Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Over the last few weeks there have been many reports about tomato leaves curling, sometimes severely, in growers’ fields. It starts with upward cupping at the leaf margins followed by inward rolling of the leaves (Photo 1). Lower leaves are affected first, and can recover if environmental conditions and cultural factors are adjusted to reduce stress. Not all leaves on a plant roll, but eventually the rolling can involve most leaves on a plant and last through the season. In severe cases, whole plants can be affected. The margins of adjacent leaflets may touch or overlap (Photo 1). Rolled leaves become rough and leathery but are otherwise normal in size and appearance. There is no discoloration of leaf veins associated with this problem. The good news is that leaf roll rarely affects plant growth, fruit yield, or fruit quality.
How bad leaf roll gets appears to be very cultivar dependent. Cultivars selected for high yield tend to be the most susceptible. Indeterminate cultivars seem to be more sensitive to this problem than determinate cultivars. Leaf roll is often seen just after plants are heavily pruned during dry soil conditions. If the tomato plant’s top growth is more vigorous than root growth and we are hit with a dry hot period the foliage may transpire water faster than the root system can absorb it from the soil, and the plant will respond by rolling its leaves to reduce the transpiration surface area. Another cause of this disorder includes growing high-yielding cultivars under high nitrogen fertility programs. Oddly enough leaf roll disorder also has been found to be caused by excess soil moisture coupled with extended high temperatures.
It has been found that sugar and starch accumulate in the lower leaves causing the leaf to roll; the more they accumulate the worse they roll. Leaf roll is usually a problem we see when we have hot dry conditions in June or July, when plants are most actively growing. Because leaf roll will seldom affect yield it is a problem that not much should be done about other than making sure it is not some other more severe problem (some viruses can look similar to tomato leaf roll, but if the symptoms suddenly appear and involve many of the plants in a field and their lower leaves it is probably leaf roll). You can reduce symptoms by maintaining consistent, adequate soil moisture (~1 inch per week during the growing season, which will also help with calcium up-take reducing blossom end rot problems). Growers also should not prune heavily during hot dry conditions or over-fertilize with nitrogen.
Photo 1. Tomato plant with mild leaf roll of lower leaves