JoAnne Skelly: Blossom end rot on tomatoes

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 and these are ugly. The problem is blossom end rot, 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 different.

This fruit disorder is directly related to soil moisture and soil calcium uptake. BER is common when plants grow rapidly early in the season. It is exacerbated by windy, dry weather. As plants grow, they need more water, especially as the fruits are developing. If soils are 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, which is important for fruit development.

BER is often attributed to calcium deficiency, with the addition of lime being a recommended solution. Since Nevada soils are naturally high in calcium, uptake of that nutrient is the problem, not availability. Adding lime (calcium carbonate) is NOT the answer. Lime makes a soil more alkaline, and since most Nevada soils are too alkaline to begin with, lime is not a good idea or necessary. Foliar applications of calcium are of little value. Another possible factor in BER is hoeing too close to the plants. This can damage or cut shallow feeder roots preventing them from adequate moisture and nutrient absorption.

What’s a gardener to do? Maintain a soil pH of 6.3 to 7. Keep plants evenly watered from planting to harvest. This means avoiding overwatering early in the season when soils are still cool, and avoiding underwatering later as the weather heats up. As it gets hotter, mulch about two feet around the plant to conserve soil moisture. If you mulched too early, soils may have stayed too cold for good root development. Over-fertilizing, particularly with high-nitrogen fertilizers, can increase BER because nitrogen causes rapid vegetative growth. Slow-release or organic fertilizers are good options.

Some tomato varieties are more prone to BER than are others. Next year 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 all at once is hard on nutrient uptake efficiency.

JoAnne Skelly is associate professor and Extension educator emerita, University of Nevada Cooperative Extension. skellyj@unce.unr.edu.