VIU Milner Gardens and Woodland

What causes rot on tomato fruit?

by Vancouver Island Master Gardeners Association

Question:  What causes rot on tomato fruit and why does it happen?

Answer:  Blossom End Rot (BER).

BER is basically a physiological disorder related to calcium uptake directly related to watering practices. It is not a disease and does not spread to other plants.  It appears as a brownish/tan watery softened blemish on the bottom (or blossom end) end of your tomatoes although it can happen to peppers, squash, cukes, eggplants and melons. It presents itself when fruit is about half grown, progresses moderately fast and can include the whole bottom half of the fruit. One can remove the damage and still consider it edible but it can be mealy.

It can happen in any tomato growing situation but often happens in restricted soil growing systems like pots or containers that tend to dry faster than those in in-ground systems.

As tomatoes grow more progressively thru the season their water demands increase substantially.  Regular, consistent moisture is a must as they reach maturity and start to set fruit.  One of the major components of fruit formation is calcium. It is in high demand and even though your soil may have plenty available it’s the water availability in your soil that gives the plant access /uptake.  If inconsistent watering occurs where there are periods of dry, then wet and then dry again, the calcium uptake flow is interrupted and cells in the walls of the tomato are incomplete and they start to die off and you get BER.  Another factor to keep in mind is too much nitrogen fertilizer. Fruit being heavy feeders, we may inadvertently apply too much Nitrogen and the tomato grows faster than the calcium needs can keep up with. More Is Not Better.


Even, consistent watering is a must.  Moisture at least six inches down is required.   Add mulch to maximize soil water retention.  Add compost to your soil to make Calcium more available and amend with Calcium by adding small amounts of wood ash, fish meal, oyster shell or gypsum.   Planting out too quickly in cold soil can also reduce sufficient uptake, so can pH if it is too high or low and some varieties are more susceptible than others.  Damaged roots can also be a factor so cultivate gently.  Never fear , if it shows up, take note, make some changes  and carry on.  Nothing is better than homegrown tomatoes!