Lars Lund

18 Jul 2022 11:41

Are your plant leaves curling?


Text and phot by: Lars Lund


Everything is growing and the heat may cause your plants to curl.


July is the main greenhouse and garden season, but it is also a season for diseases and a season for plants to look like they are sick when they in fact are not. A question I have been asked numerous times this year, is about tomatoes and their curled leaves. Naturally, people have been frightened by that clopyralid was found in liquid fertilizers and in growbags last year. Clopyralid is pesticide residues from especially the production of beets. When organic liquid fertilizer is made, sap from plants is extracted. This sap has in some cases contained clopyralid, a toxic for weeds, which has caused the delicate tomato plants deformations, and even ruined the plant’s production. Some of the deformations may look like the ones we saw in June and July – curled leaves – but often the curled leaves are caused by a too warm climate. When it gets too warm, the tomato plants cannot convert sugar to starch resulting in curled leaves. The same happens when it is too cold. The good news is that it will not affect the yield. The tomato will keep growing and producing tomatoes even though the leaves are still curly. Let them stay. It is not contagious.


Limp and thirsty

Another phenomenon, that often occurs, are cucumbers with limp leaves. Once again, the heat is the problem. When it is very hot, a lot of water will evaporate from the large leaves. And even though you are irrigating, the root system cannot keep up. Often the root system is not big enough due to transplanting the cucumber plant too soon. Transplanting too soon can harm the root system and the damages may be irreversible. Sometimes it is the start of verticillium wilt. But if you provide shade, the leaves will get back up again. Unless the verticillium wilt has already taken its hold. The roots of a cucumber are quite vulnerable. For this reason, you can in for instance Germany buy cucumbers grafted with pumpkin roots. The pumpkin roots have a healthier and stronger root system compared to the root system of a cucumber.



Verticillium wilt

Do your cucumbers often become limp and maybe even wither? Then they may have been infected by verticillium wilt. The disease can survive in the soil. Therefore, remember to change soil next time you are transplanting. If not, the disease is difficult to stop no matter how often you irrigate. Verticillium wilt begins in the roots, and it will prevent the transportation of water inside the plant. Another plant disease which also makes the leaves limp and wither is the fungus Fusarium. It also attacks tomatoes.

Mildew is another disease, that makes the leaves white and that typically occurs by the end of the harvest. The plant is destroyed by the fungus. Ventilating and lots of space is the answer to prevent mildew.


The cucumbers are falling off

Fallen cucumbers are also a problem. The issue is that the cucumber cannot produce all the fruit even though the plant has grown flowers. The same thing happens for many other fruit trees. They drop a bunch of fruit which cannot evolve. This is a natural process. The sorts that are bred are more vulnerable than the old sorts. You can either nip all the cucumbers growing near the stem, or keep all the cucumbers at the stem and remove the other ones.


The spider mites are busy

Spider mites usually show up by the end of the season and can be difficult to get rid of. In some cases, neem oil helps watch the video here.


Note that the video is in Danish, but it is possible to choose English subtitles in settings.

  1. Open the video on YouTube by clicking on the YouTube-logo in the right corner
  2. Click settings
  3. Click on ‘Subtitles/CC’ and then ‘Auto-translate’ to choose your preferred language


Spider mites do not fly but crawl instead. Therefore, you can also use a small roll from a toilet roll and place some double-sided adhesive tape on the roll. You then put the roll around the stem of the cucumber plant. Now when they try to crawl on the plant, they will get stuck on the tape. The plants should not touch each other, otherwise the mites will have a bridge to reach the cucumbers. But to do this, you will have to be aware that the plants are infected with mites, which is often discovered too late. You can also trick the spider mites by using bait. The bait is common beans. The spider mites are lured to eat the beans instead. The trick is to plant beans in a circle around the cucumbers, or just plant the beans next to the cucumber plant. Tansies are also a great plant to have among the cucumbers to keep the spider mites away.





In my experience, the best way is to use predatory mites, but it is not a cheap method. Besides there are many different spider mite species.


Tips from a reader

A reader called Bruno has given me the following tip

When the spider mites attack in the start of August, and the leaves begin to change colour and die, new shoots will continue to grow from the stem. Instead of throwing away the plant, I have tried nipping off the sick leaves, so the plant only has green and healthy-looking leaves. The end result in mid-September is a four meters tall cucumber plant, which continues to produce cucumbers from the stem. It is better than nothing, but it is not producing a lot of cucumbers – Bruno writes.



Spider mites like water on the leaves, but at the same time it is also a breeding ground for fungi diseases such as cucumber powdery mildew. At first it may look like spider mites. The leaves will get yellow dots and later the dots will merge and turn brown. If a leaf is infected, it cannot be saved. To prevent the fungus disease, you can air out and make sure the plants are not placed too close. And always remove any sick leaves.


Great growing conditions are prevention

As you see it can be difficult to determine what disease it is. Just make sure the plants are thriving. This means that you make sure they are getting the right amount of fertilizer. When it is hot, the plant uses more water for the fertilizer, and therefore, you should sometimes skip the fertilizer even though it says you should add fertilizer when irrigating. Ventilating all day and night is also important. Provide shade and use some good soil. If you have beds, I will recommend using the beds as they are the best solution for the plants. 

Om Lars Lund

Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.  

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