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Pests and diseases

06 Oct 2005

This page: Phormium mealybug - Leaf feeders - Diseases - Rippled leaves

 

Phormium mealybug

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Mealybugs are normally associated with houseplants or plants in greenhouses but the Phormium mealybug, Balanococcus diminutus, will survive our winter conditions outside. The bugs are insects which look something like small pale woodlice and they feed by sucking juices from their host plant.

 Phormium mealybug
Phormium mealybug

They also produce a white, waxy, fibrous substance (meal) from which they get their name. The bugs live within the folded base of the leaves so are not normally noticed unless the infestation is severe or the leaf is pulled back to reveal the sticky white 'meal'.

The effect on the plant is not particularly severe unless it is heavily infested but infected plants are generally slower growing and less healthy, and the leaves may become discoloured.

Mealybug damage
Phormium 'Tricolour' - damaged by mealybug

Infected plants often show poor root growth although it is rare to actually find the bugs below soil level.

There is no simple chemical control for mealy-bug but Malathion is sometimes recommended. The main problem is that the bugs are hidden in the leaf bases so it is necessary to pull back each leaf before the chemical can reach the pest.

The systemic insecticide, imidacloprid, (sold under the trade name of 'Provado') is also a chemical control method that may be effective.

It is possible to get uninfected material from an infected plant by taking small divisions, removing most of the outer leaves right down to the base and thoroughly washing off any signs of mealy-bug before re-potting in fresh compost.

Mealy-bugs do not appear to spread from plant to plant unless they are in direct contact so if an isolated plant is free of the pest it usually remains so.

 

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Leaf feeders

 

Healthy Phormium leaves are rarely attacked by slugs or snails or by leaf-feeding insects. Occasionally the soft young leaves are damaged (possibly by slugs) as they emerge from between the older leaves and this can result in a characteristic pair of holes that move up the leaf as it develops.

'Pink Strip' leaf showing two holes
Two hole effect

Sometimes, voles or mice will feed on the leaves. Generally we find that this only happens in the early spring when there is perhaps a shortage of other food.

Vole damage
Damage caused by voles

In New Zealand there are two species of Flax moth whose larvae feed on Phormium leaves but as far as we know, they are not found in this country.

 

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Diseases

 

In the book 'New Zealand Trees and Shrubs' by L.J. Metcalf, there are descriptions of two diseases of phormiums: Phormium leaf spot; a fungus disease which causes brownish or greyish, purple-edged spots on the leaves, and Phormium yellow-leaf virus which causes abnormal yellowing of leaves and ultimately, death of the plant. Recommended treatment in each case is removal of infected leaves or burning of the whole plant. We are not aware of either of these diseases being found in Britain.

There is a booklet by Sue Scheele; Insect Pests and Diseases of Harakeke, in which most of the pests of Phormium tenax are described and illustrated.

 

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Rippled leaves

  One effect commonly seen on some cultivars is where the leaf appears to grow in a rippled manner as shown in the examples below.

   
Rippled leaf tips

This effect is most commonly seen in the twisted-leaf cultivars such as 'Surfer' and 'Jack Spratt'.  It also occurs on 'Rubra', another narrow leaved type. These types have narrow, tough, and deeply keeled leaves and it appears that under certain conditions the new leaf forming in the centre of the fan finds it difficult to move through the folded base of the previously formed leaf. The result is that the elongating leaf gets stuck for a while and growth causes a fold to appear. As pressure builds up, the new leaf releases but then sticks again until growth pressure causes a further release. The result is a series of ridges that produce the characteristic growth pattern.

Normally the movement of the new leaf appears to be lubricated by a wax like material so perhaps in these varieties there is less wax produced, resulting in a greater chance of the new leaf getting stuck.

The rippled growth pattern can occasionally be seen in other varieties but it is usually just a small section of leaf that shows a slight ripple.

 

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