Yellow Shoulder Disorder of Tomato
By Tim Baker

Tomatoes are sometimes subject to physiological disorders that cause uneven ripening. This problem expresses itself in uneven skin color with areas that just won’t color up like they should, no matter how long you leave them on the vine. There also may be internal quality problem in the fruit as well.

At the Great Plains Vegetable Growers Conference in St. Joseph, Dr Chris Gunter gave an interesting presentation on Yellow Shoulder Disorder (YSD), one of the most commonly seen ripening problems in tomatoes. Dr Gunter has researched this problem at his previous position at Purdue University, and will continue researching this disorder at North Carolina State University where he presently works.

There are a number of causes of YSD. Genetics accounts for roughly 10-20% of the problem. In other words, some varieties will be more subject to YSD than others. There is some interest in breeding tomato varieties that will resist YSD, but so far that goal has eluded plant breeders.

The reason is because other factors in addition to genetics influences the development of YSD. Location may account for up to 30% of the problem, and weather influences YSD as well, from 5-10%. And other factors, some of which are unknown, account for 40-50%.

Research has discovered some of these factors, many of which are related to soil and nutrient problems. PH, for example, should be properly balanced to help avoid YSD. Ideally, soil pH should be between 6.0 and 6.8. If your soil pH is out of this range, your tomatoes will be more subject to YSD.

Organic matter also plays a part. Soils with organic matter less then 1.5% will be more subject to YSD.

One of the main factors contributing to YSD is the level of potassium in your soil. If soils are too low in this essential nutrient, your tomatoes will be more subject to YSD. However, don’t pour on the potassium just yet. Always get a soil test first, since excess levels of potassium are not desirable and won’t help the situation.

If your soil potassium levels are too low, increasing them may be of benefit. It has been noted that foliar applied potassium does not help, so soil application works best. It has also been found that applications of soluble potassium in the early to mid phase of tomato growth can help.

Another factor is the amount of magnesium in the soil, especially the ratio of calcium to magnesium. Cation Exchange Capacity also figures into the equation.

Numerous researchers have been working on this problem. One of the more interesting results from this research has been the development of the Hartz Ratio, which helps predict the susceptibility of your soil to YSD. Fortunately, for us, a web site exists where you can enter the values of all these parameters (from your soil test) and have your Hartz Ratio automatically calculated. This will give you a good estimate of the susceptibility of your soil for the development of YSD.

If you do not have access to the web, contact your local University of Missouri Extension Horticulture Specialist, and we will be glad to run the numbers for you.

The web site may be found at http://www.oardc.ohio-state.edu/tomato/ysd.

Tim Baker
Northwest Region Horticulture Specialist
University of Missouri Extension
102 Main, Suite 1
Gallatin, MO 64640
660-663-3232
BakerT@missouri.edu