Leaf curling and rolling in vegetables can be caused by virus diseases, insects and herbicide drift. However, summer is when we often see leaf cupping and rolling problems in vegetable crops that are not caused by pests or chemicals. It is most commonly seen in tomatoes, but peppers, potatoes, watermelons, beans, may also show the condition. This is a physiological disorder that can be caused by a number of different environmental factors.

The most common cause of Physiological Leaf Curl is very high temperatures over an extended period of time or a lack of water. The plants curl their leaves to conserve moisture. Mulching to cool soil temperatures and maintaining adequate soil moisture will aid in preventing the problem.

In tomatoes, leaf curl starts at the margins which turn up, then roll inward. It usually starts on the lower leaves. Upward curling is also found commonly in watermelons and potatoes. Beans, peppers, and other vegetables may curl downwards. Leaves may stay rolled or curled for a short time and then return to normal, or they may remain permanently tightly curled. Rolled leaves become thick and rough but are otherwise normal. Physiological leaf curl symptoms may vary greatly between different varieties, with some showing no signs and others highly affected..

Interestingly, unlike problems caused by disease, insects or herbicide drift, the quantity and quality of the vegetable crop from affected plants is most times equal to unaffected plants. This has been demonstrated in university research trials by a number of different state universities.

Spider Mites

The high temperatures we are having are favored by spider mites, and I have found plants with moderate to high populations of them recently. Spider mites are very tiny being just visible to the naked eye. They feed on the underside of leaves and needles. They may have a tiny amount of webbing on the leaf as well. The best way to check for them is to hold a white sheet of paper under a branch, and tap quite hard on the top of the branch. This will dislodge them onto the paper where they will look like a speck of dust that moves on its own!

Spider mites feed by sucking plant juices. In doing so, they stress the plant and weaken it. Leaves may be misshaped and/or discolored. The growth rate may be reduced as well as flower or fruit production.

Since they are very tiny and do not climb easily, for ornamental plants and where only a few are affected, physically spraying them off is easiest and most effective control. A garden hose with a spray nozzle and good water pressure is all that is needed. Direct the spray toward the underside of the leaves. Try to get all the leaves sprayed, starting at the top and working downward. This should be repeated every three to four days for two weeks to get any newly hatching mites from eggs that were already present.

There are also a number of miticides registered to control spider mites. Be sure to read the label and follow the directions for amounts needed, mixing, applying and personal protective gear needed.

Ken Eraas is the Ward County Extension Horticulture Assistant. You can reach him by calling 857-6444 or emailing kendell.eraas@ndsu.edu.