope that it is not nearly as bad.