Tomatoes are fantastic fruits to grow. There is a huge range of varieties available to the gardener, and home grown, fresh picked tomatoes cannot be beaten on flavour. But tomato leaves have a tendency to develop blotches, mottles, and discolorations, giving rise to much anxiety. Generally, problems sort themselves out, and a good crop develops, however dreadful the leaves look in early summer. The gardener worries that these symptoms are indicative of a major virus attack, or lack of food - but in most cases the causes are environmental. Here are some of the classic causes of leaf marking and discoloration.
Tomato plants hate being cold. Early in the season the leaves of young plants in the greenhouse may turn purple, and the plants may grow slowly. Even though the days may be warm, the nights can be very chilly. Don't sow too early if you can't provide heat and light. Take Terry Marshall's advice in 'no fuss tomatoes'
When tomatoes are planted outside at the end of May-early June, warm days can be followed by cold nights, and this temperature fluctuation will cause leaves to roll up and turn purple. The condition is only short term and not a big problem, but should be avoided if possible as it can check growth. Keep an eye on the weather forecast. If cool nights are expected, cover plants (even those in greenhouses) with horticultural fleece. Remove during the day.
Tomatoes are herbicide sensitive. Even if you have never used a weedkiller, spray form other gardens, or pavement spraying, can drift surprisingly far, even into greenhouses. Distorted, twisted leaves and stems are classic signs of herbicide damage. Fruits will also be distorted. Once confirmed, there is no remedy.
Tomatoes hate over-feeding, yet they are also greedy plants. What a challenge! Don't be tempted to feed young seedlings that are looking sad (see above section on temperature).
Plants in ground
To provide adequate nutrients for the season, grow in well-fed soil. If your plot is sandy and free-draining, you'll need to add plenty of organic material - garden compst or well-rotted manure - to improve the growing conditions. Interveinal yellowing, together with dryish brown patches on the leaves, is often a sign of lack of adequate nutrients. Try a foliar feed such as SM3 seaweed extract.
Plants in pots and bags
Wait until the first truss has set before feeding and always keep to the recommended dose. Over feeding is more frequently a cause of problems than underfeeding. Too much potash too soon, for example invariably locks-up nutrients such as magnesium.
Tomatoes get viruses and diseases. If you've been growing tomatoes, or plants in the same family (peppers, aubergines), on the same site or in the same greenhouse soil year after year, soil can become infected. Symptoms on leaves and stems vary considerably but include mottling, rolling, distortion, and discoloration. Always use a four-year rotation pattern. If necessary, put plastic down to cover greenhouse soil, and grow tomatoes in pots or bags for a few years. Soil-borne diseases can last a long time, so prevention is the better option.
Old leaves near the base of a well grown tomato plant will tend to turn yellow and brown. Don't panic. As long as the rest of the plant looks fine, and fruits are growing well, there is no need to worry. There are plenty of other leaves to feed the plant and these old ones can be removed.
Oscillating temperatures and excess static starch result in inward-curling leaves. Typically, June mornings are cold, with temperatures quickly soaring at midday - these widely fluctuating temperatures can cause leaves to curl inwards.
Long days and short nights can leave plants struggling to process the sugars made during the day.