Chlorosis is simply another term for yellowing. You are most likely to come across the term in descriptions of nutrient deficiencies or plant viruses (another term used commonly in such descriptions is necrosis, meaning cell or tissue death). However, there are many more possible causes of chlorosis than just viruses or nutrient problems.

The type of yellowing seen (e.g. the position of the affected leaves on the plant, and/or the pattern and position of the yellowing on the leaf itself) will often vary according to the cause. The chlorosis is frequently accompanied by other symptoms giving further clues as to the cause, e.g. wilting, leaf fall, dieback, stunting. The most common causes of leaf yellowing are described below:

Nutritional problems

Yellowing of the areas between the veins (interveinal chlorosis) is usually indicative of manganese, iron or magnesium deficiency. Iron deficiency affects the youngest leaves first, whereas the symptoms of manganese and magnesium deficiency tend to start in the older leaves.

Nitrogen deficiency causes a more general yellowing or reddening, initially usually of the older leaves, often accompanied by a lack of vigour. With potassium deficiency the yellowing is often more pronounced at the leaf edges.

With many of these nutrient problems the yellow tissue can turn brown if the deficiency becomes more pronounced.

Viruses

Chlorosis is a very common symptom of virus infection in plants. The symptoms are very variable and depend on factors such as the virus or viruses involved, the host plant, and even weather conditions. Whilst a few viruses will cause a uniform yellowing of the leaves, it is more common to see patterns of chlorosis, such as mosaics, mottles, streaks, ringspots, line patterns, etc.

Here are some common viruses that can affect garden plants:

Camellia yellow mottle virus
Canna viruses
Cucumber mosaic virus
Daffodil viruses
Impatiens necrotic spot virus / Tomato spotted wilt virus
Pelargonium viruses
Raspberry viruses
Sweet pea viruses
Tomato viruses
Tulip viruses

Cultural and weather-related problems

Leaf yellowing can occur under waterlogged conditions or when the soil is too compact – yews and Narcissus are particularly prone to this type of injury. If the waterlogging persists then other symptoms such as leaf loss and dieback may occur. Consider installing drainage or relieving soil compaction.

Drought conditions may also lead to chlorosis, followed by browning and leaf loss. Such symptoms are also common in newly-planted material affected by poor establishment.

Cold-induced chlorosis is common in spring on young, actively-growing leaves. Affected leaves may remain in this condition for the rest of the season. Examples of plants that may display this type of symptom are magnolia, pieris and skimmia.

Other pests and diseases

Many diseases other than viruses can cause leaves to turn yellow. Some leaf spot diseases (e.g. rose blackspot) cause the leaves to produce ethylene, a gas that leads to rapid yellowing and leaf fall. Rusts, powdery mildews, downy mildews, etc. can all cause yellowing of all or part of the leaf. In many of these cases it should be possible to confirm the cause due to the presence of fungal growth (mycelium, fruiting bodies, spores, etc.) on or close to the areas of yellowing.

Diseases attacking the root system (e.g. honey fungus, Phytophthora root rot) or the water-conducting tissues (e.g. Verticillium wilt) can also cause leaf yellowing. Other symptoms will also be present, such as root decay or stained vascular tissues, and the plant will usually decline, exhibiting loss of vigour, wilting, dieback and in many cases eventual death.

Pests that attack the foliage or roots of plants will also often cause leaf yellowing. Examples include sap-sucking pests such as aphids, red spider mites and whiteflies, and root feeders such as vine weevil and cabbage root fly. Once again it is often possible to find the culprit on the leaves or amongst the roots.

Symptoms will vary according to the weedkiller involved. The yellowing may affect the areas between the veins, the veins themselves, or be more generally distributed over the leaf. There are often other associated symptoms such as distortion, browning, dieback, etc.

No-one is concerned when the leaves of deciduous trees and shrubs fall in autumn, with most people taking delight in the autumn colours. Yellow, of course, is one of the many shades the leaves may turn before falling.

Evergreen plants also shed their old leaves regularly, and again these leaves often turn yellow before falling. The loss of young leaves would be a cause for concern. The leaves are usually lost gradually throughout the year, but some evergreens will occasionally shed a large number of old leaves at one time, and this can cause concern. A common example of this is the yellowing and shedding of many older leaves over a relatively short period in summer by holly hedges. This is a natural process, and further investigation would only be merited if the plants started to display other symptoms such as wilting, dieback or loss of young leaves.