This year there has been a number of inquiries from both within and outside of the office from gardeners who have tomato plants with leaves that are curling up. If that is what they are doing, the problem could be environmental, chemical, or biological. There are basically four reasons that can cause the leaves to curl up; weather stress, non-parasitic leaf roll, herbicide damage, or virus (TYLCV).
The first problem, leaf curling due to the weather can and usually does take care of itself. For instance, when tomato plants grow vigorously in mild, spring weather the top growth often exceeds the root development. When the first few days of warm, dry summer weather hit, the plant ‘realizes’ that it has a problem and needs to increase its root development. The plant tries to reduce its leaf area by rolling its leaves. The leaves curl along the length of the leaf (leaflet) in an upward fashion. It is often accompanied by a thickening of the leaf giving it a leathery texture.
Though rolling usually occurs during the spring to summer shift period, it may also occur after a heavy cultivating or hoeing, high winds with blowing dust and low humidity, a hard rain, or any sudden change in weather. This leaf roll is a temporary condition that goes away after a week or so when the plant has a chance to acclimate, recover from injury, or the soil has a chance to dry out.
Non-parasitic leaf roll is another environmental problem. Irregular irrigation and improper pruning cause the leaf edges to roll inward, sometimes to the point of overlapping. When most of the leaves on the plant are showing this condition it may look serious, but it too is only temporary. The cure for the curl is deep, even watering regularly, with careful pruning.
Even if you are a conscientious gardener, and don’t use herbicides in your garden, overspray and wind-blown droplets may find their way to your tomato plants. Leaf curl is one sign of herbicide damage, and can plague the plant for some time, causing little to no growth, as well as little to no fruit production. Depending on the herbicide, leaves may tilt down with curled edges to form cupped leaves, or the leaves may pucker and curl. Vegetables can also be damaged by herbicides left in mulch or compost made with hay or manure from fields where herbicides have been sprayed. Avoid using herbicides in general and never use herbicide-treated lawn clippings as mulch. Watering affected tomato plants regularly may help them recover if the herbicide damage is mild.
And the last possible reason for your tomato leaves to be curling, is if the tomato plant has been infected by certain viruses. There are certain strains of the Tobacco mosaic virus (TMV) that can cause leaf malformation and “fernleafing”. But the most common symptom of this virus is a light- and dark-green mosaic pattern on the leaves. Another example is the yellow leaf curl disease, caused by the pathogen ‘tomato yellow leaf curl virus’ (TYLCV). An aggressive education program curbed the spread of the disease, but it has not been completely eradicated. Diseased plants are stunted, with small leaves with edges that curl upward. Blossoms drop and the plants don’t fruit. Virus, however, as being the cause of the leaf curling on your tomatoes is probably the least frequent reason of them all.

Rip Winkel is the Horticulture agent in the Cottonwood District (Barton and Ellis Counties) for K-State Research and Extension. You can contact him by e-mail at rwinkel@ksu.edu or calling either 785-682-9430, or 620-793-1910.