Are Your Tomatoes Rotting on the Vine? It Could be Blossom End Blight

Many vegetable plants develop disease-like symptoms from a lack of calcium. Tomatoes are perhaps the most dramatic.

Their disorder, blossom end blight – large dark sunken areas show on the flower end (the end opposite the stem) of the tomato – can damage as much as half of the fruit and sometimes causes it to fall before ripe.  Peppers also get blossom end blight, and similar problems occur in melons and squash – particularly summer squash like zucchini. A related problem, bitter yellows, affects the productivity and quality of cucumbers.

What’s the Solution? Apply Calcium

The solution to all of these disorders – and in many cases the difference between success and failure with these crops – is to provide calcium.  Ideally this is done by mixing in calcium carbonate lime at planting; but if you are already into the season and you didn’t lime your vegetables (or underestimated how much they would need), you can still save your harvest.  Here are three ways to apply this needed nutrient:

  • Fast-acting lime:  granular calcium carbonate lime that has been processed to release more rapidly.  Mix a handful of lime into the soil gently at the base of the plant.  It is faster than some sources, but still relatively slow – you’ll see results in a couple of weeks – and low in risk of plant stress.  If your tomatoes and zucchini are just at flowering or first few baby fruits are forming – or if you are seeing very minor symptoms – you still have plenty of time for this solution.
  • Calcium nitrate:  both a high nitrogen fertilizer and a quick acting calcium source.  You can mix it in water and spray it on the leaves, or mix the granules into the soil in a ring around the plant.  Quick and effective, and it only takes a little – best if you are short on time and have lots of susceptible plants.  Real risk of plant damage, particularly in hot weather: be cautious and follow label directions carefully.
  • Liquid calcium:  usually derived from calcium chloride, this is the fastest way to solve calcium deficiency; however it doesn’t last, so you will need to continue applying through the harvest season.  Spray on leaves, and repeat once per week.  Risk of plant damage when applied too heavily or during very hot weather.

Other Causes of Calcium Deficiency in Vegetables

Calcium deficiency is influenced by other factors beyond its presence in the soil.  Severe stress from very hot weather, excessive rain or irrigation, or significant drought can limit calcium availability.

Heavy fertilizing, particularly of nitrogen, also limits the plant’s ability to use calcium correctly.

Variety selection can matter too: plants that try to ripen a lot of fruit all at once (determinate tomatoes and zucchini, for example) often run out of calcium.  So do plants with heavy yields of very large fruit, such as many of the larger slicing tomatoes.

Here are a few ways to save your harvest from the disappointment of calcium deficiency disorders:

    • Select varieties less prone to these problems
    • Add calcium lime to the soil at planting
    • Don’t over fertilize
    • Water deeply and consistently to avoid both drought and overwatering
    • Supplement with faster acting calcium sources when needed.