Zucchini rotting on the end? Pollination failure vs blossom end rot

Iowa State Extension says rot on small zucchini fruit is either poor pollination or blossom end rot, and the fruit tells you which. A fruit that shrivels whole, flower still attached, was not pollinated; a sunken, water-soaked patch confined to the blossom end is blossom end rot. A mouldy rot that spreads from a dead flower in warm, humid weather is a third cause, a fungus.

Do not start by adding calcium or spraying. Look at the fruit first, because the three causes need different fixes (sizing the pot matters for all three). In a pot, watering swings and the lack of pollinators both hit harder than in a garden bed.

Read the fruit before you treat anything

What you see	Most likely cause	Why
Fruit yellowing and shrivelling as a whole, withered flower still attached	Pollination failure	Iowa State: the fruit begins to grow and then suddenly shrivels up and dies
Sunken, water-soaked spot confined to the blossom end, which can enlarge and turn brown to black and mouldy	Blossom end rot	Missouri Botanical Garden: distinguished from other rots by its confinement to the blossom end
Soft, fuzzy or mouldy rot spreading from a dead flower, after warm and damp days	Fungal rot	A fungus colonises failed blossoms and young fruit

Size will not settle it, because Iowa State says rot on small fruit can be either cause. Missouri Botanical Garden itself notes that blossom end rot may be confused with rot from female flowers that failed to set fruit, so look at where the rot sits and whether the whole fruit is shrivelling.

Pollination failure: fruit that shrivels whole

Zucchini carries separate male and female flowers. The female has a small swelling, a miniature fruit, directly behind the petals; the male sits on a plain thin stalk. Pollen has to travel from a male flower to a female one. Iowa State Extension describes what follows when it does not: if the female flowers are not pollinated properly, the fruit begins to grow and then suddenly shrivels up and dies. The plant has not caught a disease. It has abandoned a fruit it cannot finish.

Iowa State also links rain to this, noting that bees and other pollinators are less active in rainy weather and that rainy weather could be responsible for poor pollination and rotting of the small fruits. On a high balcony the same thing happens without any rain, simply because few bees visit.

The fix is to move the pollen yourself. University of Maryland Extension notes that squash flowers open early and stay open for less than a full day, so work in the morning. Pick a freshly opened male flower, pull back the petals, and touch its pollen-covered centre to the centre of each open female flower. A soft brush also works. The full routine, with the tell-tale features of each flower, is in how to hand-pollinate on a balcony. If your cucumbers are the ones producing flowers that do not turn into fruit, that is a different pattern, covered in cucumbers that only make male flowers.

Flowers that open and close without fruiting may simply be male; University of Maryland Extension makes that point, since male and female flowers appear separately. The male-first pattern is covered in cucumbers that only make male flowers.

Blossom end rot: a calcium and water problem

Missouri Botanical Garden describes blossom end rot on cucurbits as beginning as a water-soaked, sunken spot on the blossom end, and says it can be told from other rots by being confined to that end. Both Missouri Botanical Garden and Iowa State attribute it to a lack of calcium in the developing fruit.

That does not mean the soil is short of calcium. Iowa State says that in most cases there is no need to apply calcium to the soil, and points instead to water: uneven moisture supplies and excessive nitrogen inhibit calcium uptake. Missouri Botanical Garden, by contrast, lists calcium nitrate in the soil or a calcium chloride spray among its controls. For cucurbits in containers it recommends a fertilizer that contains micronutrients including calcium, and notes that many tomato fertilizers meet that description. Against that, Iowa State sees no need to apply calcium to the soil in most cases. The two agree on the part you can act on today, which is keeping soil moisture uniform. Missouri Botanical Garden adds mulching the plants.

Epsom salt is a popular remedy that goes the wrong way. North Dakota State University Extension says it supplies magnesium and no calcium, and that it does not stop blossom end rot but leads to more of it. The full myth is covered in Epsom salt for plants.

In a pot the mix dries fast and a big squash leaf canopy drinks a lot, so the plant swings between dry and soaked unless you set a routine. Water when the top of the mix starts to dry, soak until water runs from the drainage holes, and cover the surface with mulch. The watering rhythm for vegetables in containers is in how often to water container vegetables, and mulch details are in mulching container plants. The same disorder on tomatoes, where the sources and symptoms differ in detail, is covered in blossom end rot on container tomatoes.

A pot that is too small makes every one of these swings worse, because it holds too little water to buffer a hot afternoon. Zucchini wants far more root room than the seedling suggests, as set out in what size pot zucchini needs.

Fungal rot on dead flowers

Dead flowers that stay stuck to young fruit are food for fungi. A University of Maryland Extension answer on rotting squash blossoms named Choanephora, a common fungus that can kill blossoms or young fruits when the weather is warm and humid, as the likely cause in the case it was shown. It was working from a photo and did not confirm the diagnosis, so treat that as a pattern to watch for rather than a certainty.

The advice is practical either way. Maryland says to remove failed blossoms promptly so they do not harbor fungi that can spread to other buds, and to make sure the flowers do not get wet when you water. Water the soil, not the plant, and give the pot enough space for air to move around the leaves. Maryland adds that no fungicide is useful against it during the growing season; clear crop debris at the end of the season so spores do not overwinter.

A week of checks on the pot

1. Pick every rotting or shrivelled fruit and every withered flower, including those hanging on healthy-looking fruit.
2. Hand-pollinate every open female flower each morning until the plant is setting fruit on its own.
3. Put the pot on a steady watering schedule and mulch the surface.
4. Hold off on nitrogen-heavy feeding, which Iowa State links to poorer calcium uptake.
5. Watch the next three or four fruit. If the rot comes back as a sunken spot at the blossom end, concentrate on the water.

Sources

• Iowa State University Extension and Outreach, “Why are my zucchini fruit rotting before harvest?”
• Missouri Botanical Garden, “Blossom end rot of cucurbits”
• University of Maryland Extension, Home and Garden Information Center, via Ask Extension
• North Dakota State University Extension, “The Epsom Salt Myth”