Advice
RHS Help & Advice
Tomato problems
The RHS Advisory Service receives more questions concerning tomatoes than any other edible crop, and although common and easy to grow, they can suffer from a range of easily preventable problems
Temperature-related disorders
Tomatoes need warmth and will not thrive at temperatures of below 12°C. Although it is easy in summer to keep temperatures raised in glasshouses, outdoor tomatoes may still require fleece or other additional protection in June.
Often tomatoes curl their leaves up in response to cool night temperatures. This is perfectly natural and is, in itself, not a problem. However it can be an indication that night temperatures are falling too low, leading to disorders such as ‘catfacing’.
Catfacing (right), where fruits develop corky
scars and holes, is also due to fluctuating temperatures during flower set and tends to occur early in the season.
Water-related problems
Irregular watering can also cause fruits to split, especially at the end of the season when there are fewer fruits on the plant. At peak growth each plant can require about 0.8 litres of water per day, so those in growing-bags and pots need frequent watering. Do not remove the leader of cordon types; also, stop sideshooting after August to divert water away from the fruits and reduce the potential for splitting.
Lack of water reduces growth and fruit formation. In extreme cases insufficient calcium-carrying water flows through the plant to the extremities of developing fruit, leading to blossom end rot (right). Over-application of high-potassium fertiliser can also encourage this.
Fruit problems
Disorders of fruit are common; greenback, where hard, green areas develop on the shoulder of ripening fruit, is usually caused by heat injury and insufficient potassium. Good ventilation, shading, use of tomato fertilisers and choosing greenback-resistant cultivars will help avoid this. Variations include blotchy ripening and whitewall, where the skin of ripened fruit develops irregular yellow patches, with hard, white patches inside. Similar remedies and avoiding overuse of nitrogen fertiliser will help prevent these.
Fluctuating temperatures cause uneven growth - russeted or cracked fruits can result. Careful ventilation and artificial heat can avoid these problems indoors. Ripening depends on sufficient light and warmth: slow ripening at the end of summer might be helped by reducing shading. Gradually deleafing indoor tomatoes also encourages sunlight and air to the fruits.
Leaf problems
Twisting and distortion of the young foliage is often caused through contamination by hormone-based weedkillers, such as lawn herbicides. Avoid by keeping a separate watering can for herbicide application, not spraying on windy days, and not composting the first grass clippings after lawn herbicide application (if using your own compost).
Potato blight is the main fungal disease of tomatoes causing brown, rapidly spreading lesions on leaves and stems, and patches of rot on the fruit. Spores are dispersed by wind and infection occurs in wet conditions. Crops under glass usually avoid the problem, but outdoor tomato plants should be sprayed with mancozeb (Dithane 945) or copper (Murphy Traditional Copper or Vitax Bordeaux Mixture) before symptoms occur. Spraying will not prevent infection, but may slow it and save the crop.
Plants with streaked, mottled or distorted foliage should be removed as these may indicate viral infection. Mottling of older leaves is often due to nutritional factors and is not a cause for concern.
Magnesium deficiency (right) causes yellowing between the leaf veins, often more noticeable on the lower leaves. It is most likely to occur in tomatoes grown on sandy or acid soils, or where high-potassium feeds have been over-applied. In severe cases premature defoliation can occur, but mild cases are unlikely to affect yield or quality. If deemed necessary, control with two foliar sprays of Epsom salts (54g per 4.5 litres water) at fortnightly intervals.
Flowering problems
Lack of flowers can be due to young plants being etiolated (thin and weak) by spacing them too closely when young, or giving seedlings insufficient light. This can be partly remedied by deep planting. Young plants should also be mildly stressed by keeping them in small 9cm pots until they form flowers. Transplant when the first sign of yellow appears in the blooms.
Bush tomatoes (determinate) have flowers at the ends of their shoots, while vine tomatoes (indeterminate) produce flowers from the stems and sideshoots from their leaf axis. The sideshoots of indeterminate plants should be removed, but removing sideshoots from determinate plants will result in no crop.
Failure of flowers to pollinate, or ‘dry set’, can be due to a lack of humidity in a glasshouse, causing pollen to desiccate before it has a chance to fertilise the flower, or excessively high temperatures (above 38ºC). Outdoors, low temperatures (below 10ºC) may result in poor pollination. Damping-down of indoor tomatoes should produce enough air movement to fertilise the flowers and reduce high temperatures. Male and female parts are within the same flower so pollen can be transferred by simply shaking the plants gently or manually using a small brush.
Leaving glasshouse doors open in warm weather will allow pollinating insects to enter, and the movement of plants by air currents should be enough to ensure fertilisation.
Guy Barter