Root Rot of Lavender: Scouting is the Key to Prevention

The pests of lavender observed to date (four-lined plant bug, Septoria leaf spot, etc.) have caused mostly aesthetic damage to plants, affecting marketability but not killing the plant.  More aggressive diseases are known to occur in other lavender production areas like Europe and Oregon, but have not yet been reported from Ontario.

In late June an aggressive fungal root disease of lavender showed up on an Ontario lavender farm. The plants appeared to sprout as normal in early spring, but soon wilted, turned brown and died within a matter of weeks or even days sometime in June (Figure 1). The exact timing of disease progression is unknown, but after symptoms appeared on the first plant the disease quickly spread down the row in both directions, affecting three neighbouring plants.

Figure 1. A root disease of lavender. The plant at the bottom of the photo was killed first and the disease quickly spread down the row.

Three main pathogens have been associated with this root rot: Fusarium  Pythium and Phytophthora. Fusarium is a common soil fungus that can cause disease on a wide range of plants with symptoms ranging from root rots to vascular wilts. Pythium is also a common soil pathogen, usually causing damping-off of seedlings and less aggressive diseases on roots and crowns. Phytophthora is usually a very aggressive soil pathogen that can spread rapidly under wet conditions. A plant weakened by one pathogen can be easily colonized by other pathogens. More research is required to determine which pathogen(s) are primarily responsible, and to develop more detailed strategies for preventing the disease.

In this case, the disease did not spread beyond the initial four plants because the disease was observed early enough to remove affected material before it could spread throughout the field. However, it is important to note that had the grower not constantly been monitoring the field, the disease could have progressed to a point where it could not have been controlled.

There are no pest control products registered on lavender that could control any of the more serious diseases known to affect lavender.  It is therefore essential to watch for and stop this type of pest in its initial stages long before it becomes too widespread for cultural control measures to be effective, potentially leading to long-term loss of a field for cultivation of lavender. The following cultural practices can help in preventing these types of problems:

  1. If ordering new plants, always ensure that they were collected from healthy plant material. Keep newly purchased plants in an isolated place on your farm away from established plantings for a week or two and examine closely for signs of pest issues. Do not plant any lavender with symptoms of pest damage.
  2. Regularly scout lavender fields throughout the growing season, looking for any pest issues, but especially plants that completely die during the season. Overwintering issues can result in dead plants, but a healthy plant that dies mid-season is usually a sign of a root or crown disease.
  3. Once a plant with disease has been found, it should be dug up and sent to a reputable lab for diagnosis.
  4. Flag the area where the plant was removed and continue to monitor regularly for signs of spread to the neighbouring plants. If a neighbouring plant begins to develop symptoms, all affected plants should be removed, and at least one and preferably two healthy plants on either side of the diseased plants should also be removed. This may prevent the pathogen from moving from plant to plant down the row. Any grass or weeds next to the plant should also be removed, because it is unknown at this time whether these could also be hosts to the root pathogens.
  5. Clean and sanitize any tools used to remove the affected plants before using them elsewhere on the farm. Also, prevent spread of soil from the affected area to other areas of the farm by minimizing walking or driving machinery through the affected area. For farms open to the public, it is very important to block this area off from customers, since they may then travel to other fields on your farm and could carry spores on their shoes. The area could be covered in a mulch to reduce the chance of soil being picked up on boots or machinery.
  6. Do not re-plant lavender in the affected area or any other plants in the mint family (e.g. mint, thyme, oregano, basil, sage, numerous ornamentals). For farms open to the public, this may be unsightly. However, these protocols will hopefully prevent the spread of disease, which could cause numerous dead patches in the future. Use the opportunity to add a focal point to the middle of the field, such as a sculpture or un-related ornamental plant.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
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