Leaves can show symptoms as follows:

• Leaves turn brown and then drop
• Brown tips or margins to the leaves
• Leaves turn uniformly yellow, or irregularly yellow
• Leaves remain green but fall
• Spots on leaves
• Loss of variegation
• Oedema
• Nutrient deficiencies
• Leaf deformation
• Secretions

Leaves turn brown and fall

Where entire leaves turn brown and then fall, particularly the lower leaves, this is often an indication of underwatering and excessive heat. Wilting and browning of leaves at the top of the plant are also a sign of underwatering. The upper leaves of shade-loving plants will often turn brown if they are placed in too much light. Whole plant collapse (where a plant suddenly turns brown all over and dies) may be caused by sudden changes in the environment, and in particular over-watering. Leaves can also be bruised by passing pets and people. As brown leaves are a common symptom with more than one cause accurate diagnosis often depends on what other signs of ill-health are also present.
Control: Tip the plant gently out of its pot and check the condition of the roots. If roots are brown, rotted and soft consider whether you need to increase or more likely reduce watering. Move plant to a cooler or shadier position as required.

Brown leaf tips or margins

Brown tips or margins to the leaves may indicate over or more likely under-watering, which can be assessed by checking the roots. They may also be a sign of too dry an atmosphere, or poor ventilation in a greenhouse or conservatory. Over-watering often causes browning in the middle of the leaves.
Control: Increase or decrease watering as necessary. If the atmosphere is too dry (i.e. low humidity) mist plants regularly (at least three times a day) or, better, place on a tray of damp gravel, expanded clay granules (Hydroleca) or recycled lightweight aggregate (Hortag). Add water, ensuring the level is always below the top of the gravel, clay granules or aggregate. As the water gradually evaporates it increases humidity around the plant. Also improve airflow. Choose plants that match the conditions.

Yellow leaves

It is normal for the odd lower leaf on a mature plant to yellow and fall. However, yellowing of leaves in large numbers may be caused by a wide range of factors; too much or too little water, overpotting (i.e. potting up into too large a container), too shady a position, too cold a temperature, a sudden drop in temperature or exposure to draughts. If lime-hating plants (such as Gardenia) have upper leaves turning yellow but staying firm they may be suffering from excess calcium, and thus reduced availability of iron or manganese or both in the compost (see nutrient deficiencies) - or the prolonged use of hard water instead of rain water.
Control: Check the plant roots to see if they appear too wet/dry (dark, soft, brittle roots indicate poor root health), and adjust watering accordingly. Repot into a smaller container if in too large a pot (i.e not filled by plant roots). Dry rootball with kitchen towel if it is soggy when repotting. Consider the environment and place the houseplant in a more suitable position – houseplants benefit from brighter positions, south facing windowsills for example, in winter when light levels are lower. Too much light or sunshine in the middle of the day can cause leaves to droop, or scorch yellow. Many houseplants dislike draughty positions – particularly in winter. Move to a better location when temperatures drop.

Spots on leaves

Dark spots with yellow margins on leaves may be due to fungal leaf spot, but white or straw-coloured spots or rings are likely to be due to cold (i.e. not room temperature) water splashes, aerosol damage or watering with cold water. Pale coloured spots may also develop if plants have too much sun or from aerosol damage on exposed surfaces.
Control: Treat fungal diseases with approved fungicides. Avoid using cold water (allow it to become room temperature before use e.g for orchids). Avoid using aerosols (leaf shine products or insecticide products). Move plants to a shadier position if in a bright window.

Loss of variegation

Variegation may vary during the year and is often less apparent in late summer. Where there is a permanent loss of variegation, (and reversion can be ruled out) inadequate light is often the cause. Waterlogging may also lead to loss of variegation or leaf colour. Reversion can occur where more vigorous green shoots outgrow the variegated shoots.
Control: Ensure variegated plants are in good light. Improve drainage if necessary. Prune out reverted growths whenever they occur on variegated plants.

Oedema

Raised corky patches of tissue on the undersides of leaves represents the late stages of oedema. This is caused by excessive water uptake (often linked to overwatering) and is worst in situations where there is a lack of airflow and a humid atmosphere.
Control: Seasonal factors may encourage oedema but where it occurs regularly, improve airflow around affected plants. Ensure regular, but not excessive watering. Do not remove affected leaves.

Nutrient deficiencies

Lack of colour in the tips of leaves, with poor growth, may be due to a shortage of nutrients in general, particularly nitrogen. Interveinal yellowing in ericaceous plants (such as Gardenia, azalea) may also be due to lime-induced chlorosis caused by iron, magnesium or manganese being unavailable in non-ericaceous potting compost or induced by prolonged watering with hard tap water. Rain water, water from a dehumidifier or air-condition unit are ideal. However any form of root stress such as under or over watering can induce nutrient deficiencies. Over-feeding with potassium-rich fertiliser can induce magnesium deficiency.
Control: Repot using an ericaceous potting medium (if appropriate), removing as much old potting media as feasible. Water with rain water (allow to warm to room temperature particularly in winter). Use a foliar feed (liquid fertiliser diluted and sprayed on leaves) to correct the problem. Consider whether other causes are responsible - drought, over-watering, pot-bound plant etc.

Leaf deformation

Where leaves are crinkled or creased it is usually due to stop/start development. This may be caused by variation in temperatures particularly from night to day, but is usually due to irregular watering when new leaves are developing. If kept in bright light in a dry atmosphere, leaves of certain plants (Maranta for example) curl inwards (concave). Cold and dry conditions may cause leaf margins to curl outwards (convex). Saintpaulia and Gloxinia are examples.
Control: Ensure plants are kept away from draughts or fluctuating temperatures. Water regularly - perhaps checking one day a week and watering little and often in summer, maybe every fortnight in winter. The aim should be to scrupulously avoid soggy potting media and bone-dry conditions, and aim for moist potting media. Consider whether the plant is in the ideal position and move if necessary.

Secretions

When houseplants are in active growth and warm daytime temperatures are followed by cold night temperatures, stress ruptures of cells may occur, releasing sugary exudates - this is known as guttation. Some houseplants, such as Plumbago auriculata, produce a natural whitish deposit on the underside of the leaves. Some plants produce sticky deposits on the leaves in response to cultural stress. Common examples include Cymbidium, Ctenanthe, Philodendron, Tibouchina and Zantedeschia. Such symptoms indicate poor growing conditions.
Control: Improve cultural conditions; avoid excess watering. Consider repotting the plant in spring. Regular feeding and watering should help avoid these conditions.

Various pests and diseases can also cause discolouration of leaves.

Common pests include aphids, glasshouse red spider mite, leafhopper, mealybug, scale insects and whitefly.