Agricultural News: Why Are My Tomato Leaves Curling?

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   Recently I’ve received a couple calls regarding tomato leaves curling. The curling or rolling is caused by one of three things: environment stress, viral infections, or herbicide damage.

   Let’s talk first about the most common reason you’ll find your tomato leaves curling: Physiological Leaf Curl aka Leaf Roll (see photo). This has to do with the environment and can occur at any point in the growing season, but it’s typically seen when spring turns to summer.   Excessive moisture & nitrogen, heat, drought, excessive pruning, low phosphorus, or root damage can cause the leaves to curl. You’ll typically see leaves begin to roll first on the lower, older leaves with leaflets rolling upward and inward at margins. The leaves may be thickened with a leathery texture, but the color remains a healthy green. If the environmental stressor is identified and corrected, the issue can be managed. The good news is that this type of leaf roll is not known to affect yield or fruit quality. Also, indeterminate (vine tomato) varieties tend to be more susceptible to physiological leaf curl than the determinate (bush tomato) cultivars. You can manage physiological leaf curl by maintaining consistent soil moisture and avoiding over-fertilizing, excessive pruning, and root damage. Other preventative measures include hardening off transplants and planting a variety of cultivars.

   A second and less common cause leaf roll can be due to certain viral infections. Plants infected by the Tomato Yellow Leaf Curl Virus exhibit cupped and pale green leaves, stunted plant growth, yellowing of leaf edges, purplish veins on the underside of the leaves, and a decline in fruit production. This virus is transmitted by whiteflies, so controlling those will help reduce your plant’s chance of exposure. Plants infected by the Tomato Mosaic Virus exhibit mottled-colored rolled leaves, small leaflets, and internal browning of infected fruit. There’s no treatment for viral infections. Removing and destroying diseased plants is recommended. Be sure to disinfect any tools used to remove infected plants to avoid spreading the virus. Controlling weeds around your plants is also recommended as weeds can be hosts for certain viruses. 

   Herbicide Damage also causes tomato leaves to curl. Tomatoes are especially sensitive to small concentrations of herbicides. Common symptoms of chemical injury include: drooping plants, downward rolling or twisting of leaves, yellowed plants, malformed fruit, or stems that are white, split, calloused, thickened, or twisted. New growth is usually affected first. Testing for specific herbicide damage is not readily available, but if you’ve ruled out other causes of plant stress such as weather, soil, misapplied fertilizers, or insects, then you may be looking at herbicide damage.  Improper use of pesticides and using materials in your garden previously treated with certain herbicide active ingredients can cause injury. For example, picloram and aminopyralid herbicides can remain active in the soil, pasture grass, and hay for a year or more. Read and follow all pesticide label instructions, follow proper herbicide stewardship, and avoid using hay, manure, mulch, or compost in your garden that were previously exposed to pesticides. If you don’t know if a material was previously exposed, err on the side of caution and do not use them.  See UT Publication W 295-A, Preventing Off-Target Herbicide Problems in Tomato Fields for more information. Herbicide damage cannot be reversed. However, if the exposure is mild the plant may be able to grow out of it, but the yield may be lower or delayed.  

   As you can see, there are a number of reasons tomato leaves can curl, but the good news is the most common reason is due to environmental stressors, which can be prevented and managed. Knowing the symptoms of physiological leaf curl, viral infections, and herbicide damage can help you to diagnose and potentially prevent long-term damage to your crop. 

   For additional information on leaf curl or for assistance in diagnosing problems with your garden, please contact Megan Harris at 931-722-3229.