Why Is My Zucchini Rotting?

Blossom-End Rot

• Blossom-end rot is caused by a lack of calcium in the developing fruit. The soil may be deficient in calcium, or conditions may be present that make it difficult for the plant’s roots to access the calcium, such as dry soil. The main symptom is the appearance of small, light brown spots on the blossom end of the immature zucchini, which eventually expands and becomes sunken and leathery. Hard, brown areas may develop inside the fruit. Test the soil for calcium and add lime to amend, if necessary. Maintain an evenly moist soil by watering to supplement rainfall.

Lack of Pollination

• Zucchini blossoms need to be pollinated for the fruit to develop fully. The fruit will begin to grow, but will shrivel up, die and rot if the female blossoms have not received sufficient pollination. Bees generally transport pollen from the male flowers to the female flowers, but under some conditions, such as rainy weather or the presence of pesticides, this may not occur. Zucchini blossoms can be hand pollinated to ensure the female flowers are fertilized. This is best done in the morning, when the blossoms are open and high humidity creates ideal conditions for it to be transferred readily. A single male flower can be re-used to pollinate several female flowers.

Phytophthora Blight

• Zucchini is susceptible to Phytophthora foliar blight and fruit rot, caused by the fungus P. capsici. It is spread easily by wind and rain. Symptoms first occur on the zucchini plant’s leaves, but eventually spread to the fruit. The rot often starts on the underside of the fruit where it is in contact with the soil. Sunken, water-soaked areas appear on the fruit and are soon covered by white fungal growth. The rot progresses rapidly, resulting in a complete collapse of the fruit. Infested areas will remain infested for many years. Infested fruits should be destroyed and buried.

Prevention

• Prevention of fruit rot in zucchini consists mainly of cultural practices to increase plant vigor. Test the soil before planting zucchini and amend it as needed. Add a 2- to 3-inch layer of mulch in the form of organic materials such as grass clippings, pine straw or leaves, or black polyethylene mulch. Mulching maintains soil moisture and discourages weed growth. Follow recommended rates when applying fertilizers. Do not overfertilize the zucchini plants with nitrogen or potash, which may inhibit the uptake of calcium. Water to keep the soil evenly moist, particularly during extended dry periods.