What Causes Blossom End Rot On Summer Squash

Blossom end rot on summer squash is caused by a calcium deficiency in the developing fruit, which is almost always triggered by inconsistent watering rather than a lack of calcium in your soil.
 
This frustrating disorder shows up as a dark, sunken, leathery spot on the blossom end of the squash, turning what looked like a perfect harvest into something you have to toss.
 
The good news is that blossom end rot on summer squash is completely manageable once you understand what’s really happening beneath the surface.
 
It’s not a disease, it’s not a pest, and it’s not something that spreads from plant to plant.
 
Instead, it’s a physiological problem, meaning it comes down to how the plant moves nutrients and water through its system.
 
In this post, we’ll break down what causes blossom end rot on summer squash, why calcium matters so much, and how to stop it from ruining your crop.
 

What Causes Blossom End Rot on Summer Squash

Blossom end rot on summer squash happens when the fruit can’t get enough calcium during its rapid growth phase.
 
Calcium is essential for building strong cell walls, and when it’s missing, those cells break down, creating that telltale dark spot.
 
Here’s the twist most gardeners don’t realize: your soil probably has plenty of calcium already.
 
The real problem is that calcium can’t travel to the fruit fast enough, usually because of uneven moisture.
 
Calcium moves through the plant dissolved in water, so when watering is erratic, the fruit gets starved of it even in rich soil.
 
Several factors can cause blossom end rot on summer squash by disrupting that calcium delivery, and we’ll look at each one below.
 

1. Inconsistent Watering

The number one cause of blossom end rot on summer squash is inconsistent watering.
 
When the soil dries out completely and then gets drenched, the plant’s calcium uptake gets thrown off balance.
 
Summer squash grows fast, and the fruit needs a steady supply of moisture to pull calcium into its developing cells.
 
If you let your plants wilt between waterings, you’re basically rolling out the welcome mat for blossom end rot.
 

2. Calcium-Poor or Unbalanced Soil

Sometimes the soil genuinely lacks calcium, especially in sandy or acidic ground.
 
But even then, the calcium deficiency that causes blossom end rot on summer squash is often about balance rather than absence.
 
Too much nitrogen, potassium, or magnesium can compete with calcium and block its absorption.
 
That’s why dumping fertilizer on your squash hoping to fix the problem can actually make it worse.
 

3. Drought Stress and Extreme Heat

Hot, dry weather speeds up transpiration and pulls moisture away from the fruit.
 
When summer squash plants are stressed by drought, they prioritize survival over fruit development.
 
This is why blossom end rot on summer squash tends to spike during heat waves or dry spells.
 

4. Root Damage or Poor Soil Structure

Healthy roots are how squash plants drink up calcium in the first place.
 
If roots get damaged by hoeing too close, pests, or compacted soil, calcium delivery suffers.
 
Poor soil structure that drains too fast or holds water too long can also cause blossom end rot on summer squash.
 

How to Prevent Blossom End Rot on Summer Squash

Preventing blossom end rot on summer squash comes down to keeping moisture and nutrients steady.
 
The fix isn’t complicated, but it does take consistency.
 

1. Water Deeply and Regularly

Give your summer squash about an inch of water per week, ideally in one or two deep soakings.
 
Deep watering encourages roots to grow downward where moisture stays more stable.
 
This steady supply is the single best defense against blossom end rot on summer squash.
 

2. Mulch to Hold Moisture

A layer of straw, grass clippings, or compost around your plants keeps soil moisture even.
 
Mulch also keeps the soil cooler, which helps during hot spells when blossom end rot on summer squash is most likely.
 

3. Test and Amend Your Soil

A soil test tells you whether calcium is actually missing or if something else is blocking uptake.
 
If calcium is low, add garden lime or gypsum, but do it before planting for the best results.
 

4. Avoid Over-Fertilizing

Stick to balanced fertilizer and go easy on the nitrogen.
 
Too much nitrogen pushes leafy growth at the expense of fruit, and it can trigger blossom end rot on summer squash.
 

5. Remove Affected Fruit

Pick off any squash already showing blossom end rot so the plant puts energy into healthy new fruit.
 
Don’t compost the diseased fruit if you’re worried about spreading spores, just toss it.
 

Can You Save a Squash With Blossom End Rot?

Once blossom end rot on summer squash appears, that particular fruit can’t be cured.
 
The damaged tissue won’t recover, and the spot will only get worse as the squash matures.
 
Your best move is to remove the affected fruit and focus on fixing the conditions that caused it.
 
New squash that develops after you correct watering and soil issues should come in clean and healthy.
 

So, What Causes Blossom End Rot on Summer Squash?

Blossom end rot on summer squash is caused by a calcium deficiency in the fruit, usually driven by inconsistent watering rather than a lack of calcium in the soil.
 
It’s a physiological problem, not a disease, so it won’t spread from plant to plant.
 
By watering deeply and regularly, mulching, testing your soil, and avoiding over-fertilizing, you can prevent blossom end rot on summer squash and enjoy a clean, healthy harvest all season long.
 
Now you know exactly what causes blossom end rot on summer squash, so you can stop guessing and start growing.