Blossom-end Rot on Tomato
by Dr. Curtis E. Swift, Colorado State Area Extension Agent (Horticulture)
In 1888, Galloway aptly described this condition when he named it black-rot. Now called blossom-end rot (BER), this appears as brown to black leathery spots on the underside (blossom-end) of the fruit of tomato (Lycopersicon esculentum Mill.), pepper (Capsicum annuum L.), eggplant (Solanum melongena L.), watermelon [Citrullus lanatus (Thunb.) Matsun & Nakai] and other vegetable fruits. Squash also can be afflicted with this problem when two to two and one-half inches long. Affected areas are often the size of a quarter or larger, sunken and gray to black in color. As this problem progresses, one-half or more of the fruit may be affected. BER fruits ripen earlier and are usually worthless.
This disorder is caused by a deficiency in calcium (Ca), a water-soluble element. The resulting dehydration of the cells explains the parchment-like appearance of the affected tissue. In some cases, the dead tissue is colonized by various fungi producing a moldy appearance.
A number of environmental factors contribute to this problem. Planting in poorly drained soil, improper soil preparation and planting, inadequate or excessive watering, using excessive amounts of pesticide, soil pH levels below 5.5, inadequate calcium in the soil or applying the wrong form of calcium, applying too much nitrogen or using the wrong form of nitrogen, excess levels of potassium, excessive pruning, the use of plastic mulch instead of an organic mulch, and high soil temperatures all contribute to this problem. Even plant diseases such as curly-top virus are said to increase blossom-end rot problems.
Blossom-end rot is a symptom of calcium deficiency in the plant. Even with an abundance of calcium in the soil, inadequate calcium levels in the fruit can occur. Calcium movement through the plant and its accumulation in the fruit is directly related to the transpiration of water (Keiser and Mullen). If the humidity is low, excessive transpiration from leaves and stems may redirect Ca away from the fruit resulting in more BER (Kirby and Pilbeam; McLaughlin and Wimmer).
Soil moisture content plays a critical role in the movement of calcium in soil and its uptake by roots. Without sufficient soil moisture, calcium will not move to the roots. If the soil is too wet, oxygen is unavailable for root growth and calcium will not be absorbed. Calcium is absorbed only by young root tips in which the cell walls of the epidermis are unsuberized. Once a suberin layer develops in these cells, water and calcium can no longer be absorbed. Suberin is a waxy substance through which water and nutrients cannot move. These deposits form what is known as the Casparian strip. Excess soil moisture and a lack of oxygen results in the development of this suberin layer.
Since the Ca in the plant moves in the sap stream through the xylem to the fruit, soil moisture content is extremely important for Ca uptake (Hanger, 1979; Kitano et al., 1999). Franco et al. (1999) and Taylor et al. (2004) report the occurrence of BER was lower with higher quantities of water. Maintaining an even soil moisture supply is an important way to control blossom-end rot. Over-watering, however, due to the reasons described above should be avoided.
Tomatoes and many other vegetables develop fairly extensive and deep roots. Without proper soil preparation, vegetable crops are unable to develop adequate roots and calcium and water uptake is inhibited. Roots of transplants should be spread somewhat during the planting process to ensure the soil of the transplant and soil of the garden is somewhat mixed. This will alleviate problems resulting from the interface between these two soils and permit roots to spread properly. The interface between two soils is a physical barrier affecting the direction and rate of water movement, nutrient and oxygen flow in the soil, and root development.
Hoeing or cultivating closer than one foot to the plant will damage roots. This reduces water and nutrient uptake increasing problems with blossom-end rot.
Roots require specific soil temperatures to continue growth. As soil temperatures increase in the summer, root growth is reduced resulting in reduced water and calcium uptake. Mulching with an organic material helps prevent extreme soil moisture fluctuations and also helps keep the soil cool during hot summer months. The use of plastic mulch should be avoided as it has been shown to increase blossom-end rot problems. This may be due to reduced water penetration into the soil, reduced soil oxygen levels or elevated soil temperatures.
Dry soil and hot, dry, windy days create a water and calcium deficiency in the plant. Even a brief soil water deficit can disrupt water and nutrient flow in the plant. If this occurs while fruits are developing, blossom-end rot will likely develop.
Blossom-end rot is usually more severe on tomato plants gardeners have pruned or placed in cages. The unpruned, uncaged plants act as a mulch over the soil, restricting water loss by evaporation. Shading the plants is also beneficial as it reduces transpiration (by lowering plant temperature) and reduces soil moisture loss. Both of these factors allow more calcium to enter the fruit.
The use of a foliar spray is a common recommendation for the prevention of blossom end rot. A one percent (1%) calcium chloride (8 lbs. of calcium chloride per 100 gallons of water) has been thought to be beneficial. However, Drs. Hodges and Steinegger, Extension Specialists with the University of Nebraska, Lincoln, report calcium does not move from leaves to the fruits. Thus, foliar sprays of calcium won't correct blossom end rot. Tomato fruits do not have openings in the epidermis (skin) through which calcium can be absorbed. Contrary to past belief the direct application of calcium as a spray, based on information from Hodges and Steinegger, is ineffective.
The addition of limestone, gypsum or crushed eggshells to the soil well before transplanting is recommended in some states to overcome the soil calcium deficiency. The use of calcium sulfate (gypsum) at the rate of two pounds per 100 square feet of soil area is recommended in some areas. Research by Taylor, Locascio and Alligood found the form of calcium applied to the soil does make a difference on Ca uptake. The addition of calcium sulfate (CaSO4) resulted in higher soil Ca concentrations than when calcium nitrate [Ca(NO3)2], calcium chloride (CaCl2), or calcium thiosulfate were applied to the soil (listed in descending order of benefit). They also found the addition of potassium (K) reduced the uptake of calcium resulting in more problems with blossom-end rot. In high K soils as are found in much of Colorado, using a complete fertilizer containing K will contribute to problems with BER.
Liming is recommended in areas with low pH (acid) soils. The application of up to two cups of lime per plant is recommended in North Carolina. When used lime needs to be worked into the soil 12-inches deep. The addition of lime or gypsum is not, however, recommended in Western Colorado. For a more in-depth discussion on Western Colorado soils, refer to the Web Pages on this topic.
Maintaining the proper balance of potassium, phosphorus and other soil nutrients and avoiding excessive growth due to over-fertilization with nitrogen is recommended. Excess levels of ammonium (NH4--N), magnesium, potassium and sodium have been reported to reduce the availability of calcium. English et al., report the use of nitrate nitrogen (NO3- - N) stimulates Ca uptake while ammonium nitrate (NH4- - N) reduces the uptake of CA. A soil test should be conducted to help determine what needs to be added and what should not be added to your garden soil.