Blight 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. The disease caused by Phytophthora infestans, 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 annuum). . 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 infestans 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 he althy plants. In this article you will learn everything about late blight, its typical symptoms on tomatoes and the measures to combat it.
Detecting 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, brown to black, damp spots of dead tissue can be seen on the leaves, which usually spread quickly from the tip.
- Withering of the leaf and later of the whole plant.
- Underside of leaf: sometimes whitish fungal coating, somewhat reminiscent of powdery mildew.
- Stem: Black-brown, sharply defined spots that can discolour the entire stem.
- Fruits: Yellowish spots that later turn brown and sink in; Infested tomato fruits initially feel hard on the brown areas, but later they either rotof the diseased plant or shortly after harvest in storage.
The entire tomato crop can be affected by tomato blight in a very short time, because the spores are transmitted by wind and water spray during the warm season. Infection is initially stopped in dry conditions and heat above 30 °C, but 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.
Blight 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 selected for planting tomatoes in unprotected outdoor areas, because this way the fungal spores usually have no chance of gaining a foothold and infecting the plant. Some wild relatives of our cultivated tomato, such as the Humboldt tomato (Solanum humboldtii) or currant tomato varieties (Solanum pimpinellifolium) are largely resistant toPhytophthora infestans. In addition, tolerant and resistant varieties have been bred for many years, both open-pollinated and hybrid seeds. The tomato varieties ˈDe Beraoˈ, ˈSunvivaˈ, ˈPrimabellaˈ and ˈResibellaˈ, for example, have proven to be robust against late blight. Among the hybrid varieties we find other resistant crossings, such as 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, survive 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 cansignificantly reduce or delay infestation with tomato blight.
Low planting density to prevent brown rot on tomatoes
If 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.
Correct 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 he althy 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 watering and mulching tomatoes properly.
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, such as that provided by our Plantura organic tomato fertilizer, prevents diseases such as late blight and late blight. You can also find important tips for fertilizing tomatoes in our special article.
Plant tonics also help to keep the plant he althy, 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 with 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 means Chitoplant fromGarlic-based crab shells and ENVIrepel also show good resistance to tomato late blight.
Important: The egg fungus survives both in the soil and on plant debris. 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 of tomato brown rot
Fighting 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), the plant strengthener horsetail extract was effective in the biological field even after visible infection and was able to significantly reduce leaf infestation. You can either buy horsetail extracts or make them yourself.
Conclusion: The best way to combat late blight on tomatoes consists mainly of preventive measures such as choice of variety, choice of location, observing crop rotation and proper care.
Not only pathogens can affect our tomato plants and ruin the harvest. Blossom end rot, for example, is caused by a lack of nutrients. You can find out how to recognize this disease and how to effectively prevent and combat it in our special article.