Blossom End Rot

Why half-grown zucchinis rot on one end

By: Sandy Feather 2010
Penn State Extension

Q. We are having trouble with our zucchini.  The plants look great but when the fruits reach four to five inches long they start to rot out at the blossom end.  We have tried one dose of Fung-onil (chlorothalonil) and that has not stopped it. Everything else in the garden looks fine so far.  What can we do to stop this problem with the zucchini? 

A. From your description, it sounds as though your zucchini are being affected by blossom end rot. Although we usually associate blossom end rot with tomatoes, zucchini and other varieties of squash are also susceptible. Despite the name, blossom end rot is a calcium deficiency in the developing fruit rather than a disease, so fungicides such as Fung-onil will not help the problem.

Blossom End Rot

Blossom end rot develops due to one or more of the following factors: calcium deficiency in the soil, drastic fluctuations in soil moisture, over-fertilization, or root damage caused by cultivation. Calcium may be present in your soil in sufficient quantity, but is unavailable to the plant due to lack of or excessive soil moisture, or because the fine feeder roots have been severed by hoeing near tomato plants. It is often worse on plants that are carrying a heavy crop of fruit because they have a higher need for calcium.

Soil Test First

To start, have your soil tested to make sure that soil calcium levels are adequate. Soil test kits are available from your local Penn State Extension office for a nominal fee. Send a check made payable to Penn State Extension to Penn State Extension, 400 North Lexington Street, Pittsburgh, PA 15208. Write Attn. Soil Test Kit in the lower left corner of the envelope. You may also pick them up in person, Monday through Friday from 8:30 a.m. to 4:30 p.m. Take the sample according to the directions in the kit, and send it to the soil analysis laboratory in University Park, PA.  Follow their recommendations for limestone and fertilizer applications.


Watering Zucchinis

Maintain even soil moisture by watering when there is not sufficient rain and by mulching your tomato plants. You may use an organic mulch such as straw, or an inorganic mulch such as black plastic. Mulching helps maintain soil moisture, moderate soil temperature, and it also keeps weeds under control.

Precautions when Cultivating

Be very careful about cultivating near squash, tomatoes, peppers and eggplants because you can sever fine feeder roots and make the plant think that it is under moisture stress - which will cause blossom end rot. It is better to hand weed around these plants since it does not cause as much disruption of desirable plant roots.

Easy on the 'N'

Zucchini are classified as light feeders, which means they do not require a high level of nitrogen fertilizer. Too much nitrogen results in a lot of vegetative growth and little or no fruit, and it also can cause blossom end rot because excessive nitrogen blocks the plant from taking up calcium.

There are calcium sprays available that can be helpful with tomatoes suffering blossom end rot, but they may not be as useful for squash and other susceptible fruits.


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