Combating Blossom-End Rot

Blossom-end rot is a serious disorder affecting vegetables. It is seen most commonly on tomatoes but can also be found on peppers, eggplants, melons, and squash. This is a physiological problem, not a disease caused by bacteria, viruses, fungi, or insects, but by fluctuating soil moisture that causes calcium deficiencies and hampers cell formation in the fruit; therefore, blossom-end rot cannot be cured with common treatments such as fungicide or insecticide.

The symptoms usually begin as a small wet spot at the blossom-end of the fruit that appears just as the fruit is ripening. As the spot develops, it becomes a larger, sunken, black, and leathery lesion. It is rather gross-looking, especially as the lesion grows, clearing the way for secondary pathogens to enter the fruit and further decay. To avoid the loss of your summer vegetables, you need to learn more about the factors that contribute to blossom-end rot.

Factors including rapid plant growth, soil moisture fluctuations, ambient air temperature, and climate contribute to a lack of calcium in the plant and can cause blossom-end rot. Here’s how: calcium and other minerals are dissolved in water and taken throughout the vascular system of the plant. Under high moisture stress, the water moves rapidly through the leaves, causing much of the water to be lost through transpiration and leaving minerals behind in the leaves.  Because fruit does not transpire as much as the leaves of a plant, fewer minerals can be left in the fruit. Most of the calcium in a mature fruit, contained in its waxy skin, is deposited when the fruit is the size of a thumbnail. When a calcium deficiency happens, the cells collapse on the area of rapid growth (the end of the fruit), producing the sunken end symptom of blossom-end rot.

Subsequently, the season’s first fruits are most susceptible to blossom-end rot.  By the time the second set of fruit is developing, the plant has a larger root system capable of supporting the plant’s needs for water, calcium, and other minerals.

Remove affected fruits when the symptoms first appear. Avoid over-fertilizing with much nitrogen fertilizer, and instead use tomato fertilizer of 5-10-10.  Excessive use of nitrogen fertilizer yields greater leaf growth, but remember we are growing these plants for fruit. Keep the soil moisture level consistent, being sure the soil does not get too dry between watering or rain showers.  Plants generally need one inch of rain each week.

So, what should you take away from all of this? When growing tomatoes, peppers, eggplants, melons and squash be consistent with watering, keeping the soil evenly moist. When selecting a fertilizer, pick one lower in nitrogen and higher in phosphorus.  To determine the best fertilizer, look for those labeled with a lower first number, which represents nitrogen, and a higher second number, which represents phosphorus, as with the fertilizer suggested above, 5-10-10.  Spraying calcium does not really help because little is absorbed by the plant and even less reaches the fruit where it is really needed. Varieties of plants that are resistant to blossom-end rot have yet to be developed, so be diligent and remember this advice to produce the prize vegetables you’ve always wanted.

Barbara Arnold is green corps coordinator at Franklin Park Conservatory.