Why Do Zucchinis Rot on the Vine?

In most cases, zucchinis rot on the vine due to poor pollination, calcium deficiencies, and squash vine borer infestations. When this happens, you’ll see your fruit start to set only to turn out brown and withered. Your entire harvest could end up rotting away if you don’t resolve the underlying problem in time. Some growing seasons are worse than others, too, so it’s hard to predict when you’ll get an excellent yield.

Why do zucchinis rot on the vine?

3 Reasons Why Zucchinis Rot on the Vine

You’ll need to put on your detective hat and start an in-depth investigation upon noticing your zucchinis rotting on the vine. Consider all of the following causes to find the one that’s leaving you without this delicious squash on your dinner table.

Lack of Proper Pollination

A lack of proper pollination will result in zucchinis starting to grow, and then turning brown and dying. Poor pollination doesn’t affect every single fruit on the plant either. Some will grow fine while others shrivel up and die within just a few days.

Zucchinis rely on bees and other pollinators to transfer pollen from their male flowers to the females. Their deep flowers do not let the pollen fly through the wind and land in the right places for fruit growth. If pollinators do not properly spread the pollen, then you will have dying zucchinis on your hands.

Thankfully, you can make up for the absence of active pollinators by following these steps to hand pollinate your zucchini plants:

  • Take a moment to tell the male flowers apart from the females (The males have a single stamen in the center while the females have a baby zucchini at the base).
  • Get a clean cotton swab and gently brush it against the stamen inside a male flower to collect the yellow pollen.
  • Rub the yellow pollen on the cotton swab against the pistils inside the female flowers.

Repeat this process for all the female flowers on your zucchinis to see if the rotting issue goes away. If it does not, then you’ll need to explore other causes to get to the root of the issue.

Calcium Deficiency

A lack of calcium can cause blossom end rot, resulting in your zucchinis rotting on the vine. This problem is most closely associated with tomatoes but affects squash plants just as often. The developing fruit needs calcium to properly grow or it will shrivel up and die before reaching its full potential.

In addition to the dying fruit, other signs of blossom end rot include:

  • Misshapen leaves
  • New leaves turn yellow
  • Overall stunted growth

You can complete a soil nutrient analysis using a rapid test kit from your garden center to determine if your soil calcium sits below the ideal range. Or you can just treat your plants with this balanced fertilizer to see if the problem resolves.

Common home remedies include watering plants with milk and adding crushed-up antacids to the soil. Although these methods work from time to time, a balanced fertilizer prevents other nutrient imbalances while boosting calcium. Fertilizing is typically the best remedy for a calcium deficiency in zucchinis.

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Vine Borer Infestation

Squash vine borers quickly wreak havoc on zucchini plants and leave their fruits rotting on the vine. Affected plants usually start wilting first. Then, the zucchini rots on the vine before the plant dies.

In most areas, vine borers start appearing in June and continue causing problems through July. As soon as they make their appearance, an adult squash vine borer will lay eggs in the base of zucchini and other plants. 

The larvae start hatching one week later and begin eating the vegetation along the middle of the vine. Water fails to properly flow through the vines as a result, killing the leaves and eventually the entire zucchini plant.

Signs of squash vine borer infestations include:

  • Wilting leaves, dying fruit, and dead plants.
  • Holes near the base of the vines.
  • White grubs inside the vines.
  • Presence of ½-inch long, winged insects with black and orange coloring.

Plants affected by squash vine borers are as good as dead. You must eliminate these insects from your garden with pesticides or cover your garden beds with barriers before replanting.

Alternatively, you can wait to replant your zucchini until after late July. This is when the borers stop laying eggs for the year. However, this method will only work if your growing season is long enough to let you harvest your zucchini before the frost comes.

How Do You Keep Zucchini from Rotting on the Vine?

The only way to keep zucchini from rotting on the vine is by treating the underlying cause. You’ll need to do some investigative work to try to pinpoint the cause. Once you think you’ve found the cause, begin a treatment plan based on the information above. Give each treatment method at least a week or two to work before exploring other reasons for the rot.

  • Treat lack of pollination by manually pollinating zucchini flowers.
  • Improve calcium deficiency by feeding your zucchini an organic fertilizer.
  • Place these traps near your Zucchini in June and July—they’ll catch and kill adult squash vine borers before they can lay eggs.

In the end, you may lose a few zucchini plants, but you’ll be more prepared to halt the problem in its tracks the next time around. It’s best to treat every growing season as a chance to add to your knowledge and skills. By doing so, you’ll improve the health of your garden year by year and keep breaking your harvest records as time goes on.

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Why is Your Zucchini Dying on the Vine?

Zucchinis can start rotting on the vine at any given time for one of these common reasons:

  • Improper pollination
  • Calcium deficiencies
  • Vine borer infestations

To avoid getting caught off guard, check your zucchini plants often for signs of rot. Then, use this guide to identify the cause and resolve it before the issue takes your entire harvest.

With quick action, you can preserve the majority of your harvest and stay rich in delicious zucchini each year. Your efforts will result in bigger and bigger harvests that reward you for staying diligent and responsive to the needs of your garden.

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