Control late blight and late blight in tomatoes

Late blight is a highly contagious tomato disease that can spread rapidly. We give tips on how to prevent and combat fungal disease on tomatoes.

There are numerous tomato diseases, but late blight is probably one of the most serious. By Phytophthora infestans, disease caused by an egg fungus (oomycete), occurs mostly in tomatoes (Solanum lycopersicum) and potatoes (Solanum tuberosum), but also other nightshade plants (Solanaceae), such as Physalis (Physalis sp.) and peppers (Capsicum annum) on. The rot usually appears shortly before harvest or afterwards in storage and causes massive harvest losses. The genus name Phytophthora comes from the Greek and means "plant destroyer", the species name infestant indicates the high infectiousness of the disease. In 1845, this disease of potatoes caused the Great Famine in Ireland. The best way to combat the disease is prevention, because once it breaks out, it is almost impossible to get rid of it. It is just as important to recognize an infestation early on, because this is the only way to contain it quickly and prevent it from spreading to healthy plants. In this article you will learn everything about late blight, its typical symptoms on tomatoes and the measures to combat it.

Recognizing brown rot in tomatoes: symptoms and damage

The following symptoms occur with late blight on tomatoes:

• From summer to autumn: In the early stages of tomato brown rot, you can see it on the leaves first brown to black moist patches of dead tissue, usually spreading rapidly from the tip spread.
• Wilting of the leaf and later of the whole plant.
• Underside of leaf: Sometimes whitish fungal coating that mildew remind.
• Stem: Black-brown, sharply defined spots that can discolour the entire stem.
• Fruits: Yellowish spots that later turn brown and sink in; Affected tomato fruits initially feel hard on the brown areas, later they either rot on the diseased plant or shortly after harvest in storage.

The entire tomato crop can be affected by tomato rot in a very short time, because the spores are transmitted by wind and water spray during the warm season. In dry weather and heat above 30 °C, the infection is initially stopped, but it progresses rapidly at lower temperatures.

Prevent tomato rot

Once symptoms of late blight are seen on tomatoes, combating them is extremely time-consuming and many of the means of choice are anything but environmentally friendly. The best way to combat it is therefore prevention. In the following paragraphs we explain various measures that can prevent the infestation in advance.

Brown rot resistant tomato varieties

The targeted choice of variety is one of the most important measures to prevent tomato brown rot. Resistant or tolerant varieties should be used for planting tomatoes in open ground be selected, because then the fungal spores usually have no chance of gaining a foothold and the plant infested. Some wild relatives of our cultivated tomato, such as the Humboldt tomato (Solanum humboldtii) or varieties of the currant tomato (Solanum pimpinellifolium) are largely resistant to Phytophthora infestans. In addition, tolerant and resistant varieties have been bred for many years, both open-pollinated and hybrid seeds. For example, tomato varieties that are firm to seeds have proven to be robust against late blight ˈDe Beraoˈ, ˈSunvivaˈ, ˈPrimabellaˈ and ˈResibellaˈ. Among the hybrid varieties we find other resistant crossings, like the varieties ˈPhilovitaˈ, ˈPhantasia F1ˈ and ˈVitella F1ˈ.

The right choice of location against brown rot on tomatoes

As explained above, the spores of Phytophthora infestans, the cause of brown rot in tomatoes, in the soil as permanent spores or on potato tubers or old roots. Crop rotation therefore helps immensely in preventing tomato blight. For example, tomatoes should not be planted after potatoes or other nightshade plants (such as peppers, aubergines, physalis). You should also not follow infested tomatoes. In particular, high humidity and wet leaves for several hours combined with pleasantly warm temperatures favor infestation with brown rot. Therefore, the location should ideally be protected from the weather, especially from rain showers. A rain canopy outdoors can already significantly reduce or delay the infestation of tomato blight.

Low planting density to prevent brown rot on tomatoes

If the plants are too close together, diseases such as brown rot in tomatoes can spread quickly. After rain, the plants dry out slowly because the air can circulate poorly. Wet and moist tomato leaves are ideal targets for the water-dependent fungal spores. The longer it takes for the plants to dry again after morning dew or rain, the longer the fungus has time to infest. To ensure good drying and air circulation, tomatoes should be planted about 80 to 100 cm apart.

Proper watering behavior

Tomato brown rot fungal spores are quickly transmitted to the lower leaves by splashing water from the ground. If the plant is watered upside down, the spores can also get from affected leaves to healthy ones. Here you can start with a better watering technique: The plants should only be watered in the root area. It should neither splash when watering, nor should the foliage of the tomato be wetted with irrigation water. For more tips, check out our article on the right one Watering and mulching tomatoes.

Strengthen tomato plants against late blight

If the tomato plant is well cared for and supplied with sufficient nutrients, pests and pathogens generally have a hard time. Potassium plays an important role in the stability of plant tissue. Fungi and other pathogens cannot penetrate stable cell walls as quickly or at all and infect the plant. A good supply of potassium, like ours Plantura organic tomato fertilizer offers, thus preventing diseases such as late blight. Important tips for Fertilizing tomatoes can also be found in our special article.

Herbal tonics also help to keep the plant healthy, so they have a preventive effect against tomato rot, but usually no longer when there is a visible infestation. Good home remedies for tomato brown rot include weekly spraying horsetail broth or garlic or onion peel tea. Both should prevent the growth of pathogens, supply the plant with trace elements and thus strengthen it. The biological agents Chitoplant made from crab shells and ENVIrepel based on garlic also show good resistance to tomato late blight.

Important: The egg fungus survives both in the soil and on plant remains. Therefore, never dispose of foliage and fruit infested with tomato blight on the compost or leave it in the garden. It should be disposed of with the residual waste as soon as possible. The scissors used for this should also be disinfected afterwards.

Control brown rot on tomatoes

Combating late blight in tomatoes when the first symptoms appear is complex and requires immediate action. There are some fungicides for this purpose that can also be used in the home garden. If you want to combat late blight in tomatoes biologically, the arsenal of remedies is unfortunately very limited. In experiments conducted by the State Institute for Agriculture (LfL) in the biological field, this was Plant strengthening agent horsetail extract effective even after visible infection and could the leaf infestation significantly reduce. Horsetail extracts can be purchased or you can make them yourself.

Conclusion: The best way to combat late blight on tomatoes is to take preventive measures, such as choosing a variety, choosing a location, observing crop rotation and proper care.

Not only pathogens can affect our tomato plants and ruin the harvest. the blossom end rot caused, for example, by a nutrient deficiency. You can find out how to recognize this disease and how to effectively prevent and combat it in our special article.