Phytophthora - A Head's Up

Healthy Viburnum x bodnantense 'Dawn' foliage - at least for now

It's not often that you have lunch with someone whose books you're using pretty much every day, but that's what happened to me today at the National Trust. Unfortunately I didn't know at the time (no-one told me - hrrrumph!), otherwise I'd have asked Tony Lord (no, not the one from Hawaii 5-0 he's Jack Lord, this one) lots of probing questions about the RHS Plant Finder and his Encyclopedia of Plant Combinations, both of which are very well thumbed here at VP Gardens.

Putting that to one side, the main conversation over lunch was the potential threat of Phytophthora to National Trust gardens. You may know it by its other name, Sudden Oak Death - this refers to the first known appearance of the disease in Tan Oaks in California in the 1990s, not our native oak species. This fungal-like disease first appeared in the UK in 2002, probably from plant material imported from the States. The Department for Environment, Food & Rural Affairs (Defra) have worked extremely hard since then to try and eradicate the disease by a nursery inspection process and tough measures to combat any known outbreak. However, it now looks like they're changing their stance from disease eradication to control. They've also conducted research into the disease over the past 5 years and are now poised to start a 3 month public consultation process to be launched tomorrow (July 15th) at RHS Wisley.

The main plants affected so far have been Viburnum, Rhododendron, Magnolias and Camellias in gardens. More worryingly, it has also been found in our native Bilberry (Vaccinum) and the true heather, Calluna vulgaris. Outbreaks have been mainly confined to Cornwall, Devon and South Wales, but if the Trust's experience is anything to go by, it's slowly spreading - helped by the mild wet summers we've had recently. Outbreaks have often been associated with the nearby presence of the controversial, invasive garden escapee Rhododendron ponticum, which appears to act as a host for the disease.

There are 2 forms: P. ramorum and P. kernoviae. Symptoms of affected plants include blighted leaves, wilt and die-back of branches and 'weeping canker' (an oozing, black substance on the trunk). Once infection has been spotted and confirmed, the treatment regime is harsh and includes:

Nurseries and retail premises

1. Destruction by burning or deep burial - infected plants plus susceptible plants within a 2 metre radius of infected plants and associated plant debris.
2. Disinfection of surfaces and pots.
3. Prohibition on movement of susceptible plants within a 10 metre radius of infected plants and remaining plants in the infected lot for at least 3 months.
4. Prohibition on use of Phytophthora fungicides during the holding period (as these can mask the presence of the disease).
5. Advise the cessation of overhead irrigation (as the disease can be water borne).
6. Trace-back and trace-forward of related plant material (so source and subsequent material can be traced and destroyed as necessary).

Parks, gardens and uncultivated land

1. Prohibition on movement of the infected plant and parts of the plant (e.g. must not be used for propagation purposes or foliage purposes).
2. Destruction by burning or deep burial - infected plants, susceptible plants within an appropriate cordon sanitaire and associated plant debris.
3. No planting of susceptible hosts within 4 metres for 3 years (as the disease can lie dormant for this amount of time).
4. Prevention of regrowth (as this often has the disease).
5. For infected trees, felling or pruning will be required depending on the part of the tree infected and the extent of infection.
6. Measures must be taken to prevent re-infection at the site (e.g. prohibition on planting susceptible plants in contaminated soil, removal or sterilisation of contaminated soil).

The Trust have already prepared their response to Defra for release tomorrow - I've been reading their Press Release today, ahead of the game (how cool is that?). They're calling for:

1. Research funding to be maintained, so a better understanding of the disease and its control may be found
2. Provision of additional funding to eradicate Rhododendron ponticum, so that the creep of both forms of Phytophthora are minimised (with control of this invasive species an added benefit)
3. Establishing a Heritage and Botanic Garden Action Group - to help co-ordinate effort across the gardening sector in tackling these diseases. This would also be the first time a body has been created dedicated to the care of botanic and historic collections in the UK

That's fine, but where does that leave you and me, the ordinary gardener? Firstly we need to be vigilant - ensuring that any demise of our own susceptible plants is quickly noted and dealt with. The Defra website has some good photographs of diseased plants for you to check, an information leaflet and a list of all the susceptible plant species found in the UK thus far. We also need to be more careful about how we buy our plants - steer clear of any nursery or garden centre etc. that doesn't source their plants from suppliers using the Plant Passport Scheme. Once purchased, place your plants in a quarantine area for 4 weeks before planting out. That's plenty of time for your plants to show any symptoms if they have the disease.

The Defra Plant Health website has lots more information for you to check out on Phytophthora and other serious pests and diseases - it does tend to make rather dry reading (though the Phytophthora leaflet is in plainer English), but forewarned is forearmed eh?