Calcium deficiency to blame for blossom-end rot

UC Master GardenerJune 12, 2013 

Blossom-end rot appears as a black, leathery blemish on the end of tomatoes.


Q: There are dark spots on the bottoms of my tomatoes. What is causing this and how do I prevent it?

A: Like a horror movie, you find yourself walking out to visit your tomato bed. From afar, you see the rosy red fruits of your labor. But, as you get closer you see something. It’s dark and foreboding. It’s yucky. Maybe you emit a silent scream. Maybe you are tougher. But, no matter, the disappointment is palpable. You’ve got rotten tomatoes.

More specifically, you have a case of blossom-end rot.

Blossom-end rot affects the blossom end of maturing fruit. It begins as a small lesion and progresses to a black, leathery blemish. But before you get your “bloomers” in a knot, have no fear; the solution is simple.

Blossom-end rot is a physiological disorder related to calcium uptake of the plant. Sandy, high-salt soils and lack of moisture lead to deficiencies in calcium. While some variation exists between plant types, no tomato plant has been found to be resistant to blossom-end rot.

Creating a good environment for your tomato plant can eradicate this problem. Amend soil with organic matter to increase its ability to hold nutrients, oxygen and water.

Go sparingly when using manures to fertilize soil; they are often high in salts which interfere with calcium absorption. Instead, use fertilizers that are high in phosphorus, such as 4-12-4 or 5-10-10.

To keep your tomatoes firm and juicy, you need just the right amount of water. This is where a “farmer’s consistency” is key. Overwatering or drought conditions affect the plant’s ability to access calcium and form healthy fruit. A drip system makes watering a snap and ensures consistent moisture levels. Set a timer on your drip and spend your time mulling over salsa recipes instead of watering.

Take care when weeding near your tomato plants. Small roots that assist in water and nutrient absorption may get damaged while pulling weeds. Mulching around your plants retains moisture and keeps weeds at bay.

With a little vigilance, you can nip those little “horrors” in the bud.

• • •

The June Advice To Grow By Workshop will be held from 10 a.m. to noon June 22. Tom Spellman, a professional orchardist with Dave Wilson Nurseries, will discuss summer pruning.


Contact the University of California Cooperative Extension Master Gardeners at 781-5939 on Mondays and Thursdays from 1 to 5 p.m. in San Luis Obispo; at 473-7190 from 10 a.m. to noon in Arroyo Grande; or at 434-4105 on Wednesdays from 9 a.m. to noon in Templeton. Visit the UCCE Master Gardeners website at or email

The Tribune is pleased to provide this opportunity to share information, experiences and observations about what's in the news. Some of the comments may be reprinted elsewhere in the site or in the newspaper. We encourage lively, open debate on the issues of the day, and ask that you refrain from profanity, hate speech, personal comments and remarks that are off point. Thank you for taking the time to offer your thoughts.

Commenting FAQs | Terms of Service