Blossom End Rot

With warm-season vegetables in their peak production at this time of year, inquiries pertaining to blossom end rot have been abundant. Prevalent in tomatoes, but also found on peppers, squash, cucumber, eggplant, and watermelon, this physiological disorder is purely the result of a calcium deficiency within the plant itself. The causes of said deficiency, however, can differ –making their treatments differ as well.

The first, most obvious, cause is a simple lack of calcium in the soil. Though this is rare in the area due to an abundance of limestone, the amendment comes in the form of fertilizer following label instructions. It should not be assumed, however, that your soil is actually lacking calcium. More likely, the plants themselves are lacking calcium uptake due to a high soil pH. In this instance nutrients may be abundant in the soil but they are essentially locked in place and unabsorbable because of the soil alkalinity. Lowering this pH with a sulfur fertilizer, the nutrients become unlocked. A pH close to 6.5 is recommended for vegetable gardens for calcium uptake, and this information and the amendments necessary can be relayed to you through a soil test.

Looking elsewhere, a lack of calcium uptake, ultimately leading to blossom end rot, can also be caused by sharp changes in outdoor temperatures, extreme fluctuations in soil moisture, drought, root damage, waterlogged roots, and too much growth from overfertilization with nitrogen. The list is extensive. However, noting your gardening practices and the environmental conditions your garden is facing can be quite useful in this instance as you work backward to address the problem.

As always, a mulched garden bed will assist with soil moisture and temperature issues. The more consistent you can keep these soil factors, the better off your plants will be. Remember as well, it is best to somewhat mimic rainfall patterns to ween plants off excessive moisture through successive watering efforts. If an abundance of rainfall has fallen, followed by extended drought, this fluctuation is likely to cause blossom end rot. This itself can be mitigated by not withholding irrigation events after rainfall, but slowly decreasing the amount of subsequent water applied. Essentially, slow down the drying out process. Of course, this practice should still be done with water conservation practices in mind, aiming for a non-oversaturated soil.

If fertilizing, opt to utilize a calcium nitrate fertilizer over ones that release nitrogen in the form of ammonium, as excess ammonium ions in a soil profile can result in reduced calcium uptake. Also do your best to avoid overfertilization, so to avoid too rapid of plant growth. This is especially import as fruit are small, as the rapid growth will show up in them as well, likely in the form of blossom end rot, cracking, or catfacing.

And then, remember as well, there is only so much we can do to prevent blossom end rot. If weather conditions for it are favorable, it will likely appear. Remove these fruit and allow your plants to push their nutrients toward trying again.

By Anthony Reardon, Horticulture Small Farms Agent, K-State Research and Extension/Johnson County

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