Zucchini Blossom End Rot Caused by High Nitrogen

Blossom end rot is one of the most common diseases of many different host food plants. It is capable of causing extensive yield loss in both agricultural and home gardening settings. Though most commonly associated with tomatoes, blossom end rot infects a wide range of host plants including zucchini, peppers and watermelons. Understanding blossom end rot's causes, including over-abundance of certain nutrients such as nitrogen, is key to protecting your plants from infection.

  1. Causes

    • Blossom end rot is sometimes described as a disease, but this is somewhat inaccurate. Blossom end rot's chief cause is thought to be calcium deficiencies in the developing fruit of fruit-bearing plants. This is not necessarily caused by a calcium deficiency in soil, but rather a conglomeration of natural factors combining to affect the plant's ability to uptake calcium available in soil and divert it to developing fruit. For this reason, gardeners sometimes treat blossom end rot infections by adding calcium to soil although this treatment rarely offers effective control of the disease.

    Nitrogen and Blossom End Rot

    • The key to managing blossom end rot is managing the natural and cultural factors that contribute to a plant's inability to uptake sufficient amounts of calcium. These factors include extreme fluctuations in available moisture in the soil, high salt content in soil, and excessive root pruning or damage. Another leading contributor is an over-abundance of plant nutrients, such as magnesium, potassium, sodium or nitrogen in soil, which is typically caused by an over-reliance on fertilizers during the growth process.

    Preventing Infection

    • Avoiding high-nitrogen fertilizers whenever possible is the easiest way to avoid blossom end rot in zucchinis or in any other host plant. Have your soil tested by a local university extension office to see if it is actually deficient in nitrogen enough to warrant fertilization. If it is, use only as much fertilizer as needed to bring nitrogen to ideal levels. For general fertilization, use low-nitrogen formulations such as 4-12-4 or 5-20-5. For side-dress applications needed to maintain green color and normal growth, use calcium nitrate or ammonium sulfate at the rate of 1/4 lb. per 100 square feet.

    Other Considerations

    • Zucchini plants suffering blossom end rot can sometimes be treated with calcium chloride. However, the fruit harvest is difficult to save. Avoiding over-fertilization is critical to avoiding blossom end rot. It is also necessary to manage the other factors contributing to this disorder, such as salt content in soil, protecting the zucchini plants from extreme cold or heat, and regular soil testing.

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