Along with the tomato foliage diseases that can really challenge the gardener, there is one fruit problem that really can be frustrating. Blossom end rot can show up especially on the first fruit set. We have waited a long time to get our very own tomatoes and those first fruit sets are likely to be in trouble.
Blossom end rot hides pretty well at the bottom or end of the tomato fruit and begins to cause problems on developing fruits. Blossom end rot is not caused by a pathogen, while decay can come into play later. Research tells us blossom end rot is caused by a lack of a steady supply of calcium while the fruits are developing from the time they are very immature. If we are looking for this problem early, the signs that show up are lesions that look black and somewhat leathery in appearance.
The causes of blossom end rot are believed to be one or more of the following: a root system that has yet to develop well enough to supply the needed calcium to the fast growing fruit, perhaps the soil itself is lacking in calcium, or weather conditions have not been favorable for calcium uptake by the tomato plant.
Sometimes it can be that fertilizers used limit the needed uptake of calcium. Others address rapid foliage growth, where the calcium will go first before making its way to the fruits. Yet another potential cause and one we as gardeners can modify is an even supply of soil moisture to ensure adequate uptake of calcium. This will help eliminate the potential for drought stress, which does affect calcium uptake.
Points to remember about watering are:
- Never let the tomatoes wilt (slight wilting on a hot day even with enough soil moisture is common)
- Water deeply and wide enough to supply water to the entire root system
- Water the soil, not the plant for foliage disease prevention
- The use of mulch or composts are recommended to further control of water loss and keep a more uniform amount of water in the soil over time
A soil test can help you determine if your garden soil is lacking in calcium and adjustments can be made using finely ground limestone. This process is not immediate and can take a year or more to become available. Clean egg shells can be added to the compost bin to break down as well.
Lastly, this condition cannot be reversed, so any fruits showing blossom end rot, even if looking great otherwise, should be removed so the plants energy can be directed elsewhere for future fruit set. Removing them early on is better than watching the fruit grow knowing you will be composting it in the end.
About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.