Matthew Orwat
County Extension Agent – AG/NR
Email: matthew.orwat@ag.tamu.edu
If you have ever watched a nice tomato grow only to watch in horror as the bottom of the tomato gets a rotten spot that rapidly spreads, you have been a victim of blossom end rot.
Symptoms of Blossom End Rot
Blossom end rot is a physiological disorder that occurs in cucumbers, squash, melons, pumpkins, tomatoes, and peppers. The tissue at the blossom end of the fruit begins to break down, causing a wet sunken lesion. The lesion rapidly enlarges and can encompass half the fruit in no time at all. Bacterial and fungal infections often colonize the lesion, causing the fruit to rot faster. In peppers, blossom end rot may look like sunscald.
Causes of Blossom End Rot
Blossom end rot is caused by a lack of calcium in the tissues near the blossom end of the fruit. However, this does not mean there is a shortage of calcium in the soil. In fact, calcium is often abundant.
Water is a plant’s blood, circulating minerals from the soil into every part of the plant, including the fruit. The water is drawn up into the plant by open pores in the leaves and fruit that allow water to leave the plant, like sucking fluid through a straw. This is called transpiration. When a plant is short of water, it cannot get the necessary minerals to the entire plant. Since foliage does more transpiration than fruit, it captures more calcium.
Drought stress, inconsistent soil moisture, a long stretch of rainy weather, hot and windy weather, and either high relative humidity or low humidity can limit the plant’s ability to transpire, causing blossom end rot. High levels of ammonium (NH4+), potassium (K+), and magnesium (Mg++) can prevent absorption of calcium. Lots of foliage growth from over application of nitrogen can make the problem worse.
Treatment of Blossom End Rot
There is no treatment for blossom end rot. Once it starts, you can’t stop it. As soon as you notice blossom end rot, pick that vegetable. Since it won’t be usable if you leave it on the plant, don’t make the plant continue to send resources to it. People often ask me if they can eat the parts of a tomato or other vegetable if they cut off the bad part. From a food safety standpoint, I would say no. While blossom end rot is not toxic, secondary infections of bacteria and fungus that are toxic often colonize fruit with blossom end rot. I put damaged vegetables in the trash.
Note: Not the compose pile – you don’t want to accidently spread bacteria and fungi across your garden by inoculating your compost pile. Most home compost piles do not get hot enough to kill the bacteria and fungi on the rotten fruit.
Prevention of Blossom End Rot
Inconsistent watering is the most common cause of blossom end rot. Your goal should be to keep the soil consistently moist. Don’t overwater, or you will have other problems. The easiest way to tell when to water is to buy a cheap moisture meter (around $10, so not expensive). Stick it in the ground two to three inches deep. Your goal is to keep the moisture meter in the green area. When it starts to fall to the dry side of the green zone, water your vegetables. If the meter is at the high end of the dial, don’t water then.
Remember that different areas of your garden can have different soil moisture levels, so I keep the meter with me and check the soil in several parts of the garden when I inspect my plants. I only water if the meter shows the soil needs it, so some parts of the garden get more irrigation than others.
Rarely, blossom end rot is caused by too much potassium and magnesium that prevent the plant from absorbing calcium. If you use ammonium as a nitrogen source and over apply it, that can also cause the plant to have problems absorbing calcium. A soil test of your vegetable patch once a year can tell you how much potassium and magnesium to apply to your garden, so you don’t cause absorption problems for your vegetables. The soil lab rarely recommends adding potassium or phosphorus to our black land soil, but raised beds with purchased topsoil may need them.
Blossom end rot is a frustrating problem because by the time you see symptoms, the damage can’t be stopped. Consistent soil moisture is the best way to prevent the problem.
Healthy, Developing Tomato. Image Credit Matthew Orwat, ANR Agent, Texas Agrilife Extension
Blossom End Rot, Ripe Tomatoes. Image Courtesy of Larry Williams, Horticulture Agent, UF / IFAS Extension.
Further Reading
Aggie-horticulture has several vegetable resources available. There are easy growing articles on all the vegetables that grow here. General gardening guides on growing vegetables and other plants are also available. There are cucurbit , tomato, and watermelon problem solvers with pictures of affected plant parts. Finally, you can always submit something to the Plant Pathology Lab, although that costs money.
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