There are have been multiple reports of lavender suddenly dying over the past few weeks. Symptoms match those of Phytophthora root rot, with portions of plants wilting and dying with other portions still appearing healthy. However, samples of these plants have not shown any presence of Phytophthora when submitted to the University of Guelph Pest Diagnostic Clinic. I have similar issues occurring in my cultivar verification trial at the Simcoe Research Station (Figure 1). Similar issues also occurred last year in the plot.
Figure 1. Sections of an angustifolia ‘Melissa Lilac’ plant dying in the cultivar verification plot at the Simcoe Research Station in 2020.
All the samples of these plants so far have been diagnosed as Fusarium root rot. Fusarium is a very common soil fungus present in virtually all soils. Some strains can cause disease while others feed on dead and dying plant matter in the soil. It is often not possible to distinguish between the two types even with genetic tests, so it is unknown if the Fusarium caused the problem or it just invaded the dying tissues. Most of the time Fusarium is more of a secondary problem, taking advantage of an already weakened plant.
Fusarium can also cause a wilt disease after invading the vascular system of the plant. This could cause infected branches to wilt and eventually die. There have been reports of Fusarium wilts affecting lavender in other regions, but they usually cause the vascular system to turn brown within the stems. There is no evidence yet that this is occurring. If you are curious if this might be going on in your fields. Cut off a dying and a healthy branch as low down as possible and slice it open lengthwise. If you see discolouration in the dying branch alone, please send me some photos.
This appears to be a new phenomenon in lavender. Previously, these symptoms could be directly attributed to Phytophthora and/or excessively wet soils. While many areas have seen significant rains recently, much of the damage being reported pre-dates the recent rains. The winter and early spring were much drier than normal. I have had lavender research plots consistently for the past 14 years and have only noticed this problem in the last two.
I can only speculate as to the cause of the problem, but it would have to be something unique to the past two years. The only thing that comes to mind is that both years were preceded by mild winters followed by cold and highly variable springs. The last two Mays have contained especially cold periods with snow and repeated frosts. Mild winters can reduce the hardiness of plants and cause them to break dormancy earlier in the spring. This may make them especially vulnerable to the cold later in the spring. The damage may have been subtle at the time of the damage but could have caused branches or plants to collapse under the high moisture demands during bloom. What supports this theory is that many of cultivars affected have been shown to be less hardy in Ontario.
It should be kept in mind that there is always the possibility the damage is caused by Phytophthora (Phytophthora nicotianae) in your field. It is best to confirm the cause is not Phytophthora by sending a sample into a diagnostic lab before making any decisions on how to manage the issue. Phytophthora can spread rapidly under these wet conditions resulting in the loss of entire fields. Managing Phytophthora takes much more extreme measures than other potential causes of the symptoms.
Since the cause is not entirely known, there is not much you can do about the problem for plants showing symptoms. If the symptoms are showing up in your field, here are some things to consider to avoid spreading the disease in case the problem is caused by a fungus or Phytophthora:
- Sanitize pruning/harvesting tools regularly, even after pruning asymptomatic plants. This can be done with rubbing alcohol (70% alcohol) or a household disinfectant. Clean the tools of any obvious debris before sanitizing. Wipe off excess disinfectant before using the tool. A 10% bleach solution (1 part household bleach to 9 parts water) is highly affective as well but will damage the tools and is difficult to work with in the field. Having two sets of tools with one soaking in disinfectant while the other one is in use will prevent down time.
- Prune/harvest symptomatic plants last to reduce the chances of spreading the disease to healthy plants.
- Avoid moving soil from affected areas to unaffected areas. Rinse and sanitize digging tools and equipment between locations. Avoid walking in the area of affected plants when soil or foliage is wet.
- If removing plants due to the issue, avoid planting lavender back in the same spot for at least a year or until the cause of the issue is known.
- Improve drainage and create drainage channels that prevent surface water from moving from affected areas into unaffected areas.
- Avoid collecting cuttings from plants exhibiting symptoms.
Angustifolia cultivars are mostly past peak this week while lavandins are near peak. This the best time for harvesting for essential oil extraction. Oil quality will only slowly degrade after this time, while oil yield will remain steady as long as buds remain on the plants. Some cultivars are prone to shedding their buds after bloom (e.g. Melissa Lilac) and this will significantly decease oil yield. Excess and prolonged rainfall can also result in the development of mould on the buds and this will reduce oil quality. Most agri-tourism operations wait until after bloom to harvest for oil, and in most years oil yield and quality are still good.
This is a good time to fertilize lavender with nitrogen. The plants will be focusing on vegetative growth for the remainder of the summer, and nitrogen is a key nutrient for leaf growth due to its presence in chlorophyll. With the recent heavy rains, nitrogen already applied earlier in the year may have leached out of the root system.
Noticing red inch worms crawling all over the flowers? These are quite common and appear to feed on nectar and/or pollen within the flower. They do not appear to damage the flower itself.
Japanese beetles are also commonly seen on flower heads. If you watch them closely, they appear to chew through the sides of flowers to get at the nectar within. They generally do not cause enough damage to warrant a control measure.