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Diagnosing leaf problems in woody plants

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Pests and diseases may damage leaves, but environmental causes are far more common.

Most trees and shrubs will have a few leaves that suffer damage or disfigurement during the growing season. Following severe winter weather or prolonged summer drought, however, there may be more extensive leaf damage on these plants.

Foliage can be damaged in a number of ways but identifying the causes from leaf symptoms is not always easy. Take into consideration:

  • When and where a plant is growing - new plants take time to establish.
  • Soil and root conditions - always examine the condition of the roots to check that they are healthy and have grown out into the soil (established)
  • Site conditions over the past 12 months - especially if the ground has been waterlogged or very dry.

In some cases, such as when leaves are uniformly yellow, there may be a range of factors to consider and all available information should be assessed. Possible explanations may vary from too much or too little water, too sunny a position or simply the shredding of older lower leaves. Bear in mind that even evergreens discard older leaves, which usually function for three to five years then deteriorate and drop.

Possible causes

Establishment problems

Leaves turning brown and desiccated on young trees and shrubs in the first summer after planting, often remaining firmly attached, indicates that plants have not established correctly. This may be due to the poor quality and condition of the plants purchased, poor planting technique or lack of aftercare.

How to avoid

Always buy healthy-looking, good quality plants with a good root system. Make sure they are suited to the soil and situation in your garden. Always prepare the site thoroughly and ensure roots are spread out when planting. Avoid planting too deeply - or too shallowly. Plant in autumn so the roots can establish in cool conditions before the following summer. For the first two years water well once a week in dry spells and keep weed free.

Drought

Browning of leaf tips or margins, especially on young growth, may suggest drought and a failure to establish. Brown leaf tips are common when dry weather follows spring planting before new roots have established properly.

How to avoid

Water as necessary and maintain a 5-7.5cm (2-3in) mulch around shrubs and young trees, particularly on lighter, drier soils.

Cold

Frost can damage young shoots in spring distorting growth and causing leaves to turn brown. Plants will recover but re-growth may be slow. Where leaves turn dark purple, common in camellias and clematis, it is usually due to low night-time temperatures. The older leaves of Photinia often show vivid dark red spots and discoloration following cold winters while Garrya leaves often have large areas of black spotting. Cold and drought conditions may cause leaf margins to roll up or the leaves to droop and this often occurs in hardy evergreen shrubs, particularly Rhododendron. Leaves that are white and chlorotic have usually experienced low temperatures and are especially common on tender plants placed outdoors too early in spring.

How to avoid

Use fleece or other material to protect against frost. Give careful thought to the planting position and, where necessary, erect temporary windbreaks using stout posts and windbreak fabric. In exposed gardens consider planting a shelter belt of tough trees and shrubs.

Scorch

Scorching of foliage caused by bright sun is usually obvious, especially as it is worse on the sunny side. It often occurs where water has collected on the leaves. Cold winds in spring or warm, dry winds in spring and summer can cause browning and desiccation of young foliage on deciduous trees and shrubs and may even damage evergreens. Japanese maples (acers) are particularly susceptible. Cold winds in winter, especially if the soil is frozen, may even cause scorch on hardy conifers.

How to avoid

Erect windbreaks and site susceptible plants in sheltered situations. Ensure plants are mulched and never dry out.

Waterlogging

The blackening of leaves, usually starting along the vein, is due to waterlogging and is particularly common on heavy soils after wet winters. It is frequently seen on Aucuba. The roots are a bluish-black and fall apart when teased out. There is often a sour smell to both the soil and roots. Waterlogging can also result in reduced variegation or leaf colour.

Evergreens, such as hollies, may lose all their leaves following periods of environmental stress, such as waterlogging, but often re-sprout if conditions improve and root damage has not been too severe.

How to avoid

Either, improve drainage, plant in ridges or mounds or choose plants that thrive in wetter soils

Physical damage

Abrasion or other superficial tissue damage (often on Clematis and other wall-trained plants) or the shredding of softer leaves is usually cause by wind buffeting or leaves blowing against a hard surface, such as a fence or wall. Hail damage, represented by small white spotting or flecking, may occur occasionally, mainly to the upper surfaces of leaves.

How to avoid

Tie in young growth regularly and provide shelter in more exposed situations.

Oedema

Raised corky patches of tissue on the underside of leaves represents the late stages of oedema. This is caused by excessive water uptake and is worst in situations where there is a lack of airflow and humid atmosphere.

How to avoid

Improve airflow around affected plants by pruning and reduce watering.

Loss of variegation

Variegation may vary during the year and is often less apparent in late summer. Where there is a permanent loss of colour inadequate light is often the cause. Waterlogging can also lead to loss of leaf colour. Reversion may also occur where more vigorous green shoots outgrow variegated shoots.

How to avoid

Ensure variegated plants are in good light and improve drainage where necessary. Prune out reverted growth.

Leaf deformation

Where leaves are crinkled or creased it may be due to stop/go development, often cause by a cold spell in spring or early summer. Where herbicides have been used consider the possibility of spray drift.

How to avoid

Use fleece to protect young growth and ensure regular watering in dry spells during early growth.

Nutrient deficiencies

Lack of leaf colour with poor growth may simply be due to a shortage of nutrients in general, particularly nitrogen. This is more common on light, sandy soils than on clay soils. Interveinal yellowing is usually caused by iron, magnesium or manganese deficiencies. Yellowing of foliage will also occur where hard water is used on ericaceous plants, such as rhododendrons and camellias. Yellowing of foliage may, however, be a secondary symptom and there may be other causes for the deterioration, such as drought, pot-bound roots or a general lack of root function due to poor establishment.

How to avoid

For a general lack of vigour apply a balanced fertiliser in spring. For specific mineral deficiencies apply sequestered iron, Epsom salts or manganese sulphate. Where yellowing is due to excess calcium in an alkaline soil it is better to choose lime-loving plants.

Chemicals

Brown, dead areas on foliage can be caused by using pesticide sprays at the wrong strengths or in hot, sunny conditions; also by misjudged application of some weedkillers. Path weedkillers are residual and heavy rain can wash them into adjoining beds. Spray drift may cause distorted growth, particularly on roses.

How to avoid

Read the manufacturer's instructions.

Pests and diseases

Various pests and diseases can also cause browning and other damage to leaves and these possibilities cannot be ruled out. The following is a list of the most common pests and diseases:

 

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