Tomato Diseases And Pests: An Overview Of The Most Common Problems

Having problems cultivating tomatoes? You can find ‘first aid’ tips here for the most common tomato diseases and pests.

Apr 09, 2021 02:26 pm
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Blight on tomatoes

Brown rot damages the tomato plants and makes the fruit inedible

Various tomato diseases and pests can become a serious problem when cultivating tomatoes. You can find advice here about what to do if your homegrown fruit suddenly develops brown spots, the foliage dries out or bugs spread on the plants - including tips for damage limitation, prevention and combatting diseases.

The most common tomato diseases at a glance:
  • Leaf blight and brown rot
  • Didymella lycopersici and stem rot
  • Alternaria solani
  • Powdery mildew
What tomato diseases are there?

Leaf blight and brown rot

Leaf blight and brown rot on tomatoes
Leaf blight and brown rot can destroy entire tomato crops

Leaf blight and brown rot is by far the most common tomato disease. The cause is a fungus called Phyrophthora infestans, which is often transferred from infested potato plants to outdoor tomatoes. The rot spreads particularly quickly across the entire plant in damp weather. This results in gray-green to brown-black spots that spread wider and wider and cover the leaves, stems, and fruit. The infested tomato fruit develops deep, hard spots and is not longer suitable for consumption. You can prevent the rot by ensuring there is plenty of distance between the plants in a greenhouse or film tent. A covered place on a sunny balcony or patio is also suitable. Ensure that the plants are protected from the rain and the leaves cannot dry out quickly in the event of drought. If the tomatoes are growing in a mixed vegetable bed, it is essential to maintain plenty of distance from early potatoes when planting. Never water tomatoes over the foliage! There are now many varieties of tomato that are highly resistant against leaf blight and brown rot, for example, ‘Phantasia’, ‘Golden Currant’, ‘Philovita’ and ‘De Berao’.

Didymella lycopersici and stem rot

Another tomato fungus known as Didymella lycopersici causes fruit and stem rot. This is initially visible on the stem base of older tomato plants, where the bark becomes black and subsides close to the soil surface. This interrupts the water transportation in the stem. Some time later, the fruit begins to waste away from the stem base in concentric circles and the leaves turn yellow. The spores of the sac fungus spread on the wind and in humid weather via water spray, infecting other tomato plants. Chafing points from cord ties or other injuries are gateways for pathogens. So you should try to avoid injuring the tomato plants by using soft tying material and handling with care. If a tomato is infested with the fungus, it should be removed, and the plant rod as well as fixings should be disinfected with methylated spirits.

Alternaria solani

Alternaria solani
Alternaria solani on a potato shrub

A tomato disease that initially manifests on the tomato plant leaves in dry, very warm weather, is triggered by the Alternaria solani fungus. The infested leaves develop round, gray-brown spots. As the fungus is transferred to the tomato plants from the soil, Alternaria solani affects the lower leaves first, spreading later to the upper leaves. Ultimately, the diseased tomato leaves roll up and die completely. Brown, oblong-oval spots can also be found on the tomato stem. The fruit turns soft and mushy. As Alternaria solani is also frequently transferred from potatoes to tomatoes, the same precautionary measures apply here as in the event of leaf blight and brown rot. However, the fungus does not affect the entire plant but travels from leaf to leaf. Removing the diseased leaves early on can stop the spread. Caution: The tomato fungus remains attached to plant rods (particularly on wood) for a long time. You should therefore thoroughly disinfect the material after every season!

Powdery mildew

Powdery mildew appears as white spots on leaves and stems. The disease in tomatoes cannot be treated with a spray

Tomatoes are unfortunately not immune to powdery mildew either. The Oidium neolycopersici fungal spores cause the typical floury-white coating on tomato leaves and stems. The leaves whither and drop over time. Powdery mildew spreads in humid weather in particular and is extremely difficult to combat in hobby gardens. The fungus does not attack the tomato fruit itself, however the plants often die off completely in extreme cases of mildew. Remove the infected leaves immediately in order to curb the spread. Even fairly mildew resistant varieties are rare; ‘Philovita’ and ‘Phantasia’ are considered relatively resistant.

Which pests infest tomatoes?

In addition to the numerous fungal diseases that tomatoes can unfortunately succumb to, there are also animal attackers that seriously threaten tomato crops if infestations are heavy. In addition to classic garden pests such as aphids, and nematodes, there are a few that specialize in tomatoes.

Tomato leaf miner flies

Liriomyza bryoniae is the Latin name for the tunneler that chomps its way through the inside of tomato leaves. In English they are know as: Tomato leaf miner flies. The flies lay their eggs on the top and bottom of leaves. The actual pests are the larvae, as they dig clearly visible, winding tunnels through the tomato leaf tissue. With a overall development period of 32 days from egg to fly, infestations spread rapidly, particularly in greenhouses. Leaves should be immediately removed in order to prevent the spread of tomato leaf miners. Beneficial insects such as parasitic wasps help to combat them naturally.

Tomato leaf miner moths

A tunnel eaten by leaf miner moths
A tunnel clearly eaten by the larvae of the tomato leaf miner moth

Tomato leaf miner moths (Tuta absoluta) go about their business in a very similar way to tomato leaf miners. The inconspicuous, nocturnal, gray-brown moths with long, backwards arching feelers, are only about 0.28 inches long and spend their entire life on the tomato plant. The females lay around 250 eggs on leaves, in flowers, and on young fruit. The damage caused by life miners on tomato plants first appears in the upper areas on the young shoots and is easily recognized. Not even the fruit is safe from the leaf miner moth larvae. A secondary infection with fungus and bacteria is often the result of damaged fruit husks. Upon discovery, the tomato leaf miner moths should be combatted with pheromone traps. Beneficial insects such as pirate bugs and parasitic wasps can also be used.

Bright line brown eye

Bright line brown eye
A nocturnal pest: Bright line brown eye caterpillars love eating tomato plants

The name sounds funny, but the creatures are far from it: Bright line brown eye is an inconspicuous brown moth whose caterpillars have an insatiable appetite for tomatoes and bell peppers. The 1.57 inch long caterpillars can be recognized by their green-brown coloring with thin, yellow stripes on the sides and black warts.

Similarly to the grown moths, these pests are active at night and chomp their way through tomato leaves and fruit. Insect nets and closed greenhouses are a precautionary measure to protect against moths. In the event of a caterpillar infestation, you should collect the larvae as quickly as possible and relocate them to stinging nettles. Pheromone traps and natural, Neem-based protective measures help to combat bright line brown eye.

Tomato russet mite

Fruit intested with tomato russet mite
Corked tomato fruit infested with tomato russet mites appear rusty

The russet mite Aculops lycopersici is a significant tomato pest. Its life-cycle barely lasts a week, so it has a tremendous propagation rate. The mite often migrates from potatoes to tomatoes. It is difficult to combat an infestation, as tomato russet mites often only become visible on plants very late on. Leaves turning yellow and the main shoot becoming brown are indications of a russet mite infestation. The flower stems also become discolored, young fruit corks, bursts and drops and the entire plant dies. The only effective way to combat tomato russet mites is to dispose of the entire plant.

Physiological damage to tomatoes

Growth defects in tomatoes are not always cause by plant diseases or pests. It is often poor cultivation conditions, unfavorable weather or an unsuitable location that damages the plant. The following typical symptoms can be attributed to environmental influences and poor care.

Blossom end rot

Signs of a calcium deficit
Large, black spots on the underside of the tomatoes is a clear sign of a calcium deficit

Blossom end rot appears primarily on tomato fruit that is cultivated in a plant bed. Discs of brown-back rot that spread and harden form around the flower base. The newly formed leaf shoots are clearly under-sized and deformed.

Blossom end rot, however, is not a fungal infestation, but a calcium deficit. This results in particular from drought stress. If the plant is not watered sufficiently in severe droughts, the nutrient salts concentrate in the substrate and the tomato’s fine roots can no longer absorb sufficient amounts of the calcium required from the soil. Preventing blossom end rot is very simple: Ensure an even supply of water during hot summers in particular and do not allow the tomato plants to whither. In more extreme cases, improve the soil in the plant bed with calcium carbonate or algal lime.

Green or yellow shoulders

Yellow shoulders on tomato fruit
Pronounced yellow shoulders on tomato fruit

If the tomato fruit is not ripening properly and green or yellow rings remain around the stem base then the tomatoes may have become too hot. These appear on the outer fruit that are directly exposed to the rays of sunlight in particular. Excess nitrogen or a potassium deficit can also cause green shoulders. The fruit is edible, but not very appealing to look at. Help to prevent this by shading plants in extremely exposed locations during the hours of midday. and choose less sensitive, light fruit varieties such as ‘Vanessa’, ‘Picolino’, ‘Culina’ or ‘Dolce Vita’.

Burst fruit

Burst tomatoes
It is annoying when beautiful, red tomato fruit bursts shortly before harvesting

Almost every gardener has experienced this: Just before they are finally ripe, the fruit skin bursts in several places and with it the dream of the perfect tomato harvest. Burst tomatoes on otherwise healthy plants are not a disease but also the result of an uneven water supply. If the tomatoes are suddenly heavily watered after a dry period, they swell right up and then their skin bursts. The same thing applies here: Water tomatoes evenly. If you want to be on the safe side, you can choose a non-bursting variety, such as ‘Green Zebra’, ‘Corianne’ or ‘Picolino’.

Leaf roll

If the tomato leaves roll inwards like a spoon, this is a sign of over-fertilization. The phenomenon is also known as leaf curl. The trigger is usually excess nutrients or drought stress and can be easily resolved with even watering and a long-term, organic fertilizer.