Pest & Disease Management - Phytophthora ramorum

Sudden oak death has caused extensive damage to a wide range of hosts on both sides of the Atlantic.

Phytophthora ramorum has proved particularly problematic on rhododendron - image: Forestry Commission
Phytophthora ramorum has proved particularly problematic on rhododendron - image: Forestry Commission

This fungal disease has caused extensive tree loss in California, USA, affecting mainly tanbark oak and some Quercus species, giving rise to the name "sudden oak death".

In the UK, damage has been found in nurseries, garden centres, woodlands and heathlands as well as in managed parks and gardens on a wide range of hosts. This includes arbutus, calluna, camellia, choisya, cornus, Garrya, Griselinia, hamamelis, ilex, kalmia, Laurus, Leucothoe, lonicera, magnolia, Michelia, Osmanthus, Parrotia, Photinia, pieris, rhododendron, Ribes, syringa, pot-grown Taxus, Umbellularia, Vaccinium and viburnum.

It has also been found in many European countries and caused dieback on rhododendrons in Germany and Holland in the 1990s, although scientists did not identify it until 2001.

Intensive research efforts funded by Defra, the Horticultural Development Company (HDC) and the EU have assessed the susceptibility of plant species, how the fungus spreads and how it can be controlled. Practical guides on managing notifiable Phytophthora diseases in nurseries, parks and gardens can be downloaded from the Food & Environment Research Agency (FERA) website at www.fera.defra.gov.uk.

Other notifiable Phytophthora spp. include Phytophthora kernoviae, first discovered in Cornwall in 2003, which has been found on plants including Fagus, liriodendron, magnolia, Quercus ilex, Q. robur, rhododendron and Vaccinium causing similar symptoms to Phytophthora ramorum, but on rhododendron, infections appear to be more serious.

Phytophthora lateralis has been found on Chamaecyparis lawsoniana with other potential hosts including Taxus brevifolia and Thuja occidentalis.

Symptoms

Shoots become darkly discoloured. The blackening spreads into the leaves through the leaf stalks, discolouring the leaf base and tip. Shoot cankers may lead to wilting.

In viburnum, the disease infects the stem base, which results in rapid wilting, and leaves, which blacken. In some plants, such as camellia, magnolia, laurel and lilac, the leaves are mainly affected and browning or blackening starts at the leaf tips and margins, and progresses towards the base. In rhododendron, infection seems to start at the leaf petiole end forming an arrow-shaped infected area.

Bark infection in trees takes the form of dark cankers low down on the trunk that seep dark sap and can be colonised by bark beetles. The tree dies when canker girdles the trunk. The cankers are believed to appear at least several weeks after infection.

Some tree species, such as beech, are susceptible to bark infection only, others to bark and foliar blight. On conifers, the main symptom is needle blight of young foliage.

Phytophthora lateralis infects the roots and stem base of host plants.

How to recognise it

The main symptoms on nursery stock are shoot and leaf dieback and sometimes bark cankers. Growers who suspect a disease outbreak must contact FERA on 01904 465625 or email planthealth.info@fera.gsi. gov.uk.

Where the notifiable disease is confirmed, infected plants and all susceptible plants within two metres of the infected plants will be destroyed under statutory action. Standing down beds are sterilised and, where plants are field-grown, soil is sterilised or removed.

All known susceptible plants grown within 10 metres of the infected plants cannot be moved for three months nor sprayed with fungicide against Phytophthora. This is because it could suppress symptoms of infection. These plants are subject to further inspections. Mature trees are pruned or felled and the infected wood destroyed.

Treatment: cultural control

- Keep secateurs and all other pruning equipment clean (using approved disinfectants) and sharp.

- Avoid mechanical and physical damage on susceptible species.

- Avoid pruning susceptible species in wet weather.

- Water plants in the morning to allow foliage and compost to dry during the day.

- Make sure that there is good air movement around crops.

Spores have been isolated from streams and ponds so irrigation water, if contaminated, could be a source of spread. Nurseries where the disease is present are advised to stop overhead irrigation. Growers are advised to monitor for signs of infection on plants known to be a host and to implement rigorous hygiene practices.

HDC project HNS 123 looked at the role of chemical treatments in disease control. Other work has found that products such as Hyperox, Virkon S, Jeyes Fluid and Menno Florades are capable of eliminating the pathogen (under certain conditions) on nursery standing beds where infected plants have been grown.

Research work in the USA on the effect of the composting process on Phytophthora ramorum was included in a research round-up in Waste & Resources Action Programme project STA0012. It was found that the pathogen could be eliminated at 55 degsC after 14 days. In the UK, this would be achieved using the PAS 100 protocol.

Biology

Phytophthora ramorum produces two types of asexual spore, sporangia and chlamydospores. Which spore is produced, and how many, depends on the plant host and environmental factors, such as light and temperature.

Research has isolated both types of spore from infected leaves but neither from bark cankers. Sporangia are spread by water splash and they germinate in moisture, releasing swimming zoospores that penetrate the plant through leaf and stem surfaces or wounds.

Chlamydospores are thick-walled and are the mechanism by which the fungus survives hostile conditions - they can survive in temperatures as low as -2 degsC.

Phytophthora kernoviae has not been found to produce chlamydospores but has formed long-lived sexual spores, or oospores, in the laboratory.

Fully updated by Dove Associates.

Use plant protection products safely. Always read the label and product information before use.

Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.


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