What causes the tops or shoulders of ripening tomatoes to remain green or turn a bright yellow instead of red? This is a case where beauty is not just skin deep. The fleshy part of the tomato just beneath the yellow shoulders is less flavorful and may have a different texture. A number of different factors have been shown to play a role in causing this. Some tomato cultivars are more likely to have the problem than others. In the early 1900’s, scientists discovered that a genetic mutation in tomato resulted in uniformly red tomatoes and this gene was bred into many commercial varieties. This has been reported by some to be associated with a less flavorful tomato, possibly because of a lower level of sugar and lycopene in the fruit.
Yellow shoulder is consistently associated with high heat or overexposure to sunlight. Fruit removed from the vine and placed out of the hot sun just as it starts to pink will develop uniform red color while fruit left on the vine and exposed to a lot of sun might develop yellow shoulder. Uniform red color is expected by most of today’s shoppers. But did you notice that most nice, uniformly red supermarket tomatoes don’t have the same flavor as heirloom or other varieties? Could be related to that ‘uniform ripening gene’.
What this means is that while the shoulders might be a bit less flavorful, the rest of the tomato has great flavor potential! If the fruit is otherwise firm and healthy, don’t reject those yellow shouldered tomatoes. If you grow your own tomatoes, you can choose from the many varieties including heirlooms that don’t have this trait.
To prevent yellow shoulder, you can try to avoid over exposure of the ripening tomatoes to excess sun by taking steps to prevent pest and disease problems that cause a loss of foliage (and shade), avoiding excessive pruning, and harvesting early to finish ripening in a more moderate environment. Ripening off the vine may result in a slight flavor difference. Reduce disease by using practices that promote dry foliage. Create good airflow around the plants by using ample plant spacing, supporting plants using stakes, cages or trellises, and pruning indeterminate varieties to 1-2 main stems per plant. Avoid overhead irrigation or use it early in the morning. All this helps because many fungi that cause foliar diseases of tomato can only cause infection if there is a film of water on the leaf surface. In addition, make sure fertility is adequate. Lower potassium and calcium levels in tissue analysis studies have preceded the development of yellow shoulder symptoms. More info is available in this 2011 University of Delaware crop update.