Rip Winkel
written by: Rip Winkel -Horticulture Agent, Cottonwood Extension District
Are some of your tomatoes showing a sunken, brown leathery patch on the bottom of the fruit? If so, you probably have what is called blossom-end rot. Appearing most frequently on tomatoes, blossom-end rot can also affect peppers, squash and even watermelons.
Though not a disease, this condition is caused by a deficiency of calcium during the formation of the fruit. And, as a calcium deficiency, it is often assumed that this is due to a corresponding lack of that element in the soil. This is not necessarily the case, especially in this part of Kansas. That is because our Kansas soils are mostly derived from limestone, which is partially made up of calcium. So, what causes blossom-end rot?
Actually, there are a number of possible causes, especially on tomatoes. Some of these causes are listed here below:
1) Tomato tops often outgrow the root system during cooler spring weather. As long as it is cool, the root system can keep up with this growth. But when the weather gets hot and dry, the plant becomes stressed, sending water — which contains calcium—to the leaves, bypassing the fruit. The plant eventually responds with new root growth and the condition corrects itself after a couple of weeks.
2) Heavy fertilization, especially with ammonium forms of nitrogen, can encourage this condition. Heavy fertilization encourages more top than root growth and the ammonium form of nitrogen competes with calcium for uptake. If blossom-end rot has been a perpetual problem with your tomatoes, try using calcium nitrate (15-0-0) when fertilizing.
3) Anything that disturbs roots such as hoeing too deep can encourage blossom-end rot. Mulching helps because it keeps the soil surface cooler and therefore a better environmental condition for root growth.
4) Inconsistent watering can be a factor also. Keep the soil moist, but not water logged. Mulching can help by moderating moisture levels over time. You should also avoid doing any damage to the roots as well as watch the type and amount of fertilization you apply. Soils with adequate calcium will not benefit from adding additional calcium. However, if your soil is deficient in this nutrient, add 1pound of gypsum per 100 square feet. Gypsum is calcium sulfate and will not affect pH, (though calcium tends to raise pH, sulfate lowers it and the two cancel each other out). Even if calcium is not needed, the gypsum will not hurt anything. It has also been observed that spraying plants with calcium has no effect in preventing blossom-end rot. The fruit’s waxy surface doesn't allow absorption of the material, nor does calcium move from the leaves to the fruit.