Experiencing extreme stress due to unfavorable conditions can negatively affect tomatoes, whether they are grown in a greenhouse or outdoors, after transplanting seedlings or during the growth season. We have already discussed why tomato leaves curl up, and now let's look at the reasons why they curl downwards.
Oftentimes, the curling of leaves indicates a deficiency in certain nutrients. Usually, this is preceded by leaf yellowing - chlorosis, followed by curling. For example, if the leaves curl downwards, and the undersides and stems turn purple, it may indicate a phosphorus deficiency. In such cases, tomato seedlings after transplanting often experience a lack of phosphorus combined with the stress of relocation. Tomatoes usually recover when their roots establish fully, or you can apply phosphorus foliar spray directly on leaves. Additionally, adding phosphorus fertilizers to the soil before the flowering period is advisable.
In open ground, tomato leaf blades can curl downwards during rainy seasons or due to overwatering. This is a plant's protective reaction to root rot in excessively moist soil conditions. The plant limits water uptake by the roots, leading to the curling of leaves downwards. Prolonged overwatering can cause root rot, which can result in tomato plant death. (It's worth noting that waterlogging can allow the development of Fusarium wilt, which will be discussed later in this article.)
This type of leaf curling occurs when the plant stops water uptake until it is safe to drink. In this situation, it is best to stop watering in the greenhouse until the soil completely dries out, and in the garden, take necessary preventive measures, such as improving drainage before the new season, temporary soil covering with compost that absorbs water quickly, and then removing the moist compost. Additionally, using plastic covers and carefully cultivating and aerating the soil around the plants can help avoid root damage.
Mid-season in a greenhouse, especially during high temperatures and bright sunlight, tomato leaves may curl downwards at 32°C due to sunburn. In such cases, providing shade and watering the plants only at the roots while avoiding leaf wetting are recommended.
Towards the end of the season, a dangerous disease may manifest, characterized by leaf and stem curling and some recovery at night - fusarium wilt of tomatoes. This disease is caused by the fungus Fusarium oxysporum sp. lycopersici.
Lower leaves turn yellow and curl downwards, sometimes only on one side of the plant or branch. Upon closer examination of the affected stem, darkening of the vascular tissue (tissue closer to the "skin") can be observed. The inner part of the stem remains healthy. Vascular blockage, through which the plant moves food and water, leads to wilting and yellowing. Verticillium wilt develops more slowly, and symptoms on the plant are more uniform. Bacterial wilt of tomatoes leads to gradual wilting, unlike fusarium and verticillium wilts, where symptoms begin from the plant's lower part.
All these wilts are caused by pathogenic microorganisms transmitted through the soil and can persist in it for a long time, even if such plants were not previously grown in that area. They can be introduced to the garden through infected seedlings or contaminated soil. Fusarium wilt does not spread through soil from plant to plant. Each plant becomes infected individually when the organism enters the root system. Infected plants should be destroyed.
If you have diagnosed fusarium wilt on your tomatoes, in the next season, consider purchasing only varieties resistant to this disease and its variations. Improving soil drainage to prevent problems is also advisable.
In today's world, where many gardeners live next to enthusiasts of a perfect lawn, there may be cases of herbicide exposure when treating plants in the garden. Tomatoes are not tolerant to herbicides.
Unlike leaves curling upwards or inwards, tomato leaves affected by herbicide always curl downwards. Not only tomatoes, but other plants near the treated area may exhibit similar symptoms. This is an important factor in determining herbicide impact. If you encounter such symptoms, wait for new leaves to emerge and analyze if the situation improves. Using greenhouse tomato cultivation methods can be used as a preventive measure.
In a greenhouse, herbicide residues can come with manure from a barn or stable if animals consumed hay treated with aminopyralid or clopyralid, active ingredients in herbicides used to control weeds in pastures. Residues of these agrochemicals can persist in treated hay and hay products, mown grass, as well as in manure and compost for an extended period. If concerned, it is recommended to use chicken manure, which does not pose such problems.
