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Blossom end rot is the scourge of many tomato growers. In addition to tomatoes, it can also be found in peppers, eggplant as well as squash and watermelons. When it comes to tomatoes, it is most commonly seen on larger fruited varieties, with long-fruited varieties (Roma type) being more susceptible than round varieties. Blossom end rot starts as a light tan water-soaked lesion (spot) at the end of the fruit where the blossom was (opposite end from the stem). The lesion will continue to grow and eventually turn black and leathery. Additionally, fungi and bacteria will often move in and case tissues to decay.

Contrary to popular belief, blossom end rot is not caused by a disease or insects. Blossom end rot is actually a physiological disorder. The cause of blossom end rot is poorly understood, it has commonly been believed that it is caused by low levels of calcium in developing fruit.

There are several reasons why plants may not be getting enough calcium. A lack of calcium in the soil is usually not the issue. Most of the time there is sufficient calcium in the soil, but the ability of the plant to take it up can be a problem. Calcium uptake by plants depends on active transpiration (leaves loose water in order to take up more water as well as nutrients from the soil, among other things) and anything that inhibits transpiration can inhibit calcium uptake. Additionally, leaves transpire more than the fruit so calcium moves into the leaves easier than the fruit. Factors that can contribute to blossom end rot include drought stress, fluctuating soil moisture, cold soils, waterlogged soils, or rapid vegetative growth due to excessive nitrogen fertilization. High concentrations of ammonium, potassium, and magnesium in the soil can also inhibit calcium uptake.

Others argue that cause of blossom end rot is abiotic stress to the plants, not a lack of calcium. Some experiments have shown that fruit in the early stages of developing blossom end rot have similar calcium levels to healthy fruit (here is a review article discussing this). These stresses, whether they be drought, high light intensity, heat cause cells in the fruits to die. The death of these cells results in blossom end rot.

Regardless of what school of thought you prescribe to, there are several different things you can do to try and prevent blossom end rot.

  • Avoid excessive fertilization of plants, especially with fertilizers using ammonium as their nitrogen source (ammonium competes with calcium uptake), during early fruiting.
  • Make sure to provide adequate moisture from fruit formation to maturity. Using mulch can help conserve soil moisture an even out the moisture in the soil.
  • Avoid frequent shallow watering. Instead, water deeply. In general, plants need about one inch of moisture per week, whether that is from rain or irrigation. If hot dry weather is predicted make sure plants are well watered before it arrives.
  • Choose varieties that are less prone to developing blossom end rot such as Celebrity, Mountain Pride, and cherry tomatoes.
  • If you feel like a lack of soil calcium is your problem make sure you conduct a soil test to see if your soil actually needs any.
  • Foliar applications of calcium don't help much because there is poor absorption and movement of the calcium to the fruit. Unfortunately once a fruit is suffering from blossom end rot there is nothing you can do, so it is best to remove the fruit and discard them.

Good Growing Tip of the Week: Because pots dry out quicker than the soil does we often see blossom end rot more frequently in container grown tomatoes (fluctuating soil moisture). Depending on the size of the pot and the tomato plants it may be necessary to water daily, if not more often, during hot, dry weather.