Scott C. Ockey/Plant Disease Diagnostician
Blossom end rot is a non-parasitic disease affecting Open Pollinated / Heirloom tomato, pepper, and watermelon fruit. Fruits are usually affected when about one-third or more grown, but the disease can occur during any growth stage of the fruit. Losses caused by blossom end rot vary from negligible to severe.
Blossom End Rot Symptoms
Symptoms: The first symptom of the disorder is a slight, water-soaked discoloration on the blossom end (opposite of the stem) of the fruit. As the lesions enlarge, turning leathery and dark brown or black, they often become sunken into the fruit. Although the affected tissue is normally dry, bacteria and fungi may invade the lesion, producing a soft, watery rot.
Cause: Blossom end rot is a physiological disorder due to a shortage of calcium in the young fruit. Moisture imbalances or water stress aggravate the problem by interfering with calcium uptake in the plant.
During the day, the pores on the leaves are open and water transpires from them drawing sap into the leaves. Since the fruit do not lose much water by transpiration, they receive little of the calcium containing sap. At night, the leaf pores close, root pressure forces sap into the plant, and the developing fruit get their share of calcium and other nutrients. If the plant is water stressed at night, the system fails and the fruit receive very little calcium, causing blossom end rot. A lot of salts in the soil, including ammonium, potassium, and magnesium interfere with calcium uptake by plants and can result in blossom end rot. Root damage caused by improper cultivation practices or excess soil water may also lead to blossom end rot. The incidence of blossom end rot varies with the variety and is greatly influenced by environmental conditions.
Control: To avoid blossom end rot, the heirloom gardener should use cultural practices that allow for proper uptake of calcium by the plant. Avoid water stress of fluctuating soil moisture by using infrequent, deep irrigation to keep the soil uniformly moist. Do not allow garden plants to be water stressed at night. The best way to maintain even the soil moisture is by using organic or plastic mulch to prevent wide fluctuations in soil moisture. Avoid over fertilizing; do not use ammonium based nitrogen fertilizers. To avoid injuring roots, do not hoe or cultivate near plants. Pull weeds next to garden plants or use a plastic mulch. Do not overwater, especially in heavy clay soils.