Blossom end rot is a common disease on tomatoes and peppers. It is characterized by a lesion that forms on the tip of a young fruit ready to ripen, on the opposite side to where it is attached to the plant. This is the point where the flower was originally found, thus the name “blossom end” rot.
The lesion is light brown, small and watery at first, then grows and becomes dark brown or black and sunken. The lesion may eventually cover more than half of the fruit and can be invaded by other organisms.
Blossom-end rot occurs when fruits are growing rapidly and therefore have a high need for calcium, yet are unable to get enough. The solution might seem to treat the plant with a calcium-rich fertilizer, such as chicken manure or almost any other organic fertilizer (nearly all contain calcium), and in fact, that recommendation is commonly made. However, studies show that simply applying calcium has no significant effect. Even if the calcium-rich fertilizer is sprayed directly on the plant’s foliage and no other treatment is applied, the calcium tends to remain in foliage and very little reaches the fruit.
The Real Culprit: Moisture Stress
In fact, blossom end rot is almost never due to the absence of calcium (calcium is usually abundant in garden soils), but to the inability of the plant to absorb calcium from the the soil. And that is most often due to uneven watering. If the plant lacks water during the critical period of fruit formation, less sap reaches the fruit which will therefore not receive its share of calcium and voilà! Blossom end rot sets in. Typically, blossom end rot occurs when the plant is repeated stressed by irregular watering or rainfall, going from moist to dry to moist again.
The solution? Always ensure constant moisture to the roots of tomatoes and peppers and blossom end rot is unlikely to occur. Applying mulch to the soil at the base of the plant is ideal because it helps keep the soil evenly moist.
Other factors to consider are:
- Adjusting the soil’s pH to close to 6,5. Calcium tends to remain insoluble and thus unavailable in soil that is either too acid (pH below 6) or too alkaline (pH above 7).
- Avoiding the excessive use of nitrogen-rich fertilizers (those with a higher first number, such as 15-10-10). They cause overly rapid green growth, draining calcium to the plant’s foliage rather than its fruit.
- Avoiding cultivating at the foot of the plant. This severs plant roots and thus disrupts the flow of calcium-bearing sap to the fruit. Here again, mulch comes to the rescue! A good mulch prevents weeds from growing, so there will be no need to cultivate the soil around the plant and blossom end rot will therefore be less likely.