Phytophthora kernoviae: Symptoms and diagnosis

Leaf and stem necrosis and dieback in rhododendron and other shrubs. Bleeding cankers in trees.

Typical symptoms: leaf necrosis

Infected shoots (buds and young stems) and leaves of plants become necrotic. Entire tufts of foliage often wilt at the end of the shoots and then desiccate. Leaf necrosis occurs in many infected species. In rhododendron, infected leaves often show blackening down the mid-vein and stalk (IMAGE: CIMG0861 severe dieback of rhododendron caused by P. kernoviae).

Shoot tip dieback and leaf wilt of Rhododendron ponticum infected with P. kernoviae,
Cornwall, UK, 9 Aug 2005

Beech and oak: bleeding canker

Bark invasion by P. kernoviae In European beech (Fagus sylvatica) kills extensive areas of inner bark, often extending 10 m or more up the stem. This necrotic zone oozes a black, sticky fluid hence the phrase bleeding canker (IMAGE CIMG0907: Trunk of beech tree with bleeding cankers caused by Phytophthora kernoviae). Two English oak trees (Quercus robur) have also picked up infections from neighbouring host rhododendron bushes. These trees have also developed bleeding cankers.

Our monitoring of infected trees suggests that the bleeding cankers are not contagious, and the trees may recover from infection.

Bleeding canker caused by P. kernoviae on beech,
May 2006

Bilberry (Vaccinium myrtillus)

Studies by Forest Research found P. kernoviae infecting plants of bilberry (Vaccinium myrtillus) in oak woodland and also open heathland in Cornwall. Bilberry is a native plant to Britain. It grows in lowland and upland heathlands, but can also be found in some woodlands, grasslands and peat bogs.

Symptoms include stem blackening and necrotic leaves, although the affected leaves are quickly shed.

Magnolia: leaf spot

A wide range of different magnolia species and hybrids have been infected by P. kernoviae, picking up the infection from affected rhododendrons in the same garden. Our research shows that P. kernoviae can persist on infected tissues over winter and then re-infect the new flowers and leaves that are formed the following spring. The most common symptom in Magnolia is leaf spot; some species also show bud blast and blossom blight (IMAGE: P kernoviae infection of magnolia foliage).

Magnolia tree foliage infected with P. kernoviae,
Cornwall, UK, 22 Oct 2004

Diagnosis

The general symptoms of P. kernoviae infection make it difficult to diagnose without laboratory testing. If you find a rhododendron or tree exhibiting these symptoms, please contact our Tree Health and Disease Diagnostic Advisory Service. We may recommend you send us leaf or tissue samples so we can screen for P. kernoviae, P. ramorum and other possible pathogens.

More information

• For more information about symptoms download the Forestry Commission-Fera Fact Sheet

If you suspect a plant has P. kernoviae or a similar infection, please report your sighting using the Forestry Commission’s Tree Alert form.