Yellow Shoulder

 Yellow, orange, or green shoulders on tomatoes indicate a common, preventable disorder.

Yellow shoulder of tomato (Kate Russell)

Yellow shoulder, also known as greenback and green shoulder, appears as discolored areas on the tops of tomatoes. Unfortunately, these discolorations enter the fruit as well. Fruit affected by yellow shoulder tends to be hard and white, not exactly what you expect from a sun-ripened tomato. Despite what everyone says, extreme sunlight is not what causes yellow shoulder, though it will cause sunscald if there isn’t enough leaf cover.

When scientists tried to figure out yellow shoulder, they discovered the red, neatly arranged cells found in healthy tomatoes were smaller and erratic. The chlorophyll in these deformed cells failed to turn red, which is bad news in the world of tomato growers, so they set out to find out what was going on.

What causes yellow shoulder?

I would love to say that the solution is simple. Most garden gossip says that high heat and too much sunlight cause yellow shoulder, but that is false. Research has demonstrated it takes insufficient potassium, too much magnesium relative to calcium, and a pH above 6.7 to trigger yellow shoulder.

Lab-based soil tests can give you this kind of information. Luckily, those tests cost about the same as a large bag of fertilizer and provide a wealth of information. In case you hadn’t noticed, I recommended a soil test every 3-5 years. Now, back to your tomatoes. 

Preventing yellow shoulder

A soil pH of 6.4 to 6.7 can help prevent yellow shoulder. Plants can absorb many more nutrients when the soil pH is between 6.2 and 6.5. And this is probably where the extreme sunlight and heat myth comes in. When plants are too hot or have insufficient water, they become less able to absorb nutrients. blossom end rot works the same way in many cases. There’s probably enough calcium in the soil but not enough water in the plant to move the calcium around. Yellow shoulder also occurs when temperatures get too low for the same reason.

Maintaining a magnesium/calcium ratio of 1:6 is good, but 1:4 is ideal. Too much calcium or magnesium in the soil can trigger yellow shoulder. [I bet you never thought gardening included being a chemist! You may want to read up on Mulder’s chart to see how intricately these chemicals interact. It makes me realize high school chemistry class could have been much more interesting than it was.] Ideally, potassium levels of 144 ppm are best at the seedling stage and then 350 ppm as the plant starts fruiting.
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Since this disorder starts developing as soon as fruit appears, adding amendments later in the season does not help. Researchers are developing tomato cultivars less prone to yellow shoulder, but that takes time. Until then, get your soil tested, water your tomatoes regularly, and aim to keep your soil pH within a healthy range.