Have you noticed tomatoes that have round, water-soaked spots on the bottom? Blossom end rot (BER) is a frustrating problem that can be quite common in vegetable gardens, as it can affect tomatoes, peppers, and squash. You might notice it after a few very hot days. It usually starts on green fruit as a small, discolored spot on the end opposite the stem that might look water soaked. The spot enlarges and tissues dry out, becoming sunken, collapsed, and leathery.

Some tomato varieties, especially paste tomatoes, are affected more than others, but all varieties except cherry tomatoes seem to be susceptible if conditions are conducive. It is more likely to occur earlier in the season when plants are rapidly growing, but it can also show up later in the season when we have hot, dry weather.

Blossom end rot results from a low level of calcium in the fruit and a lack of water balance in the plant. Most of our soils have plenty of available calcium so there is no need to add more. Fluctuations in irrigation and/or heavy application of nitrogen are often the culprits in this condition. To reduce blossom end rot, monitor soil moisture to make sure the root zone neither dries out nor remains saturated. Follow recommended rates for vegetable fertilizers, using organic fertilizers.

Improvement in irrigation practices usually results in the problem going away. Rather than watering on a schedule, you should check the soil to determine its moisture level. Either by digging down into the soil without disturbing the roots or using a moisture meter, check to make sure the entire root zone is moist but not soggy. Use mulch around the plants to help maintain soil moisture, but make sure the mulch isn't right against the base of the plants.

Blossom end rot is not caused by a pathogen, so there are no pesticide solutions.

To reduce blossom-end rot:
• Use cover crops in the off season to increase water-holding organic matter in the soil.
• Irrigate before periods of hot weather.
• Don't disturb the root area.
• Don't over fertilize. Too much nitrogen during the early fruiting period can tie up calcium and make BER worse. Manure, especially poultry manure, can make it worse.

Vegetables affected by blossom end rot are still edible if they haven't begun to get moldy. Just cut away the discolored area.

More information about blossom end rot:
http://ipm.ucanr.edu/PMG/GARDEN/VEGES/ENVIRON/blossomendrot.html

Information about growing tomatoes in the home garden: https://anrcatalog.ucanr.edu/pdf/8159.pdf

Help Desk of the UC Master Gardeners of Contra Costa County (SEH)

Advice from the Help Desk of the
UC Master Gardener Program of Contra Costa County

Subject: Summer Tomato Problems

Client: Came to our Help Desk office with multiple problems with his tomatoes and maybe on his samples of rosemary and rhododendron

Help Desk Response: Thank you for coming to the UC Master Gardener Program Help Desk with your multiple tomato and plant problems. They appear to be all typical of growing tomatoes in the home garden in the County – early blight, “catfacing”, and blossom end rot. We also made an assessment of the health of your rosemary and rhodedendrons.

First, we discussed the early blight problem on your tomato plants. In particular, we discussed that spores of the early blight fungus in your case were spread by splashing water, so it is important to keep the leaves as dry as possible. We also discussed using mulch to reduce the spread of spores from old infected leaf material in the soil, and using crop rotation techniques. Here is a link to additional information and photographs http://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/tomearlyblight.html. 

catfacing green tomatos

We further identified a disfigurement on the bottom of the tomato fruit as a physiological problem called catfacing. It occurs when weather is too cold during flowering. There is not any way to prevent this but the problem will not spread and the fruits are safe to eat.  A link to that information is here http://ipm.ucanr.edu/PMG/GARDEN/VEGES/ENVIRON/catfacing.html 

We also discussed the physiological problem called blossom end rot on your tomato. It occurs when the soil is allowed to dry too much, causing a calcium deficiency in the tomato plants. The way to reduce this condition is to make sure the soil moisture stays even and does not dry out too much between waterings. Adding calcium to the soil is not necessary. More information and photos can be found here http://ipm.ucanr.edu/PMG/GARDEN/VEGES/ENVIRON/blossomendrot.html

Finally, I looked at the specimens of rosemary and rhododendron leaves you brought in, and confirmed that there were no pests or diseases there. However, both plants appear to be water stressed and should perk up with additional irrigation.

I hope that this information is helpful. Please do not hesitate to ask for advice again. We are here to help.

Help Desk of the UC Master Gardener Program of Contra Costa County (JL)
(all pictures from UCANR links cited above)

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Note: The  UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog  (http://ucanr.edu/blogs/CCMGBlog/).