Phytophthora Root Rot

Given the high rainfall experienced in many areas over the past two weeks, it is not a surprise that Phytophthora root rot has been found in at least one commercial field. What is surprising is that it is not Phytophthora nicotianae, which is the most common species and most devastating for lavender. Instead, Phytophthora cactorum was found after a row of lavender collapsed and died within a week after heavy rains and flooding.

Phytophthora cactorum has been reported on lavender in other areas before, but this is the first time I have encountered it on lavender in Ontario. It is a very common pathogen of ginseng in Ontario and is also found on numerous other crops and landscape plants including cucurbits, strawberries, and apples. The fact that it is commonly found in Ontario but has not been found in lavender before suggests that the extreme wet conditions may have allowed it to develop on roots when it normally would not.

The problem with encountering a new pathogen is there is no information on how much it will continue to spread and how best to manage the issue. Unlike P. nicotianae, which does not survive a typical winter in Ontario, P. cactorum definitely survives winter because it is a perennial issue in ginseng. The hope is that it is only pathogenic under extreme conditions and when the soils dry out it will not be able to spread much further.

There are no fungicides registered for control of Phytophthora root rot in lavender as a culinary herb. There are a few products available for lavender as a cut flower. The best way to manage any Phytophthora disease is sanitation and removal of affected plants. Here are some quick tips for identification and management of any Phytophthora disease:

Identification:

• Rapid wilting and death of plants following wet to very wet conditions, depending on the species of Phytophthora.
• Sometimes sections of plants collapse over time across a plant.
• Multiple plants are affected all at once followed by continued spread to neighbouring plants in wet conditions.
• Laboratory diagnosis is the only way to confirm the cause and identify the species of Phytophthora.

Management:

• Remove all dead and dying plants and any plants with symptoms including as much of the root system as possible.
• Ideally, remove at least one healthy plant on each side of the damaged plants because they may already be infected. With a new species of Phytophthora, it is unknown if this step is necessary, because it is unknown how quickly it could spread again in wet conditions or how long it will linger in the root zone in the absence of symptoms. If you have a patch and it stops spreading in dry conditions and then continues to spread in the next wet period, then it is a good idea to remove all of the plants in the area of the infection.
• Avoid re-planting lavender on the same site. For Phytophthora nicotianae, which is known to only survive very mild winters in Ontario, it is best to avoid replanting in the same spot until two winters have passed. For other species, we don’t know how well they will survive, but it is best to follow the same replant interval and monitor the new plants closely for any signs of disease development.
• For all phytophthora species, spores can be transferred to new areas by movement of water, splashing of soil during rains or overhead irrigation, or on boots, clothing, tools or machinery. Use cover crops/grass or mulch over infested areas to reduce splashing and soil transfer on machinery and boots, clean and sterilize tools after using them in the infested area, and do everything necessary to avoid surface waters running from the infested area towards healthy plants.
• Fungicides registered for lavender as a cut flower (under outdoor ornamentals) can be applied to control or suppress Phytophthora root rot. It will be difficult to get full control of a root rot pathogen. To find the available options visit the PMRA Label Search (https://pr-rp.hc-sc.gc.ca/ls-re/index-eng.php) and search for “outdoor ornamentals” or “outdoor cut flowers”, find products with Phytophthora on the label, and then confirm that the product is registered for all outdoor ornamentals and/or outdoor cut flowers without having a defined list of crops (if there is a defined list of crops it is only registered on those crops). If applying one of these products, the lavender cannot be used for culinary purposes.

Unknown Disease Symptoms

Unusual symptoms have developed both in my research plot in Simcoe and in grower fields. Some branches within most plants in the plot are turning yellow, while other branches on the same plant appear to be healthy (Figure 1). It has been difficult to determine a cause of these symptoms. There is some discolouration of the vascular system when the stems of affected branches are cut. This suggests it may be a wilt pathogen like Fusarium that grows within the xylem and blocks the transport of water and nutrients.

Figure 1. Yellowing on some branches within lavender plants.

It is possible that the wet conditions this year have led to more than usual root dieback and invasion of the vascular system by pathogens like Fusarium. When this coincides with full bloom, which is the time that the water demands of the plant are the highest, it may lead to more symptoms. It is possible that once plants are pruned, they will start to recover. If you have these symptoms in your field, it is probably best to fertilize properly, ensure the plants do not dry out, and check for any insect pests like garden fleahoppers that might put added strain on the plants. Insect controls may be necessary to avoid additional strain on the plants.