Blossom-end rot is a common physiological problem associated with growing conditions that can affect susceptible plants anywhere. It affects fruit. Stems and leaves show no symptoms.

Causes

Blossom-end rot is a non-infectious disease or disorder of fruits caused by low levels of calcium in the fruit. For example, a tomato’s fruit needs calcium to grow. Calcium acts like glue, binding cells together. Tomatoes absorb calcium through water but calcium isn’t fast moving. If a tomato grows rapidly, or if some other conditions slow water absorption, then calcium doesn’t circulate evenly throughout the fruit. The tomato’s tissues breakdown and leave the telltale damage of blossom-end rot on its bottom end.

Blossom-end rot on tomatoes.

Blossom-end rot on tomatoes.

Several factors can limit a plant’s ability to absorb enough calcium for proper development. These include:

  • fluctuations in soil moisture (too wet or too dry)
  • excess nitrogen in the soil (lowers calcium uptake)
  • root damage
  • cold temperatures/cold soil
  • soil pH that’s markedly acidic or alkaline
  • excessive heat
  • soil high in salts (lowers the availability of calcium)

Plants Commonly Affected

Tomato, pepper, cucumber, squash, eggplant, watermelon and other fruiting vegetables.

Blossom-end rot on a green tomato.

Blossom-end rot on a green tomato.

Damage and Symptoms

Blossom-end rot occurs on both green and ripe fruits and is characterized by a water-soaked, sunken, or brown spot on the blossom end of the fruit when the fruits are approximately half their full size. The spot then enlarges and turns into a leathery brown or black patch. If the problem is severe, the fruit will have a flattened or somewhat concave bottom end. Blossom-end rot will not spread from plant to plant or from fruit to fruit. The plant will generally show no signs of damage, yet the fruit will show tell-tale signs of blossom-end rot.

Organic Control and Prevention of Blossom-End Rot

Prevention:

  • Protect young seedlings from extreme temperatures and conditions by gradually hardening them off.
  • Grow plants in soil with good drainage.
  • Plant outside at the right time when soil is warm enough. Avoid setting plants out too early in the season, which can expose them to cold soil and temperatures.
  • Soil worked with organic matter and compost will allow the plant’s root system to grow strong and deep.
  • Avoid excessive nitrogen fertilization. Ensure adequate phosphorous levels.
  • Test your soil pH; tomatoes grow best at a soil pH of about 6.5. If needed, add quick-release lime to provide plenty of absorbable calcium.
  •  Supply water evenly throughout the season to facilitate regular uptake of calcium. Tomatoes need 1-3 inches of water a week. They are known to prefer deep watering a couple of times a week rather than everyday superficial watering.
  • Avoid close, deep cultivation after fruit set, especially in dry weather. Scraping the soil lightly with a hoe is usually sufficient to control weeds in home gardens.
  • Mulch established plants to conserve moisture and provide a more uniform water supply. Suitable materials are straw free of weed seeds, corncobs, grass clippings, peat moss and newspaper.
  • Cultivate carefully around plants to avoid damaging root systems.
  • Choose resistant vegetable varieties whenever possible.

Control:

Blossom-end rot cannot be reversed on fruits once it’s set in, but steps can be taken to slow and halt it.

Blossom-end rot on a pepper.

Blossom-end rot on a pepper.

  • At the first sign of blossom-end rot, pay attention to watering and mulching. Even out water supply to the plant so that it is never super dry and then super wet, instead make sure it has a steady, even supply of water.
  • Preserve affected plants by applying calcium immediately. Spray plants with natural calcium solutions such as Enz-Rot and Rot-Stop, which are specifically meant to treat blossom-end rot by correcting calcium deficiency. Follow label directions for application. Or mix 1 tablespoon calcium chloride in 1 gallon of water. Apply in the morning when temperatures are cool.
  • Reduce stress on the plant by picking affected fruit and thus allowing it to direct its energy to other tomatoes.
  • Cut out rotted spots on harvested fruit as blossom-end rot does not make the rest of the tomato inedible. However, if tomatoes have been infected by fungi or mold, discard them.
  • Spray with seaweed extract help supply some calcium to affected plants.

Most plants usually grow out of the problem later in the season when growing conditions have been corrected. Determinate varieties are more prone to blossom-end rot because they set fruit in a short period of time, whereas indeterminate and semi-determinate varieties set fruit throughout the season, allowing for easier calcium regulation for plants.

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