Your beautiful tomato plant is setting fruit and soon has nice green tomatoes. Then, to your horror, a dark, sunken spot shows up at the bottom of the fruit.
You might think an insect or disease has attacked your plant. But that’s not the case. It’s actually a disorder called blossom-end rot that is caused by a calcium imbalance.
Plant cells have rigid cell walls. To build those walls, the plant needs calcium.
“Calcium is to cell walls what the cement is in a brick wall,” said University of Missouri Extension horticulture specialist David Trinklein. “It gives it structural strength.”
After being taken up by the roots, calcium moves through the plant. This requires water. If there isn’t enough water to carry the calcium, or enough calcium to be carried by the water, then the mineral cannot reach the blossom end of the fruit, and those walls break down. The result is a dark, sunken spot.
To control blossom-end rot, test the soil before you plant your tomatoes.
“In most Missouri soils, if the pH is at the ideal level of 6.2-6.5, there likely is sufficient calcium in the soil,” Trinklein said. “If a soil test shows the pH is fine but the soil still needs calcium, you can add something like calcium sulfate, also called gypsum, to boost calcium without changing the pH.”
But the problem might not be the amount of calcium in the soil.
“Oddly enough, a plant that is overwatered has difficulty taking up water,” Trinklein said. “It’s called water wilt.”
With all the rain this year, our tomato plants are nearly drowning. When plants are standing in water, their roots don’t get enough oxygen. This damages the cell membranes, so the roots can’t take up sufficient water to deliver needed calcium to the fruit.
We can also cause blossom-end rot by damaging the roots while weeding.
“You will find that gardeners who get a new rotary tiller will often have blossom-end rot,” Trinklein said. “They cruise up and down the rows churning the soil with their new toy, which frequently results in excessive root pruning.”
Improper fertilizing can cause the disorder too. Trinklein said there are two forms of nitrogen that can be taken up by the plant: ammonium form, which has a positive charge, and nitrate form, which has a negative charge. He says the nitrate form is preferable for tomatoes.
“If we feed excessive amounts of ammonium-form nitrogen to tomatoes, its positive charge confuses the plant because calcium has a positive charge,” Trinklein said. “We call that ‘nutrient antagonism.’ Ammonium will keep the plant from taking up adequate calcium.”
Avoid urea fertilizer, which converts to ammonium form, he said. Many products sold as tomato fertilizer contain urea, so be sure to read the label.
An ideal fertilizer to feed tomatoes is calcium nitrate, Trinklein said. We’re giving the plant the nitrate it wants, and as we’re doing so, we’re adding some calcium to counter blossom-end rot.
The MU Extension guide “Growing Home Garden Tomatoes” (G6461) is available for free download athttp://extension.missouri.edu/p/G6461.
Soil tests for lawns and gardens are available for a nominal fee through MU Extension’s Soil and Plant Testing Laboratory. For more information, go to http://soilplantlab.missouri.edu/soil.
For more information from MU Extension on lawn and garden topics, including guides, articles and online resources, go to http://extension.missouri.edu/LawnGarden.
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