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Reading the Tomato Leaves: How to Nip Diseases in the Bud

How to Spot Diseased Tomato Leaves

Leaf Spots

Flea Beetles

These are small dark-colored jumping insects found happily munching on your tomato leaves early in the season, while they’re still young and tender. They look rather more like a beetle than a flea, beautifully colored, often shiny or metallic.

The flea reference comes from the enlarged hind legs of the insect, which gives it its jumping abilities. Adults feed on the leaves, stems and flowers of many common garden plants, including cabbage, beans, peppers and tomatoes.

flea beetle

Source: Wikipedia

The damage can be seen by the small BB sized holes chewed by the insect, which can coalesce into large holes if you have a heavy infestation.

5 Minutes to Better Tomatoes

FREE science-based report shows how to increase your tomato yield this season.

Septoria Leaf Spot

Also called Septoria leaf blotch or canker, this disease is caused by the fungus Septoria lycopersici, and if left untreated can result in the death of the plant. It attacks plants of the Nightshade family, to which tomatoes belong, and is very contagious. It thrives in moist, warm conditions where there is little airflow.

Septoria Leaf Spot of tomato

Source: www.extension.umn.edu

The fungus lives in the soil over winter, and can also live on dirty garden tools. It makes its way onto the leaves generally by getting splashed with infected ground water. The spots are round, with dark brown edges and small black fruiting structures in the middle of them. The fungus spreads upwards, from the oldest leaves to the youngest, and they turn yellow, then brown, and then they die. Fruit is largely unaffected.​

Early Blight

Caused by the fungus Alternaria solani, early blight is a very common disease of tomatoes, and may also affect potatoes. Like most fungus, it overwinters in the soil, and grows quickly in moist and wet conditions. It is commonly mistaken for Septoria leaf spot as it infects the plant at the same time, but it has different symptoms.

Early blight

Source: www.extension.umn.edu

The brown spots on the plant’s leaves will have obvious concentric circles inside them, and are often surrounded by yellowing leaf tissue.

Early blight in tomato

This fungus also attacks the fruit and stems. The black spots on the fruit start at the stem end and spread from there. This disease is also common at the end of summer, when the air and soil gain some moisture.

Late Blight

Yet another fungus out to get your tomatoes is Phytophthora infestans. This one can again also infect potatoes, and was responsible for the Irish potato famine in 1845. It hides over winter in infected tubers and weeds of the Nightshade family, and spores spread when the infected plants grow in the new season.

late blight in tomato

Source: www.longislandhort.cornell.edu

It affects all parts of the plant, the leaves developing bluish gray spots, which turn brown and eventually fall. The stems show dark lesions, often surrounded by white fungal growth, and the fruit has brown lesions which spread all over it.

Treat Fungus Organically

As you can see, fungus can be a big problem for tomatoes and there are many treatments for it. Using a standard fungicide that's copper-based isn't really an organic method. However, the treatment in the video below is effective and 100% organic.

Bacterial Speck

Bacterial speck is caused by Pseudomonas syringae pv. tomato. This infection causes 2 mm black spots on leaves often surrounded by a yellow halo. Both mature and immature fruit has small black spots on them, about 1-2 mm across. The disease occurs in cool and wet weather, from about 64 – 75 F.

5 Minutes to Better Tomatoes

Free report shows how to increase your yield

Bacterial Spot

Bacterial spot is caused by Xanthomonas campesiris pv, vesicatoria. It causes slightly smaller (1 mm) spots on leaves, but larger and scaly looking spots on the fruit. If the lesion is small and not yet become scaly, it is easy to mistake for bacterial speck. Both spot and speck lesions only start on immature fruit.

Source: Flickr

Verticillium Wilt

This is a soil borne bacterial infection, and under the correct conditions will cause yellow blotches on the tomato’s leaves, then the appearance of brown veins culminating in dead spots of a chocolate brown. These are commonly confused with early blight, but the spots are not definite and have no concentric rings inside them.

The leaves wilt, die, and drop off the stem, leaving only the top few green. Fruit growth is retarded, and may suffer sunburn due to the lack of leaves. To properly diagnose this problem involves slicing the main stem vertically just above the ground, and see the brown discoloration in the conducting tissue just underneath the bark.

Distorted Leaves

Aphids

Aphids are the bane of many plants, and the tomato is no different. They are small, usually greenish insects, active during spring and summer. They are small, about 2-3 mm long, and you will find them in clusters all over the leaves where they feed on the leaf tissue. A light infestation will usually cause no visible damage, but large numbers can cause curling, stunting and wilting of leaves.

aphids in tomato leaves

Source: www.extension.umn.edu

Virus

There are many viruses that affect tomatoes, such as cucumber mosaic virus, tomato mosaic virus, tomato spotted wilt virus and many others. Leaves are generally mottled, with yellow and light green patches, and misshapen – long and thin, or curled and deformed. The fruit will have yellow lesions or brown rings, and plants will be stunted with a short internode length.

Leaf Roll

Leaf roll is a plant response to drought stress. The plant curls its leaves up and inward in an attempt to minimize water loss through its leaves. The leaves themselves will become thick and leathery, and once rolled will not unroll even if water is restored to the plant. This condition does not seem to affect growth or yield.

Herbicide Damage

The plants leaves appear bleached or brown, and may be twisted, cupped, curled or distorted. The veins themselves appear to be thicker and closer together than normal. The side of the plant closest to the misapplied herbicide is more severely affected, and in the worst cases the whole plant may show symptoms.

herbicide damage

Source: www.extension.umn.edu

Drooping Leaves

Stalk Borers

Stalk borers are a smallish caterpillar which infects your crop from June to early August. You will find a small hole in the stem near ground level which is the entrance point of the insect, and it can also sometimes be found in the stem of the plant. The caterpillars are ¾ - 2 inches long, with a red-brown head and purple-brown body with pale stripes.

stalk borer

Source: www.extension.umn.edu

Fusarium Wilt

Source: www.extension.umn.edu

This problem generally only occurs in heirloom varieties of tomato, the newer breeds are resistant. The leaves will turn yellow and then wilt, often on one side only.

Fusarium wilt

The lower leaves wilt first, and it will eventually spread to the whole plant. If you cut the stem near the soil line, the veins will be brown and the center is green.

White Mold

This mold, Sclerotinia sclerotiorum, causes a dark, firm, waterlogged lesion on the stem, which will then die and turn white. In the middle of the dead stems are black”sclerotia” that are the size and shape of mouse droppings. Cool and moist conditions may produce white mycelia both in and outside of infected stems, and fruits are soft and rotted.

white mold

Source: www.extension.umn.edu

Cold Injury

Exactly what it sounds like – exposure to low temperatures. Below 50 F, leaves will show immediate injury although fruit may remain normal looking for up to a week. Foliage will look water soaked and soft, and then turn black. Fruit does the same – becomes waterlogged and then rots.

Discolored Leaves

Sunscald
sunscald

Source: www.extension.umn.edu

Verticillium Wilt

This is a soil borne bacterial infection, and under the correct conditions will cause yellow blotches on the tomato’s leaves, then the appearance of brown veins culminating in dead spots of a chocolate brown. These are commonly confused with early blight, but the spots are not definite and have no concentric rings inside them.

The leaves wilt, die, and drop off the stem, leaving only the top few green. Fruit growth is retarded, and may suffer sunburn due to the lack of leaves. To properly diagnose this problem involves slicing the main stem vertically just above the ground, and see the brown discoloration in the conducting tissue just underneath the bark.

Virus

There are many viruses that affect tomatoes, such as cucumber mosaic virus, tomato mosaic virus, tomato spotted wilt virus and many others. Leaves are generally mottled, with yellow and light green patches, and misshapen – long and thin, or curled and deformed. The fruit will have yellow lesions or brown rings, and plants will be stunted with a short internode length.

Cold Injury

Exactly what it sounds like – exposure to low temperatures. Below 50 F, leaves will show immediate injury although fruit may remain normal looking for up to a week. Foliage will look water soaked and soft, and then turn black. Fruit does the same – becomes waterlogged and then rots.

Chewed Leaves/Holes in Leaves

Flea Beetles

These are small dark colored jumping insects found happily munching on your tomato leaves early in the season, while they’re still young and tender. They look rather more like a beetle than a flea, beautifully colored, often shiny or metallic.

chewed leaves

Source: www.extension.umn.edu

The flea reference comes from the enlarged hind legs of the insect, which gives it its jumping abilities. Adults feed on the leaves, stems and flowers of many common garden plants, including cabbage, beans, peppers and tomatoes. The damage can be seen by the small BB sized holes chewed by the insect, which can coalesce into large holes if you have a heavy infestation.

Slugs

Active during spring and summer, slugs can chew holes in your fruit and completely defoliate plants in a very short time. They are soft bodied creatures, usually brown or gray, about ¼ to 2 inches long. They generally feed at night, so may not be physically seen, but their effect on your crop can be disastrous.

slug

Source: Flickr

Tomato Hornworms

Another caterpillar pest that can wreak havoc in your garden. They’re a bright green caterpillar with a little spiky tail and white markings that will chew holes in your fruit and leaves, and can completely defoliate plants very quickly.

tomato hornworm

Source: Flickr

Cutworms

Yet another hungry beast, the cutworm is a greyish or brownish color, up to 2 inches in length, which is usually found hiding in the soil in spring and summer. This is the seasons they do most damage, chewing stems close to the soil line, and also leaves and fruit.

tomato cutworm

Source: Flickr

About the author

Sabrina Wilson

Sabrina Wilson is a staff writer for the Organic Daily Post.

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