This story was originally published in August 2020.
For home gardeners, nothing quite compares to the crush of disappointment of cultivating a tomato plant over the course of a growing season only to have the fruits of your labor ripen with a squishy dark splotch spreading across the bottom of the fruit where the flower once bloomed.
Despite its bruise-like appearance, the affliction isn’t from bumping into your plants. It’s known as blossom end rot, and it’s common, especially during hot and dry growing seasons like this year’s in Maine. A few simple steps can help address the problem in your current crop and prevent it going forward.
Blossom end rot is a physiological disorder caused by a lack of calcium, which in turn is caused by an environmental imbalance in the plant’s growing conditions.
“It happens because the fruit is growing faster than the plant can bring calcium to it,” said Caleb Goossen, Organic Crop and Conservation Specialist at the Maine Organic Farmers and Gardeners Association. “The cell walls in that part of the fruit get very weak [and] break down. It usually just looks like a black circle on the fruit that then can lead to secondary infection from decay organisms.”
Alicyn Smart, assistant extension professor and extension plant pathologist at the University of Maine Cooperative Extension, said that gardeners in Maine are probably seeing blossom end rot more this year than in the past because of the lack of rain.
“With the lack of rain that we’re receiving, folks are having to water a lot more often and with the fluctuation of moist to dry soil that makes it so that the plant has a hard time taking up calcium,” Smart said. “Calcium is considered a nonmobile element; [a plant] doesn’t just take it up without a water source.”
Blossom end rot usually appears on tomatoes when the fruits are quickly going from the green just-pollinated stage to to their final, ripe size.
“It’s typically worse in the beginning in the first few fruit clusters because the plant doesn’t have a well-developed root system, so it can’t have access to all the water in the soil that transports the calcium,” Goossen said.
Blossom end rot is not a condition exclusive to tomatoes, though. It can also occur in crops like pepper, squash, cucumber and melon. On peppers, Goossen said, blossom end rot can also appear on the side wall of the pepper, not just where the blossom was connected, so it is often confused with sunscald.
What to do about blossom end rot
Blossom end rot will not spread from fruit to fruit like a fungus, but crops that exhibit blossom end rot should be picked and discarded before secondary pathogens and pests are attracted to the area.
“It’s never going to get better,” Goossen said. “Take it off the plant. You could possibly cut it off [and eat around it]. It’s not going to be harmful to you to eat it but there is a risk that it’s going to let in secondary decay, mold, fungus or bacterial soft rot that will make the tomato turn into a gross mess.”
To prevent other fruits from exhibiting blossom end rot, the best thing you can do is make sure you are watering consistently and thoroughly.
“Really, it comes down to the regular watering schedule,” Smart said. “Watering in the morning is best so that the roots don’t sit in the water throughout the night.”
Some garden centers sell soil additives or foliar sprays that purport to add calcium to suffering gardens, but Goossen said that they likely won’t work as well as simply watering more thoroughly and effectively.
“It’s questionable whether that has any impact, [though] it might feel like it does,” Goossen said. “In a normal year somebody might see blossom end rot, spray a product and the next set of fruit — which would be fine anyway — looks good. It’s unlikely that [the spray] is actually helping.”
Prevent blossom end rot in the future
As with many garden woes, the strongest line of defense against blossom end rot is preventative. When planning your garden for next year, Goossen and Smart agreed that the most important thing is to get a soil test and adjust your soil according to the professional recommendations.
“There is a certain pH sweet spot, and if you’re above or below that, specific elements don’t become as available,” Goossen said. “They might be bound up by the soil making it unavailable for the plant to take it up. You might have to [add] lyme if the pH is off. ”
If you are fed up with the heartbreak of blossom end rot, you can also try to choose varieties that are less susceptible to it, such as sungold and other cherry tomato varieties. Keep garden records of which varieties experienced blossom end rot.
“There’s going to be variability between varieties, which are more or less susceptible to blossom end rot,” Goossen said. “Paste tomatoes are particularly susceptible to it, [while] cherry tomatoes hardly ever experience blossom end rot. It can show up pretty bad in some of the heirloom tomatoes, which can have unusual blossom ends anyway.”
