News release: Physiological Disorders of Tomatoes

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640
660-663-3232, bakert@missouri.edu

Release Date: July 7, 2016
Headline: Physiological Disorders of Tomatoes

Growing tomatoes in the home garden is easy, but occasionally they do have problems.  I will cover diseases and insects in future columns, but this time I would like to discuss disorders that can occur in tomatoes from other causes.

One of the most common questions that I hear from homeowners is “Why are my tomato leaves rolling?”  In this case, they see leaves rolled up from side to side.  In other words, the leaf is still the same length, but the sides of the leaf have rolled up, sometimes pretty severely.  This condition is known as physiological leaf roll, and normally causes no problem.  It won’t affect fruiting.  Some varieties are worse than others.

Leaf rolling can also be caused by herbicide drift or virus diseases.  In these cases, yields and plant health could be affected.  But usually, leaf roll is just the physiological type, and should be of no concern.

A disorder that does cause fruit losses is blossom end rot.  Typically, you will see a rotten area on the tomato fruit, at the end of the fruit directly opposite from the stem, where the blossom used to be attached.

In this case, the problem is a calcium deficiency in the fruit.  This could be caused by a calcium deficiency in the soil, but often the soil has plenty of calcium.  The usual cause is fluctuation in watering.  The solution here is to water as evenly as possible, and try to avoid situations where you go from very wet to very dry soil.  Water frequently, but not excessively.

Blossom end rot can be made worse by high temperatures and breezy conditions, root damage from excessive cultivation, excessive fertilizer, nematodes, or excessively wet soil.  Ammonium containing fertilizers may worsen the problem.  Calcium nitrate would be a better choice, since it avoids the ammonium and provides calcium.  Some varieties may be more susceptible to blossom end rot.  And finally, if this is a continual problem, you should get a soil test to see if you soil needs more calcium.

To control blossom end rot, watering consistently, as mentioned above, is the first thing to try.  You may want to mulch your soil to conserve moisture.  Increased soil organic matter always helps, and if a soil test indicates a need for more calcium, add the amount and type suggested by the test.  In the meantime, you might try a calcium spray, which may offer some relief for this year’s crop.

Another physiological disorder of tomatoes is sun scald.  The tomatoes may not develop good color, and often look like they have been scalded in the sun.  Usually, there are not enough leaves to cover the fruit, and in this case, the sun is literally scalding them.  Be sure to encourage good foliage development through proper nutrition early in the season, but be careful not to overdo it.  Remember that excess nitrogen can cause fruit not to set.

Cracking can be caused by several factors, but variety is often the key.  Some tomato varieties are simply more susceptible to cracking than others.  If this is causing a major problem for you, try changing to crack-resistant varieties.  This is another problem that may be helped by keeping soil moisture as even as possible.  If you suddenly add a lot of water to a developing fruit, it may expand too quickly and crack.

Catfacing describes tomato fruits that are badly formed on the blossom end, sometimes with scarring.  This may occur in tomatoes that are subjected to cold weather at the time of blossom set.  This seems to be a greater problem in large-fruited varieties.

A final physiological disorder of tomatoes that I will mention is yellow shoulder.  This refers to a discoloring under the skin of the fruit.  This is apparently nutritional in nature, and may involve insufficient quantities or poor ratios of potassium, magnesium, and calcium.  For home owners, the best bet is a good soil test to start with, to see if you have any nutrient deficiencies.  Increased organic matter has been shown to help, along with a pH of 6.4 or less.  As soil pH approaches neutral (7.0) or higher, the problem apparently worsens.

I still plan to write a few more articles in this series on growing tomatoes, discussing diseases and insect pests and other matters.  In the meantime, if you have any problems with your tomatoes, don’t hesitate to give me a call.