Fighting tomato diseases – late blight, late blight, gray mold & Co

For the sake of clarity, the following explanations focus on the most common tomato diseases in the home garden.

Late blight (Phytophthora infestans)

Late blight hits particularly hard during cool, rainy summers. The causative algal fungus chose to exist on land rather than in water ages ago. Nevertheless, moisture plays an essential role in his way of life. If low temperatures are added, the tomato disease spreads with breathtaking speed in a tomato culture. As a rule, the fungal spores have overwintered on potato plants and can be carried to the tomato plants by the wind .

damage picture

Grey-green spots appear on the leaves, which quickly turn brown and black. At the same time, a gray-white coating forms on the underside of the leaves, so that the affected parts of the plant are caught in a life-threatening pincer. If the tomato plant is already bearing fruit, they are not spared. The infection manifests itself here in the form of black, sunken spots that reach deep into the pulp.

control and prevention

• Do not grow tomatoes and potatoes next to each other
• Keep plants dry, preferably under a canopy
• Do not let the irrigation water splash, e.g. B. through a mulch layer
• Spray prophylactically with copper salt preparation or broth made from rhubarb leaves

Fruit and stem rot (Didymella lycopersici)

The sac fungus thrives particularly luxuriantly in warm, humid weather. It uses the slightest injury to a tomato plant as an entry point for its spread. The pathogen survives the winter preferably on older infested foci, which can also be found on potatoes, peppers or aubergines. The spores of this tomato disease reach their actual destination, the tomato plants, by means of wind, water, irrigation systems, unclean tools or work clothes.

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The infection is first visible on the bark tissue, just above the ground. You can see black, sunken spots that will also catch your eye later on on the tomatoes. The stems soften while a pink tissue develops on them. At this point at the latest, the leaves begin to turn yellow and wither. A heavy infestation eventually causes mummification of the fruit.

control and prevention

• Do not use seeds of infected fruit as seed
• Make sure you rotate crops regularly
• Do not pour overhead with rippling water
• Only work with disinfected tools
• Keep a sufficient planting distance
• Fertilize regularly with comfrey manure to strengthen
• Dust the leaves and stems with algae lime

Dürrfleckenkrankheit (Alternaria solani)

We are talking about another fungal infection that is closely related to the parallel cultivation of tomatoes and potatoes. From August at the latest, the heat-loving pathogens spread in the garden via the known paths. They are often already in the tomato plant because infected seeds were used. As soon as the temperatures are permanently above 25 degrees Celsius, the spores become active.

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In the lower part of the plant, concentric gray-brown to brown spots are revealed on the leaves. Unlike other deformities, here the leaf veins seem to act as borders. From a pronounced infestation stage, the leaves curl up and fall off. Now take a look at the stems, you will see elongated, black spots. The end of the song is a diseased tomato plant in its entirety with soft, rotten fruit.

control and prevention

• No neighborhood of tomatoes and potatoes in the bed
• Prefer growing under a rain shelter
• Do not irrigate tomato plants
• Clean and disinfect support poles regularly
• Only use seeds from healthy fruits
• Remove the leaves up to a height of 40 cm
• Dust plant parts with rock flour
• Strengthen with horsetail tea, comfrey or onion peel brew

True powdery mildew (Oidium cycopersicum)

As if the powdery mildew fungi didn’t find enough victims in the garden, they seem to have discovered tomato plants for themselves in recent years. Hobby gardeners, as well as commercial tomato farmers, are increasingly reporting problems with the pathogen. If summer temperatures of 16 to 22 degrees Celsius and humidity of 70-80 percent come together, the fungal spores multiply explosively.

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A white, mealy coating develops from the initially white spots on the leaves. The entire leaf surface is quickly occupied with it. The foliage withers and falls off. Although the tomatoes themselves are not affected, the harvest is at risk because the fruits are no longer properly cared for when infested with this tomato disease.

control and prevention

• Cut off affected plant parts immediately
• Favor resistant varieties such as ‘Phantasia F1’ or ‘Philovita’
• Use natural liverwort extract to strengthen
• Plant garlic, marigolds, chives and thyme in mixed cultures
• Do not administer nitrogen-rich fertilizer
• Spray repeatedly with a milk-water solution

Grauschimmel (Botrytis cinerea)

The prey pattern of this mold species spans an impressive 235 host plants, including tomatoes. The mushroom owes its name to the greyish fruiting body, which can be perceived by the human eye at an early stage. The warmer and wetter the weather, the more severe the infestation. Even tomato plants in the greenhouse are not safe from the pathogen.

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Gray-green spots appear on leaves, stalks and stems in summer. They quickly turn into a grayish lawn of fungus that, over time, covers the entire plant. The parts of the plant above the diseased stems no longer receive water and nutrients, so they die. The gray mold begins to produce dust and thus carries a high allergenic risk for the gardener. The fruits are not unaffected by this disaster. Often they first assume a glassy appearance before softening and rotting.

control and prevention

• Look for an airy, dry location
• Ventilate greenhouses and polytunnels regularly
• Do not leave any green clippings on the bed throughout the winter
• Never water or sprinkle the foliage
• Remove diseased leaves and shoots and dispose of with household waste
• Give the roots moss extract, horsetail broth and nettle stock right from the start
• Repeatedly powder the leaves and shoots with bedrock powder
• Only carry out trimming and other care work with disinfected scissors

Bakterienringfäule (Clavibacter michiganensis ssp. michiganensis)

Since this bacterial disease primarily targets potatoes, it is also relevant for tomato plants, since both species belong to the nightshade family. The bacterium is characterized by an immense stubbornness, because it is able to survive the winter in the bed on plant material.

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Small corked dots 2-4 mm in diameter form on the tomatoes. In the early stages you can see a dark eye in the center of these spots. After some time, the deformities cork and sink into the fruit in the form of a crater. If you cut open such a tomato, there are often small brown cavities in it.

control and prevention

• Only grow tomatoes in the same bed every 4 years
• Only use healthy seeds for sowing
• Soak your own seeds in 0.6% vinegar for 24 hours
• Keep the bed clean of plant matter throughout the winter
• Use as late as possible to avoid wounds
• Remove infected fruit immediately and dispose of with household waste

spoon leafiness

No illness in the narrower sense and no pest infestation have overtaken a tomato plant when gardeners speak of spoon-leaves. Rather, it is deficiencies or omissions in care that are expressed in this way.

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Starting at the base, the leaves curl up like a spoon. Yield losses are usually not to be feared.

control and prevention

• A balanced supply of nutrients
• Do not intermittently apply organic or liquid fertilizer
• Don’t over-fertilize the soil
• Only use shoots moderately
• Avoid drought stress

blossom end rot

The scientists puzzled for a long time what actually triggers the blossom end rot. What is certain so far is that it is a metabolic disease in connection with deficiency symptoms, especially of calcium. In a saline soil, the nutrient calcium is not available to a tomato plant in the required amount. The fruits, which are naturally less supplied with calcium anyway, finally receive a dose that is far too low and react with the following damage picture.

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The catastrophe is indicated in the form of too small, young leaves that appear dark green and deformed at the same time. The older foliage is beset by necrotic spots, mostly in an unappetizing black-green. The main damage manifests itself on the tomatoes themselves. Watery spots appear at the bases of the blossoms, which gradually enlarge and turn black. Thus, the affected fruit turns black in color from the tip and hardens.

control and prevention

• Prefer organic, humic fertilizer to a mineral preparation
• Supply the soil repeatedly with carbonate of lime
• Make sure the soil is evenly moist

Green Collar/Yellow Collar

Tomatoes caught by the green collar don’t look particularly inviting; however, they can be consumed without hesitation. This does not prevent experienced hobby gardeners from getting to the bottom of the cause in order to harvest deep red, tempting fruits.

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Tomatoes do not take on the desired red color as they ripen. A green or yellow collar remains at the base of the stem, as if the fruit were not ripe. This assumption is reinforced by the hard flesh in this area. It is highly probable that the lighting conditions and the temperature have a part in this damage. Tomatoes in the outer, full-sun regions of the plants show the greatest deficits.

control and prevention

• Cultivation under a tomato house is ideal
• Provide shade at least during the midday hours
• Avoid intense exhaustion
• Fertilize with potassium and magnesium, less with nitrogen
• Favor light fruit varieties in cultivation, such as ‘Culina’ or ‘Vitella’
• Do not settle tomatoes in the summer heat build-up on a southern wall

Magnesiummangel

When looking more closely at important tomato diseases, we often encounter deficiency symptoms as the cause of various problems. In this context, the magnesium deficiency in particular plays a significant role if you practice cultivation on light, sandy soil.

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In the lower and middle regions of the plant, the leaves lighten while the leaf veins remain green. If you don’t diagnose and fix the magnesium deficiency now, the foliage will turn brown and fall off. Although the symptoms are initially harmless, they ultimately endanger the tomato harvest because the supply of the fruit is no longer guaranteed.

control and prevention

• Avoid growing in dry, sandy, stressed soil
• Do without or reduce fertilization with potassium and nitrogen
• Fertilize specifically with magnesium-rich preparations
• Raise an acidic pH below 5 with leaf compost and a little Epsom salt

Conclusion
Your tomatoes can be afflicted by a whole range of diseases, such as late blight, gray mold and the like. Of course, this is no reason to throw in the towel and head straight for the unique treat of freshly harvested tomatoes from your own garden dispense. Keep your eyes peeled and familiarize yourself with the symptoms of the major tomato diseases. The earlier you diagnose an ailment, the more effective it is to combat and heal. You can usually save yourself the use of chemicals. In the meantime, hobby gardeners have accumulated an immense wealth of experience on how to combat tomato diseases in a way that is not harmful to health and the environment.