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What is Tomato Rot?

By Helga George
Updated May 16, 2024
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Tomato rot is a disorder of tomato fruit that is not caused by an organism, but by a lack of calcium in the fruit. In it, tomatoes develop a lesion at their blossom end, and are generally inedible. This disorder can be prevented through cultural methods. Tomato rot is more commonly known as blossom end rot.

Blossom end rot is a troublesome disease that can cause losses of up to 50%. It affects growing tomatoes both in home gardens and commercial operations. Initially, there is usually a small, water-soaked area in green and ripening fruit. As the lesion grows in size, it becomes sunken, turning black and leathery. At this point, pathogens can invade, totally destroying the affected tomatoes.

Fortunately, this is one of the tomato problems that can be prevented. Factors that affect the plant’s uptake of calcium and water also impact the severity and incidence of tomato rot. This disease is due to a lack of calcium in the soil, or calcium not being transported to the fruit. This lack of transport is often due to drought or overly moist soils.

Tomato plants that have been planted in cold, heavy soils are often prone to tomato rot. Delaying planting until the soil is warmer can often solve the problem. An excess of nitrogen fertilizer causes the tomato plants to produce an excessive number of leaves. This can exacerbate blossom end rot. Cultivating around the plants can damage the roots and inhibit water and calcium uptake, also causing this disease.

Preventative measures include treating low calcium soils with lime. Also, the pH of the soil should be monitored and kept between 6.0 and 6.5. Ammonia inhibits calcium uptake, therefore, ammonium nitrate fertilizer should be avoided, and alternate forms of nitrogen, such as calcium nitrate or potassium nitrate, should be applied. One should also be careful not to apply too much fertilizer at once, so as not to contribute to this disease.

It is very important to have a constant water supply to prevent tomato rot. Tomato plants should receive weekly increments of 1 to 1.5 in (2.5 to 3.8 cm) of water from rain or irrigation. Mulching can be helpful to help keep the soil moist. Plastic mulches, however, may cause problems under drought conditions by causing the soil to overheat. This can increase tomato rot if the soil is not watered properly.

Proper cultivation techniques are the most important way to prevent blossom end rot. One can also spray the fruit with calcium chloride while they are young, as a preventative measure. This can be done by using four level tablespoons per gallon of water every seven to ten days. Treatments should be repeated until this has been done three or four times. Be careful not to spray during the hotter times of the day, as this can burn the plant.

Gardeners and vegetable lovers can consider themselves lucky that tomato rot is one disease that can be prevented by using appropriate cultural techniques. Blossom end rot can also be a disorder of peppers, eggplants, and watermelons, if they are planted early. As with tomatoes, delaying planting these crops until the soil is warmer will reduce the incidence of this disease.

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