PHYTOPHORA - an increasing menace

by Dr John Howells

From: Clematis International. 2002. p.53

This fungus phytophora has been with us a long time and has done countless damage to many plants worldwide. What is new today is that its attacks are becoming more widespread and it is moving more into our genus, the clematis. I have personal knowledge that it can be found in plants exported from the UK to the continent, and also on plants from the continent imported into the UK. It's effects can be confused with that of stem-rot (clematis wilt).

There are at least 80 species of phytophora; Phytophora cinnamomi is the one most common in the UK. The effects of phytophora are found world-wide. It affects a wide range of plants from oak trees to petunias. It was responsible for the 19th Century potato blight in Ireland which was so devastating. It is usually soil borne. It occurs naturally in most soils. It mainly affects roots. The roots are killed, the stems wilt and the plant dies. Woolf and Hall1 often found phytophora in the roots of clematis plants both in gardens and nurseries. Other organisms may also move in and be found on examination. It starts in the roots and spreads to the collar and the lower stems of the plant. Its activities are encouraged by poor drainage, water-logging and warm temperatures. It attacks all groups of clematis in contrast to stem rot (clematis wilt) that only attacks the Early Large Flowered group.

When a plant is affected, I am advised by Wim Snoeijer, that there can be noticeable black gashes on the leaves. The illustration on page ** shows such a phenomena in a Clematis alpina.

No chemical treatment is available to the gardener. Remove the plant and an area around it and destroy the plant. Disinfect the area with Jey's Fluid. Do not grow clematis in the same area for at least three years. Be careful with the watering of clematis. This is a paradox. Clematis love water but too much can encourage the fungus. Therefore strive for good drainage and avoid 'water-logging'. Nitrogen rich food tends to promote it.

Stem-rot (caused by phoma clematidina) in clematis is found exclusively in the Early Large Flowered clematis. However, 'wilting' can occur in other groups of clematis. When this occurs extreme care should be taken to establish the exact cause of the 'wilting'. Otherwise discredit will be given to phoma clematidina when the causal agent is likely to be phytophera or some other agent. The following table may help to differentiate between the two conditions.

Stem rot (Clematis Wilt)
The fungus phytophera
The fungus Phoma clematidina
Site of attack
The roots, usually, and sometimes leaves
Leaves, with lesions spreading to
a node of the stem.
Root rot.
Stem rot.
Groups affected
All groups of clematis.
Early Large Flowered group only.
Stems attacked
All stems die.
One stem usually.
Invariable to the whole plant.
Rarely. If stem cut below damage,
new shoots appear.
Death only by gardener neglect.
Avoid over-watering and water-logging.
Buy plants with several stems.
Loss of one stem is bearable.

1. Wolff, W.G., and Hall, A.M. 'The Biology and Control of Clematis Wilt'. The Clematis. 1997. P.47.

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