Blossom-End Rot – Causes and Cures

Q. I have started to spot the dreaded Blossom End Rot on SOME of my tomatoes. A plant may have a few tomatoes with the black spots on the bottom and then the others on the same plant are fine. The leaves and the entire plant look very healthy.

I am watering regularly and feeding weekly. However, we are experiencing very high temperature currently (in the 95-100 range) and I am wondering if this is the reason, or is it lack of calcium, or too much nitrogen, too much water, not enough water…maybe all of the above? What would be the best approach to saving the tomatoes?

A. The wet and slimy blossom-end rot is a disease and is probably not what you are seeing.

The DRY blossom-end rot is caused by stress to the plant when the fruit is maturing. Any stress, including a nutrient deficiency, can contribute to this condition. Calcium and boron are common culprits.

You should consider putting another application of Pre-Plant Mix onto your tomato rows, and working it into the top 2″ of the soil, if you have not done so in the past 4-6 weeks.  One application of Pre-Plant is sufficient for each crop of a single-crop variety of plants, however, a tomato “crop” is continuous throughout several months – not like a cabbage crop that is finished in 75-90 days.

On the other hand, in hot weather (95+ daytime temperatures) like you’re experiencing the stress could simply be heat-related.

Seriously consider putting some 25-30% shade cloth placed directly above the plants, to shade them between 11 A.M. and 3 P.M.  If you can shade them a bit during these hottest hours of the day it may relieve some of that stress.

Also, don’t hesitate to water a second time each day in that kind of weather. You don’t need a LOT of water (standing water = field capacity), but it needs to be available to the plant constantly, as the plant must transpire (sweat) to stay cool in hot weather, and it uses as much as 95& of its water to do that.

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