You’ve been babying these tomato plants since May. Then one morning you go out to check on that first big beefsteak and the bottom of it looks like it’s been through something. Sunken, leathery, dark brown or black, sometimes with a weird flat spot that’s almost caving in. Your first instinct is probably “blight” or “some kind of rot fungus is going to wipe out my whole crop.” Take a breath. It’s blossom end rot, and it’s not a disease at all.
So what’s actually happening?
Blossom end rot is a calcium deficiency issue, but here’s the part that trips people up: it’s usually not that your soil lacks calcium. It’s that the calcium isn’t making it to the fruit. Calcium moves through a plant via water, and it travels slowly, mostly ending up in leaves rather than fruit. When the plant is stressed by inconsistent watering, heat swings, or root damage, calcium delivery to the developing tomato gets interrupted. The tissue at the blossom end, farthest from the water source, is the first to suffer. It literally starves and collapses.
July is peak blossom end rot season for a reason. This is when temperatures spike, afternoon thunderstorms dump an inch of rain and then disappear for a week, and plants are pushing out fruit as fast as they can. That boom-bust water cycle is the real culprit way more often than actual soil chemistry.
It’s not contagious, and it won’t spread
This is the good news. Blossom end rot isn’t caused by bacteria or fungus, so it can’t jump from one fruit to another or infect your whole plant. You’ll often see it on the first tomatoes of the season and then watch it taper off as the plant settles into a more even rhythm and its root system matures. Pull the affected fruit off (it’s not coming back, no matter how good your intentions are) and move on.
What actually fixes it
- Water on a schedule, not a whim. Deep, consistent watering two or three times a week beats a light daily sprinkle every time. Tomatoes want about 1 to 1.5 inches of water a week, delivered slowly so it soaks deep instead of running off.
- Mulch like you mean it. A 2 to 3 inch layer of straw or shredded leaves around the base keeps soil moisture from swinging wildly between soaked and bone-dry, which is exactly the swing that triggers this problem.
- Skip the eggshells-in-the-hole trick. It’s charming folklore, but eggshells break down way too slowly to add usable calcium in one season. If a soil test actually shows low calcium (rare, but it happens in very sandy or very acidic soil), garden lime or gypsum worked into the soil is a better bet.
- Don’t overdo the nitrogen. Heavy nitrogen fertilizer pushes fast, leafy growth that outcompetes fruit for calcium. If you’re feeding your tomatoes weekly, ease up and let them focus.
- Watch your container tomatoes especially. Pots dry out fast in July heat, sometimes twice a day fast, which makes blossom end rot way more common in container-grown plants. Bigger pots and consistent watering routines help a lot here.
One thing worth saying plainly: you can’t undo blossom end rot on a fruit that’s already got it. There’s no spray or additive that reverses the damage once the tissue has collapsed. All you’re doing going forward is preventing it on the next round of fruit, which is honestly good enough. Most gardeners see the problem hit hardest early in the season and then largely disappear by August once watering habits catch up and the plant’s roots are established. Consistency is boring advice, but it’s the actual fix here, not some miracle product.
