The fruit isn’t inedible, necessarily. It’s not even rotting (unless you see actual, mossy rot). A simple lack of calcium is causing the far end of the fruit to fail; to not produce viable cells.
There’s no bringing back the dead blossom end of the fruit, but quick action can keep young and future tomatoes, zucchinis, squash et al. from suffering the same fate. The lack of calcium could be due to a number of factors. Try one (or more) of our 6 steps to combat blossom end rot.
The blossom end is opposite from the one attached to the plant, where the flower petals were once attached.
Under-watering deprives plants of calcium. A lack of calcium in the plant doesn’t necessarily mean a lack of calcium in the soil. Before adding supplements to the soil, make sure the veggies are on a consistent watering schedule. Install a drip system or soaker hose, or regularly water by hand. Veggie plants need about an inch of water a week, and more when it’s hot and they’re producing lots of fruit.
Add a layer of mulch to the top of the soil. This doesn't directly combat blossom end rot, but a layer of compost, leaves, straw or fine bark slows evaporation and protect shallow roots. It also keeps weed seeds from germinating. Weeds can compete with veggies for nutrients and water. When you lay down the mulch, be sure to leave a few bare inches around the base of the plant. If mulch touches the stem it can lead to disease.
Hydrated lime provides a heavy dose of calcium. To make a slurry: Dissolve one cup of hydrated lime granules in one gallon of water. Apply in two ways: Water each plant with 2–3 cups of slurry. Or, use a spray bottle and douse the plant's leaves. If you choose to wet the leaves, DO NOT do it in the heat of the day. It can shock the plant and encourage disease. Spray in the morning, when the plant has the rest of the day to dry off.
A classic calcium supplement for humans also works well for plants. The calcium in TUMS is designed for easy, fast absorption. Make a slurry or foliar spray with dissolved tablets. Dissolve 8–10 TUMs in a gallon of water, or 4–6 in a few cups of water. You can also stick a few tablets in the soil around the base of your affected plants so that they dissolve slowly with regular watering. Water plants with the TUMS slurry or thoroughly douse the leaves in early morning when it’s not too hot.
TUMS aren't the only calcium supplements you have lying around the house. Crushed egg shells scratched into the soil around the plant can add calcium in the long term. Another old-school fix, which may be more wives' tale than effective treatment: Spray with whole milk. The foliar spray (theoretically) provides calcium which the leaves can absorb. Our take? Anything is worth a try, but what smells worse than spoiled milk in heat? Combine egg shells with a quicker fix like hydrated lime, or use milk when in dire straits.
This is more of a long-term fix. The next time you plant tomatoes, squashes or melons, add a dose of bone meal into the planting site. It will last longer than calcium from, say, TUMs, but it will also take longer for your plant to feel its full effects. It’s a great long-term aid for fighting blossom end rot but may not cure the fruit that’s forming on the plant right now.