Last year Phytophthora root rot showed up on several farms in Ontario around mid-summer. With drier weather this year, growers may not see obvious damage and may forget about the problem. Unfortunately, it is not that easy to get rid of a Phytophthora problem and the disease may continue to progress once wet weather returns.
Phytophthora root rot of lavender is a very destructive disease that can destroy whole sections of a field under warm and wet weather. Flooding that occurred in one of our cultivar trial sites several years ago resulted in rapid death of a section of around 100 plants. It is known to be caused by a few different species of Phytophthora. The outbreak in 2015 was caused by Phytophthora nicotianae, a species that has a very wide host range. Phythophthora species are members of a group of fungus-like organisms called oomycetes, which are often called the water moulds since they are often a problem in very wet weather. They produce spore filled sacs called sporangia that contain numerous swimming spores call zoospores. When the soil is saturated, these spores can swim through the soil to attack new roots. They also produce long-lived spores called oospores that can remain in the soil for several years. Symptoms will appear as wilting of sections of a plant at first and this will quickly progress to affect the entire plant (Figure 1). The disease may not show obvious symptoms for long periods but then new symptoms appear again when wet and warm weather returns.
How this disease spread in Ontario last year remains a bit of a mystery. Some disease occurred on potentially infected transplants, but Phytophthora nicotianae was confirmed as the cause of disease on three widely separated farms in southern Ontario and on four different cultivars from four different sources. There is some evidence that Phytophthora nicotianae can spread through airborne spores under warm and very wet weather, but unlike other Phytophthora diseases such as potato late blight, which is caused by Phytophthora infestans, it is not known to be the primary method of spread of this pathogen. However, it is possible that the conditions necessary for airborne spread of the pathogen were present last year. If that is the case, then the pathogen could have come from hundreds or even thousands of kilometres away if for example, a strong thunderstorm developed over an area with an infected crop and then travelled towards southern Ontario. Since the pathogen has such a wide host range, it is also possible that crop history on a site could also play a role.
Regardless of how the disease spread, it is important to scout lavender regularly for the disease, especially after periods of wet weather. If airborne spread was a possibility, then disease could be in any field in Ontario, so growers should not assume that they will not have disease this year if they didn’t notice disease last year. This is the time of year when spread of the disease is most noticeable. Growth on an infected plant could emerge as normal in the spring, and then a section of the plant may suddenly wilt and die. Any plants showing symptoms of the disease should be removed immediately and burned or disposed of in the garbage. It is a good idea to remove the infested soil from around the base of the plant as well where the organism would be most concentrated. Since the disease often moves from plant to plant down a row. The neighbouring plants should also be removed. Unfortunately, you cannot replace the plant with a new lavender plant, or you will just continue the problem. Consider planting an ornamental that is not a host of Phytophthora nicotinae. Click on the document below for a list of scientific names for known hosts of the pathogen. Avoid planting any of these in the area of previously infected plants. Unfortunately, there may be more hosts than have been identified so far.
The best way to control the disease is to avoid getting it in the first place and preventing spread of the disease once it occurs. Purchase plants from reputable suppliers and, if possible, obtain a written guarantee that they are free of Phytophthora and other pests. It is also important to ensure good drainage to prevent long periods of saturated soil in the field. Prevent water from flowing through areas that have a history of Phytophthora root rot by diverting surface water around the area. Prevent movement of infested soil on boots, tools or tractor tires or through soil erosion. There are no conventional or organically-acceptable fungicides that are registered for control of this disease on lavender in Canada.
If you grow lavender commercially in Ontario and you suspect you may have Phytophthora root rot, please contact Sean Westerveld at firstname.lastname@example.org.