A gardeners guide to why your tomatoes are dying

These are among the most common problems

Tomato blight on fruits (Alamy/PA)

At this time of year, gardeners will often experience problems with their tomatoes, whether it’s leaves turning brown or fruits becoming distorted and damaged.

August and September often present tomato problems, says Dr Anton Rosenfeld, research manager at horticultural charity Garden Organic.

“Certainly for outdoor tomatoes you are going to get more of a chance of blight, and plants tend to run out of steam a bit in the later summer months.”

These are among the most common problems:

Blight

This is among the most common diseases if you are growing tomatoes outdoors and is encouraged by wet, warm and still conditions, says Rosenfeld.

“You see brown, penny-sized lesions at the leaf margins, which are quite substantial. If you just see one or two bits on the leaves you can try pulling them off, but once it gets to about 10% of the leaves, you are fighting a losing battle.”

If the blight – a fungus which also affects potato plants and is carried on the wind – spreads to the stems, turning them black and spreading quickly, it’s likely to be too late, as it can ruin a plant in just a few days, he adds. Affected tomatoes will turn brown and will be inedible, and you will need to dispose of them.

You can try cutting off trusses of affected tomatoes and their blackened stems to stop the onslaught, but it’s often too late to save the plant.

“It is weather dependent, so if it turns really sunny and there’s only a small amount of infection, it may dry out and not spread, but if you’ve humid conditions it will spread,” he warns.

If you’ve got a plant that is infected, harvest the tomatoes before it affects them and keep an eye on them, he advises. They will ripen but they are more likely to rot more quickly.

“It’s not so much of a problem when they are growing in a glasshouse because it’s the rain getting on the leaves and spreading the spores which causes the issue.”

Creating better airflow for your plants may help to prevent it, as a mass of tangled leaves just encourages humidity, giving the spores a better chance of germinating when they land on the plant. So, remove some of the lower leaves and surplus side shoots to allow the plant better ventilation and promote better quality of fruits, he suggests.

Blossom end rot

The symptom of brown sunken circles at the base of fruits, which makes them inedible, occurs due to lack of calcium reaching the end of the fruits because of lack of water, he explains. Sporadic watering, or just insufficient watering, leads to blossom end rot.

If you don’t water enough, you may also find that the fruits split, he adds. “When you’ve let a pot dry out and then added a lot of water, the fruits grow so quickly that they start to split.”

To prevent blossom end rot and splitting, water your tomato plants regularly, he advises. You won’t heal the fruits that have split, but if you water regularly, emerging fruits won’t suffer the same demise.

Red spider mite

These little mites create pinpricks on the plant that can cause the leaves to die off. In severe cases you may see cobweb-like debris on the plants.

“It tends to be more of a problem in glasshouses, when conditions are really dry inside,” says Rosenfeld, “and is often much worse in cucumbers and aubergines if you have them in your greenhouse. Then it will simply spread to the tomatoes.”

Fortunately, spider mite doesn’t affect the fruits much, it just makes the rest of the plant look sickly.

To prevent it, increase the humidity in the glasshouse by misting them with water. Alternatively you can use a biological control, a predatory mite that predates on the red spider mite but you need to use that before it becomes a real problem, he advises. The biological control can be ordered in the post and then hung in the glasshouse where it releases the predatory mites.

“Again, this a problem that gets worse through August and September.”

Nutrient deficiency

“Tomato plants sometimes get magnesium deficiency, which results in lots of brown spots on the leaves. You can actually feed the tomato plants or spray them with a solution of Epsom salts, which seems to perk them up,” he says.

If you don’t feed tomato plants enough, they will run out of nitrogen and potassium. Feed them with comfrey or nettle feed to improve the nutrient content. Feed the plants once a week, he advises.

Ghost spot

Ghost spots

If your tomatoes have developed faint green or yellow spots or rings, the plant could have fallen victim to ghost spot, also known as the grey mould fungus Botrytis cinerea, whose spores are spread by rain or water splash and in the air.

However, this isn’t as serious as some of the other diseases because you can still eat the spotted fruits and the fungus may not spread to all the fruits on the truss.

If you are growing tomatoes indoors, reduce the potential spread by clearing plant debris that may hold the spores and increase ventilation.