Yellow, orange, or green shoulders on tomatoes is a common disorder that can be prevented.
Yellow shoulder, also known as greenback and green shoulder, appears as discolored areas on the tops of tomatoes. These discolorations are commonly yellow, orange, or green. Unfortunately, these discolorations enter the fruit as well. Fruit affected by yellow shoulder tends to be hard, white, and tough. Not exactly what you expect from a sun-ripened tomato. Despite what everyone says, extreme sunlight is not what causes yellow shoulder. It will cause sunscald if there isn’t enough leaf cover.
When scientists tried to figure out yellow shoulder, what they found was the normal red, neatly arranged cells of a healthy tomato became significantly smaller and erratic. The chlorophyll in these deformed cells failed to turn red. This 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 are the cause of yellow shoulder, but that’s not it. Research has shown that yellow shoulder is triggered by insufficient potassium, too much magnesium relative to calcium, and a pH above 6.7.
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 our tomatoes.
Preventing yellow shoulder
A soil pH of 6.4 to 6.7 can help prevent yellow shoulder. The truth is, plants can absorb many more nutrients when the soil pH is between 6.2 and 6.5. This is probably where the extreme sunlight, extreme heat myth comes in. When plants are too hot or have insufficient water, they become less able to absorb nutrients. This is something like blossom end rot where 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, 1:4 is ideal. Too much calcium or magnesium in the soil can trigger yellow shoulder. This gardening business can be tricky! You may want to check out my post on Mulder’s chart to see how intricately these chemicals interact. It makes me realize they could’ve made high school chemistry class a lot more interesting with some of this stuff, in my opinion. Ideally, potassium levels of 144 ppm are best at the seedling stage, and then 350 ppm as the plant start fruiting.
Since this disorder starts developing as soon as fruit appears, adding amendments later in the season does not help. Researchers are developing cultivars that are less prone to yellow shoulder, but that takes time. Until then, get your soil tested, irrigate regularly, and try to keep your soil pH in a good range.
You can grow a surprising amount of food in your own yard. Ask me how!
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