Tomatoes are particularly prone to what are known as physiological disorders: abnormal growth caused by non-infectious factors. This is partly due to the difficulty of controlling the sensitive requirements of tomatoes for temperature, nutrients and light levels. Outdoors, the plant is equally vulnerable to lack of warmth and variable temperatures.
Tomatoes need warmth and will not thrive at temperatures of below 12°C (54°F). Although it is easy in summer to keep temperatures raised in glasshouses, outdoor tomatoes may still require fleece or other additional protection.
In early summer, the nights can be cold and the days very warm. This fluctuation of temperatures is the main cause of what can sometimes be a very alarming degree of leaf curling. The plant is unable to cope with the accumulation of carbohydrates that occurs if nights are too cold for plant physiological functions to occur normally. Fortunately this does not seem to be a serious cause of loss of crop and usually disappears of its own accord as the nights begin to get warmer in late summer.
Small greenhouses and polythene tunnels used by home growers are much more liable to fluctuating conditions than larger greenhouses.
Mottling and discolouration of older leaves usually indicates a deficiency of nutrients, especially magnesium, and is a less serious problem providing the plant is otherwise vigorous.
Tomatoes are particularly sensitive to hormone weedkillers. Even the vapour emitted from loosely-capped bottles, or bags of lawn ‘feed and weed’, is sufficient to cause damage, especially under hot conditions. If this problem is suspected, remove all sources from the vicinity and if hormone weedkillers were the cause, the plants will produce new, normal growth. If symptoms persist the problem is probably caused by virus. These two possible causes of distorted foliage are otherwise very difficult to distinguish without specialised tests for viruses.
Weedkiller in manure