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Unfortunately, there are quite a lot of problems and diseases which can attack tomatoes.
Curling leaves caused by low night temperatures ideally the temperature should not fall below 15C, this is aesthetic only.
Yellowing leaves can be a magnesium deficiency feed with proprietary tomato food. If it persists despite feeding, it can be too little or too much water.
Problems with split fruit - literally the fruit splits caused by irregular watering. Given our variable weather regular watering is far from easy.
Greenback where the top of the fruit does not ripen fully caused by lower temperatures or lack of regular feeding.
Blight is the most serious and common problem and is often prevalent in a wet summer by which the plants develop brown patches and keel over the killer disease; you can spray them with proprietary sprays available from the garden centre but be quick off the mark, it can be too late by the time you spot the blight. If you don't want to spray, remove all infected leaves and dispose of not in the compost bin. Tomato plants grown in the greenhouse are much less susceptible to blight. If you want to grow tomatoes in an area prone to blight, ( the wetter parts of the country) it may be better to grow in a greenhouse or lean to. Blight is show in the image below left.
Halo Blight, which is small spots on the fruit surrounded by lighter rings and is a water-borne fungal disease. To minimise the risk avoid splashing the fruit when you water.
Blossom end rot: unsightly black patches on the underside of the tomato fruit. Caused by irregular watering, and more common in container-grown tomatoes than outdoors.
Botrytis, grey mould which can also be a problem. To avoid this ensure as much ventilation as possible. Do not crowd the plants and open all vents and doors in the greenhouse.
Many types tomatoes need support and this is usually done by staking. The image below right shows how the wrong type of support can cause problems. Here the stem has folded over the support and is close to snapping.
What causes a lack of Tomatoes and a small crop?
The flowers on tomatoes are very important because they become the fruit, i.e the tomatoes, so the more flowers the more tomatoes. This is show in the image where the spent flowers have become baby green tomatoes with some yellow flowers at the end of the branch which will also become fruits.
Poor pollination can cause a lack of tomatoes. The flowers need to be pollinated to produce the tomatoes, and this is more of a problem for tomatoes grown indoors in greenhouses and poly tunnels. Outside, nature does this by pollinating insects, such as bees. It is important to open the greenhouse doors, windows and vents to let the bees in to do their work. Commercial growers with large poly tunnels and glass houses will import bees to pollinate. I have seen commercial growers in Iceland growing tomatoes in the depths of winter, utilising the thermal underground heat, and importing boxes of bees to fly around and pollinate.
A lack of fruit can also occur during poor a summer if it is persistently wet and cold and the pollinators are not able to fly around as much due to the adverse weather conditions.
Top Tips for Growing Tomatoes
Regular watering is really crucial: do not let the plants dry out.For best results feed regularly as soon as the first flowers appear.Keep an eye on the plants, thin out leaves and take off side shoots.Don't give up on them: Tomatoes will crop in a greenhouse until November after which cut the vines and leave somewhere warm to ripen.To avoid diseases, if growing under glass make sure there is good ventilation and air circulation, and avoid splashing the leaves when watering.