Blossom end rot shows up as a dark, sunken, leathery patch on the bottom of the fruit, the end opposite the stem. It usually hits the first fruits of the season, and it can appear on tomatoes that look otherwise perfectly healthy.
What Causes It
Blossom end rot is a shortage of calcium in the developing fruit. The surprising part is that the solution usually has enough calcium. The problem is getting it there:
- Calcium moves through the plant with water. It travels to wherever the plant loses the most water, mostly the leaves.
- A fruit loses little water, so it gets calcium only when the plant's water supply is steady and plentiful.
- When water uptake swings, the fast-growing end of the fruit runs short, and those cells collapse.
Why It Happens in Kratky
- The container runs low. As the level drops, the water roots can end up barely reaching the solution, then a top-up floods them again. Those swings are exactly what blossom end rot follows.
- Strong solution. As water is drunk faster than nutrients, the solution gets stronger, which makes water uptake harder.
- Heat and humidity extremes. Very hot weather or very humid air changes how fast the plant moves water.
- Damaged roots. Roots hurt by root rot can't take up water steadily.
- pH out of range. Low pH makes calcium harder to absorb. See Kratky tomato pH.
How to Fix and Prevent It
- Pick off affected fruit. It won't recover, and removing it lets the plant put its effort into new fruit.
- Keep the water supply steady. Check the level more often once fruit is forming, and top up before the water roots dry out. See topping up a Kratky tomato reservoir.
- Check strength and pH. If EC has climbed, top up with plain water or a weaker mix. Keep pH between 5.5 and 6.5.
- Make sure the nutrient has calcium. Use a complete hydroponic nutrient; some growers add a calcium-magnesium supplement with soft water. See Kratky tomato nutrients.
- Use a bigger container next time. A larger reservoir changes level more slowly. See Kratky tomato container size.
Back to Kratky tomato problems.
Frequently Asked Questions
Is blossom end rot a disease?
No. It's a disorder caused by too little calcium reaching the fruit, not an infection, so it doesn't spread from plant to plant.
Can you eat a tomato with blossom end rot?
You can cut away the damaged part and eat the rest, but the fruit often rots further, so most growers remove it from the plant.
Will adding calcium fix blossom end rot?
Only if the solution is actually short of calcium. More often the cause is an uneven water supply, so steady water levels matter more than extra calcium.