Yard and Garden: Rot on Tomatoes and Zucchini

AMES, Iowa – There’s something rotten in the garden and horticulturists with Iowa State University Extension and Outreach answer questions about what may be causing those black or brown spots on tomatoes, zucchini and other garden produce. The horticulturists are available through the ISU Hortline to answer additional questions. Contact them at hortline@iastate.edu or 515-294-3108.

A blackish spot develops on the bottom of my tomato fruit. What is the problem and how can it be prevented? 

Blossom end rot is probably responsible for the blackish spots on the tomato fruit. Blossom end rot is a common problem on tomatoes. It appears as a brownish black spot on the blossom end (bottom) of the fruit. Secondary organisms invade the brownish black spot and cause the fruit to rot. Blossom end rot is most common on the earliest maturing fruit that ripen in July and early August.

Blossom end rot is caused by a calcium deficiency in the developing fruit. Wide fluctuations in soil moisture levels impair calcium uptake by the root system of the tomato plant. Excessive nitrogen fertilization also may contribute to blossom end rot.

To reduce blossom end rot, water tomato plants on a weekly basis during dry weather to provide a consistent supply of moisture to the plants (tomato plants require about 1 to 1½ inches of water per week during the growing season). Mulch the area around tomato plants to conserve soil moisture. Avoid over-fertilization. There is no need to apply calcium to the soil, as most Iowa soils contain more than adequate levels of calcium.

Pick and discard fruit affected with blossom end rot. The removal of the affected fruit allows the tomato plant to channel all of its resources into the growth and development of the remaining fruit.

Blossom end rot also can occur on pepper, eggplant, summer squash and watermelon.

The fruit on my zucchini squash begin to grow, but quickly turn brown and rot. Why? 

The rotting of the small squash fruit could be due to poor pollination or blossom-end rot.

For squash fruit to develop fully, bees and other pollinators must transport pollen from male flowers to female flowers. If female flowers aren’t pollinated properly, the fruit begin to grow and then suddenly shrivel up and die. Bees and other pollinators are less active in cold, rainy or windy weather. Fruit production may be poor during periods of unfavorable weather. Fruit production also may be adversely affected by the use of insecticides in the garden. Insecticides may kill bees and other pollinators visiting the garden. To avoid harming bees and other pollinators, apply insecticides in the garden only when necessary. Also, apply insecticides in the evening when bees are less active.

Blossom-end rot is a physiological disorder that occurs on tomato, pepper, eggplant and summer squash. On zucchini and other summer squash, the blossom end of the fruit begins to rot and within a short time the entire fruit has rotted. Blossom-end rot is caused by a lack of calcium in the developing fruit. In most cases, there is no need to apply calcium to the soil. Try to maintain an even moisture supply by watering once a week during dry weather. Also, do not over-fertilize plants. Uneven moisture supplies and excessive nitrogen inhibit calcium uptake.

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