What’s the black leathery spot on my tomatoes?
August 14, 2012
The long-awaited tomato harvest has begun.
Tomatoes are ripening, but what’s with the blackened leathery spot on the bottom?
We put in all kinds of time and energy to grow lovely delicious tomatoes, but these are ugly.
The problem is blossom end rot (BER), a physiological issue. It’s not a disease, so the unaffected parts of the tomato are still edible, once you cut away the blackened part.
BER can also affect peppers and eggplants, although the discoloration may look a little different.
This fruit disorder is directly related to soil moisture and soil calcium.
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BER is common when plants grow rapidly early in the season. It is made worse by windy dry weather, of which we have had plenty.
As plants grow, they need more water, especially while the fruits are developing. If soils get too dry or too wet, roots can’t absorb the water or nutrients they need and plants can’t grow properly. Moisture fluctuations reduce calcium uptake.
BER is often attributed to calcium deficiency with addition of lime a recommended solution.
Nevada soils are rich in calcium and adding calcium as lime is NOT the answer.
What does help is keeping the plant root system healthy and providing enough water to get calcium up to developing fruits. Foliar applications of calcium are of little value.
Another possible factor in BER is hoeing too closely. This can damage or cut shallow feeder roots preventing them from adequate moisture absorption. Cutting the roots reduces water absorption.
What’s a gardener to do? Keep plants evenly watered from planting to harvest.
This means avoid overwatering early in the season when soils are still cool, and avoid underwatering later as the weather heats up.
As it gets hotter, mulch about two feet around the plant to conserve soil moisture. Mulching too early will keep soils too cold for good root development.
Over-fertilizing can also increase BER, particularly with high nitrogen fertilizers, because this causes rapid vegetative growth. Slow release or organic fertilizers are good options.
Some varieties are more prone to BER than others are. Plant indeterminant or semi-indeterminant varieties rather than determinant bush varieties. Indeterminant types grow fruit over the whole season while determinant types produce many fruits late in the season. The flush of fruit production reduces the amount of available calcium.
Keep your tomatoes happy and irrigate efficiently.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at email@example.com or 887-2252.