Blossom End Rot

and Related Conditions in Vegetables: What It Is and How to Deal With It

(a summary of selected literature on blossom end rot)

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There are few tomato and watermelon growers who are not familiar with the symptoms of Blossom End Rot (abbreviated as BER). As the name implies, it is a problem that appears near an area of the fruit that corresponds with a point associated with the flower blossom. Opposite the end of stem attachment, it is the area on the fruit where the pollen-receptive portion of the flower was located before the fruit began to develop. Other fruiting vegetables including peppers and squash may also be affected. Similar conditions are also found in leafy and cabbage family vegetables such as lettuce and cauliflower. In this case, localized areas of leaf or flower stalk tissue are affected.

 

In the fruiting vegetables the problem first appears as a small blemish, or “lesion” near the end of the fruit opposite the point of stem attachment. As the fruit enlarges and matures the blemish gets larger. It may remain fairly small or may cover half, or even more, of the fruit. This lesion is basically dead tissue which is often described as leathery. Sometimes the area may become infected by fungi or bacteria that cause additional decay and development of a wet appearance. In some cases, BER may remain internal and only become obvious when the fruit is cut open. BER typically begins when fruit is very small and only becomes obvious as fruit enlarges.

 

What causes BER? For many decades it has been attributed to a calcium deficiency in the fruit. This is easy to understand because calcium is an essential component of plant cells. Plants with severe calcium deficiency cannot grow normally and have new shoots that are deformed. However, if this is the case, why are healthy tomato plants often found to have BER on the fruit that are first set? To explain this it is necessary to understand how calcium gets into plants from the soil. Calcium enters the plant with the transpiration stream (soil water that enters the plant through the roots) and becomes part of the plant tissue somewhere along the way. Once incorporated into tissue, calcium does not move further within the plant. This is not the case for some nutrients, such as nitrogen and potassium, which may be relocated within the plant as it develops. For this reason, if root uptake of calcium is inhibited then the newly developing shoots and leaves are the first to be affected. Many sources explain that the reason that healthy plants have fruit with BER is that it is a result of poor calcium distribution within the plant. Because of how calcium moves in the plant, some parts, such as leaves, may have many times higher calcium concentration than parts such as fruits. For the most part, calcium moves upward from the root to the stems and leaves and fruits, but there is very little movement downward. It is further explained that there is more calcium in plant parts that have high transpiration. Transpiration is the process by which water exits the plant through stomata. Leaves are specialized for having transpiration, which is a normal plant function. Fruit, on the other hand, have very little water loss. This is obvious if one compares how quickly a tomato leaf removed from the plant desiccates as compared to a fruit. As water enters the plant and moves to the leaves it carries calcium with it. By the time water exits the leaves the calcium has been incorporated into plant tissue, much of it in the leaves. Because there is not much water flow through fruits, much less calcium is moved into fruits than into leaves. Fruits are developed mainly from the phloem translocation, which tends to move downward from the leaves to the fruit.

 

Calcium cannot move readily in the phloem; thus as the fruit is developed, it is difficult for calcium to move into the fruit and calcium in the fruit is deposited there when the fruit is first being formed.

Why does this condition affect the “blossom end” of fruit? Calcium enters fruit at the stem and moves through the vascular system of the fruit. Some calcium is “dropped off” as the transpiration stream progresses through the fruit. Therefore, the concentration has decreased by the time it reaches the “blossom end”. In watermelon, calcium concentration was measured and found to be lower at the blossom end than the stem end.

 

When does BER show up in a planting? Although some generalizations can be made, BER can be influenced by a number of factors and the incidence of the condition may depend on environment, plant health status and soil conditions. Consequently, it may be brought on when any of these conditions have a strong influence. BER shows up most often in the first tomatoes that are produced in the spring. BER may cause fruit to ripen earlier, meaning that affected fruit may ripen, in a sense, “prematurely”. However, the problem is typically found in the early part of the harvest period and later fruit are not affected. While it is not easy to clearly explain why this is so, an examination of literature on the conditions associated with BER should be helpful for getting a better understanding of the factors that may favor the problem and those that may be helpful for reducing the incidence of BER.

 

Plant growth is an important consideration both from the standpoint of the overall plant as well as the growth of individual fruit. Blossom end rot often begins to appear when fruits are very small, often a fraction of an inch in diameter. Young fruit grow very rapidly, which makes them highly susceptible to BER. Ironically, vigorous plant growth may aggravate the problem. Apparently, this is because when plants are growing slowly there is more time for calcium to move into the developing fruits. For this reason, excess nitrogen fertilizer may promote vigorous growth, which in turn favors BER.

 

Soil moisture is one of the most frequently mentioned factors that influence BER. Factors that seem to favor BER are soil moisture extremes and fluctuations. It is probably safe to make a general statement that any interference with plant water uptake can lead to BER. Thus, drought conditions or excess soil moisture, which both interfere with plant water uptake, can lead to the problem. Some suggest that even mild drought stress can lead to BER, which makes sense if the stress happens to correspond with a period of rapid fruit development and a need for calcium.

 

Environmental conditions are another type of factor that is attributed to incidence of BER. This is probably because environmental factors can have a strong influence on plant water and nutrient uptake. Strong wind and low humidity cause a lot of the moisture to evaporate through the leaves, which causes the calcium to be deposited in the leaves, but not in the fruit. Each of high humidity, low humidity and hot and dry winds are factors that favor BER. It may seem confusing that such diverse conditions favor the problem. However, each of these situations would be detrimental to water uptake by plants and consequently favor reduced water and calcium uptake. Salinity is another soil environment condition that favors BER and that is especially important in arid climates.

 

Nutrient availability to plants can have an influence on BER. As already discussed, BER is thought to be a calcium deficiency in fruit that is caused more by a problem of calcium distribution in the plant rather than low soil calcium. However, there are occasions when soil calcium is low and calcium application may reduce BER. This was shown in studies conducted in Oklahoma in which watermelon fertilized with gypsum had reduced BER. Nitrogen can have several effects. Excess nitrogen fertilization leads to vigorous plant growth, which, as already discussed, can be a source of BER.

 

Nitrogen form can also be important and when it is supplied in the ammonium form in excessive amounts it can further induce BER. High potassium and magnesium levels in the soil have also been found to favor BER, probably through interactions with calcium. Cations such as K⁺, Mg⁺, and NH4⁺ can all compete with the calcium cation Ca⁺⁺. Since they all have positive charges, they all tend to compete with each other for negatively charged exchange sites. In greenhouse production using hydroponic culture, BER can result from improper nutrient level maintenance and day/night cycling.

 

Several additional factors have been associated with BER. One of these is root damage which can be the result of disease, root injury by cultivation, and nematodes. It is likely that improper use of herbicides that interfere with root growth could have similar effects. Poor soil preparation, such as the presence of shallow, impermeable soil layers that impede root growth is another factor that can favor BER. The reason these soil factors can contribute to BER is that, for the most part, calcium can only be absorbed through the extreme tip of the roots. Old portions of the root have membrane barriers which prevent calcium from moving across the membrane. Even the tip of older roots tends to block calcium uptake. Thus, the root needs to be actively growing, with healthy, young permeable root tips, in order for calcium to be taken up by the plant.

 

What can be done to help prevent BER? Considering all that has been discussed on the factors that can lead to BER, it seems that a first step is to use good planting and growing practices. This includes:

• Site selection to avoid poor drainage and wind exposure. Raised beds may be helpful in poorly drained sites. Wind exposure is a major cause of BER in Oklahoma.
• Soil testing and nutrient management, proper soil nutrient balance, avoid excess nitrogen fertilizer
• Proper liming based on a soil test. Too much or not enough can each be detrimental
• Avoid salinity problems
• Good disease control practices such as using resistant varieties and proper crop rotation
• Proper soil preparation
• Careful herbicide use
• Avoid planting when soil is still too cool, which impedes good root development
• Carefully selected and managed irrigation practices. Practices that are used should avoid excess wet or dry conditions. One should strive to maintain a uniform soil moisture environment. Drip or trickle irrigation is a good system for achieving greatest control of soil moisture.
• Use good mulching practices to reduce soil water fluctuation

There are additional factors that can also be important:

• Some cultivars and fruit types are less susceptible to BER. In watermelon, elongated fruit types are often found to be more susceptible and spherically shaped varieties are sometimes referred to as being “resistant” to BER. Some tomato varieties and small fruited types are less susceptible. Growers may want to ask seed suppliers about the availability of such varieties. Likewise, certain pepper types are more susceptible than others.
• Planting of severely “hardened-off” transplants should be avoided, as well as overly succulent plants. Overly hardened transplants would not have healthy actively growing root tips. Also, the stem would be hardened which would restrict water uptake. Overly succulent plants would probably lose too much of the transpiration stream through the foliage. The trick is to try and get the transpiration water to flow to the fruit as much as possible and not solely to the leaves.
• Remove BER affected fruits. Although BER will not spread to other fruits, subsequent fruits may be less affected. Let the plant put its energy into new fruit development.
• Avoid severe tomato plant pruning
• Foliar calcium sprays are sometimes recommended. However, research has not shown this to be a very reliable practice for most situations. A grower’s efforts would probably be better directed at careful application of the already discussed practices.