There is nothing quite like biting into what you think is a perfectly crisp, delicious apple, only to stop dead in your tracks after the first bite. Instead of clean, white fruit flesh, you are greeted by an alarming web of thin, brown lines winding through the center. It is a sight that can instantly ruin your appetite and make your stomach drop.

When people spot this, their minds immediately jump to the worst-case scenario. Is it full of worms? Is it a dangerous mold or fungus growing inside? As shown in the image, these brown streaks look suspiciously like tiny tunnels, leaving many fruit lovers wondering if a pest has taken up residence in their healthy snack.

Surprisingly, the culprit usually isn’t a bug at all. In most cases, these brown strings are caused by a non-infectious condition often referred to as a necrotic vascular disorder or “corking.” This happens right on the tree while the fruit is growing, and it is typically tied to environmental stress or a simple lack of nutrients—especially a deficiency in calcium or an uneven supply of water.

The brown “strings” you see are actually the apple’s damaged vascular system. Just like human veins, these tiny pathways are responsible for moving water and nutrients throughout the growing fruit. When the tree experiences stress or a lack of calcium, the cells surrounding these pathways die off and dehydrate, leaving behind dry, brown, fibrous strands instead of juicy fruit.

The good news is that these apples are not toxic or full of parasites, making them technically safe to eat. However, because the damaged tissue turns dry and woody, the affected sections can have a mealy texture and a slightly bitter taste. If you run into an apple like this, you can easily slice around the brown parts, toss it into a compost bin, or cook it down into an applesauce where the texture won’t matter!