Anyone else have these brown spots on their tomatoes and wonder what is going on? This is called Blossom End Rot, BER, and is a common tomato issue. It is a physiological disorder, not a disease, and directly linked to calcium, Ca. It is also more commonly seen in roma, determinate and some heirloom varieties but rarely in cherry tomatoes. 

I’ll try to sum it up and drop a little plant biology 101:

Plants have two main transport systems for nutrients. The xylem is a one way channel from the root zone where nutrients like calcium are absorbed from the soil and transported in water to the leaves and fruits. The phloem is a two way channel that transports nutrients garnered through photosynthesis from the leaves back toward the roots. 

Transpiration is the evaporation of water through the stomata, tiny pores, on the leaf surfaces. Transpiration causes water deficiency in the plant and hence more water to be pulled up through the xylem from water in the soil.

Calcium in the soil is transported via the xylem in water to the leaves and fruits of tomato plants in this manner!

Thats it, in a nutshell. Plants are so cool, right?

In regards to Blossom End Rot, calcium is key for cell growth and strong cell membranes. When there is a lack of it in tomato fruits, particularly at the blossom end we see the characteristic brown rot. 

At first look you might think your soil is deficient in Ca if you are experiencing BER. However, this is rarely the case. Instead the following reasons are usually the culprit for low Ca levels in your tomatoes. 

1. Low levels of transpiration due to cool or humid weather decrease the amount of Ca being transported to your fruits. This is often the case for tomatoes that were planted early in the season when weather is incliment and often the first fruits are affected. 

2. Fast plant growth early in the season hastened by excess nitrogen, will send most of the Ca to leaves and spread available Ca thinly through developing tomatoes causing BER in early fruits. 

3. Excessive heat causes leaves to transpire more quickly and the plant cannot transport enough water containing Ca to developing fruits to keep up with their needs. 

4. Dry soils will inhibit the flow of water and Ca to fruits and leaves. This can be a problem more often for potted tomato plants.

What to do? 

In many cases the problem of BER will sort itself out as the summer season progresses. Simply pull off the fruits you see with BER and often the next flush of fruits will not have this problem especially if you suspect it was caused by numbers 1 or 2 above.

If excessive heat is causing rapid transpiration then there are a couple of options as well. Be sure your soil is well mulched. This will keep the root zone cooler and retain water to keep the thirsty foliage and fruits quenched. 

You could also consider shade cloth. 30% shade cloth hung over plants with stakes will slow transpiration and reduce extreme summer temps that can also cause fruits to not set at all!. In seed growing circles it is common knowledge that pollen dies at temps above 95F which in turn slows and even stops setting of tomato fruits during heat waves. 

Finally, be sure you are giving your tomatoes adequate water. They require 1-2 inches per week which is about 6-12 gallons per sq yard in your garden. Aim to water deeply so the soil is uniformly moist at least 8 inches down. Deep, less frequent watering partnered with mulch is a sure bet for healthy plants! 

I found this article published by the University of California Cooperative Extension to be very helpful in understanding BER more thoroughly and recommend a close read. The author makes it clear that soil supplies of calcium are rarely the culprit but through specific types of soil testing this can be determined. 

If calcium is an issue in your soils there are many great ways, depending on your soils, to amend including gypsum, lime and oystershell. The internet also abounds with folks making their own amendments using saved eggshells that they alternately grind or soak in water. If BER persists it is also a good idea to check your soil ph which should be around 6.5.

In 2021, after a soil analysis, we amended our garden soils using gypsum at a rate of approx 30 lbs per 1000 sq ft. That year we also employed wide scale mulching using 30 bales of organic rice straw. I believe these measures both contributed to a very low incidence of BER in 2021 with only some fruits effected early in the season. 

I hope this info is helpful and wish you bountiful tomato harvests in 2022!

Kalan