Dark blemishes on the bottom of tomatoes often appear just as the fruit nears ripeness, expanding quickly and ruining the harvest. Understanding the cause of these marks is the first step toward saving the current crop and protecting future yields.
Identifying Blossom End Rot
The characteristic brown or black spot appears specifically at the blossom end of the fruit, the point farthest from the stem. Initially, the damage looks like a small, water-soaked lesion. As the fruit matures, this spot expands rapidly, becomes noticeably sunken, and takes on a dry, dark, leathery texture.
These symptoms define a physiological disorder caused by internal nutrient imbalances, not infectious pathogens or insects. Therefore, applications of fungicide or pesticide are ineffective.
Understanding the Physiological Cause
The appearance of the spot is linked to a localized deficit of calcium within the fruit’s developing cell walls. Calcium is relatively immobile, moving through the plant’s vascular system alongside water (transpiration). The primary challenge is the plant’s inability to transport calcium efficiently to the developing fruit tissue, not usually a lack of calcium in the soil.
Developing fruits have a lower transpiration rate than older leaves, so they receive less calcium when water flow is restricted. Inconsistent watering is the primary trigger, as alternating between dry and overly saturated soil severely disrupts nutrient uptake.
Without sufficient calcium, the cell walls at the blossom end collapse, resulting in the characteristic decay. High concentrations of other nutrients, such as magnesium or potassium, can also compete with calcium for root uptake sites.
High heat or rapid plant growth exacerbates the issue by prioritizing water movement to the leaves over the fruits. Physical root damage from cultivation can also impair the plant’s ability to draw calcium from the soil.
Immediate Management Strategies
Salvaging the current season requires stabilizing the plant’s moisture environment to ensure uninterrupted calcium flow to newly forming fruit.
Consistent Watering
Institute a strict, consistent watering schedule, providing deep and regular irrigation rather than frequent, shallow sprinkles. Aim to deliver approximately one to one-and-a-half inches of water per week, adjusting for rainfall, to maintain uniform soil moisture several inches below the surface.
Mulching and Shade
Applying a thick layer of organic mulch, such as straw or shredded leaves, helps regulate soil temperature and minimizes moisture evaporation. During severe heat waves, providing temporary shade during the hottest part of the afternoon can reduce excessive transpiration rates, encouraging water and calcium movement toward the developing fruit.
Foliar Sprays
Foliar calcium sprays are often only marginally effective. Calcium is poorly redistributed once absorbed by the leaves, and little reaches the interior of the fruit where it is needed most. Focus must remain on correcting the underlying water management problem rather than relying on quick-fix sprays.
Preventing Recurrence in Future Seasons
Long-term prevention requires addressing soil health and nutrient availability well before the next planting season. The most important step is conducting a soil test to determine existing calcium levels and soil pH.
Calcium uptake is dependent on soil acidity, with optimal availability occurring when the pH is between 6.5 and 7.0. If the pH is too low, incorporating dolomitic lime several months before planting can gradually raise the pH while supplying calcium and magnesium.
If the soil pH is high or neutral, calcium can be added without altering the acidity by using gypsum (calcium sulfate). All soil amendments should be thoroughly mixed into the planting area in the fall or early spring to allow time for them to become available to the plant roots.
Selecting tomato varieties that are genetically less susceptible to this disorder, particularly elongated or paste-type tomatoes, offers an extra layer of protection.