In 2010, $30 billion worth of fruits and vegetables were wasted by American retailers and shoppers in part because of cosmetic problems and perceived spoilage. That’s a poor use of about 30% of the produce on the market, not to mention the water and energy required to grow and transport it, and the landfill space getting used up by rotting fruit. So what are those cosmetic problems? You’ve probably passed over a spotty apple in the grocery store, or accidentally sunk your thumb into a mushy patch on a tomato. These blemishes can doom produce to the trash can. But what are they anyway, and are they actually bad for you?
Those spots are evidence of an epic battle between plants and microbes. Like humans, plants coexist with billions of fungi and bacteria. Some of these microbes are beneficial to the plant, suppressing disease and helping it extract nutrients. Others are pathogens, attacking the produce, still alive as it sits in a store display or your refrigerator and siphoning off molecules they can use themselves.
The good news is they’re almost never bad for you. These fungi and bacteria have spent millions of years developing strategies to overcome a plant’s immune system. But healthy human immune systems are different enough that those strategies just don’t work on us. So in a plant, what does this process look like? Microbes can reach plants in a number of ways, like getting splashed onto it during watering or fertilization. Under the right conditions, the microbes grow into large enough colonies to attack the waxy outer layer of fruit or leaves. Their target: the delicious sugars and nutrients inside.
This type of pathogen often makes spots like this. A clump of bacteria drains the nutrients and color from the fruit’s cells making that yellow halo. It then moves outward, leaving a black spot of dead cells in its wake. Each spot, which could contain hundreds of thousands of microbes is actually caused by a combination of microbial attack and the host defending itself. For example, this is the bacterial pathogen Pseudomonas syringae. Once on a tomato, it enters the fruit and leaves, multiplies in the space between the cells, and produces toxins and proteins that allow it to disrupt the plant’s immune response.
One toxin coronatine makes plants’ stomata open up, allowing bacteria to enter more freely. Coronatine also activates pathways leading to chlorophyll degradation, which you can see as yellow spots. As the bacteria continue to feed and multiply, they start to kill off the plant cells. That explains spots, but what about mushy blemishes? Those are usually caused when the fruit is attacked by microbes after it’s detached from the plant. If the plant is wounded during transport, necrotic fungi can infiltrate through the wound, kill the cells, absorb their nutrients, and leave your food looking mushy or brown.
Those spots in particular can taste pretty bad. You’re eating dead and decomposing tissue, after all. But you can usually salvage the rest of the fruit. The non-mushy spots, like the ones you typically see on apples or tomatoes, are just on the surface and don’t usually affect flavor. Of course, microbes that do make us sick, like E. coli and salmonella, can hitch a ride on vegetables, too. But because they’re not plant pathogens, they don’t typically cause spots. They just hang out invisibly on the surface.
So it’s washing fruit and veggies, not avoiding the spotty ones, that will help you avoid getting sick. So the next time you’re at the grocery store, don’t be afraid to pick up funky-looking fruit. Some stores will even give you a discount. Wash them well and store them properly, as some produce like apples and cabbages will keep in the fridge for weeks. The spotty ones may not be eye candy, but they’re safe and just as delicious.