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Pumpkin Stem Rot — Identification, Treatment, Prevention

Pumpkin Stem Rot — Identification, Treatment, Prevention

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Pumpkins have a rich history throughout the Americas. As much as 500 pounds of this fruit is harvested every year with only half of that grown as edibles.  

Half of the pumpkin crops are grown solely for decoration. The other half are harvested to be pureed to be used for the likes of pumpkin pies and soups. 

For the growers though, the term pumpkin means nothing. 

That’s a common name. Botanically, the pumpkin is part of the squash genus. The species is Cucurbita moschata. That puts them in the cucurbit family, which is one with a moderately high risk of rot.

Essentially, a pumpkin is just another variety of squash. 

The confusing part with pumpkins is that almost all types of rot (root rot, rotting vines, and blossom-end rot) start from the bottom, gradually working its way up. 

When you see pumpkin stem rot, but the vines and fruits aren’t rotting, it is a head scratcher. 

Read on to find out the main culprit of pumpkin stem rot, how to catch it early, kill it, treat the problem, and save your harvest. 


Pumpkin stem rot 

The Squash vine borer causes pumpkin stem rot. It’s the larvae of a moth. Eggs hatch in late summer, then the larvae bore into the stem. If not treated, these will pupate, exit stems, then burrow into the soil. When they emerge from soil, they bore into pumpkin vines, destroying entire harvests. 


The squash vine borer 

Numerous garden pests can ruin your crops. None is more devastating than the squash vine borer. This is the main pest that’s most likely to cause pumpkin stem rot.  

This is the larvae of a moth that bores into the stems of all squash fruits, not just pumpkins. Any plant in the cucurbit family is a target. 

The moths that cause the problem are active in early summer, which is when they tend to lay their eggs at the base of pumpkin stems. Those pupate within a couple of weeks. 

The larvae are what bore into the stems of the plant, starting to feed from the inside out anywhere from two weeks to a month after the eggs are laid.

Early signs of a squash vine borer presence are when you see the pumpkin leaves are wilting. When that happens, finger test the soil or your mulch to check the moisture level.

Wilting is a sign of the drought but when there’s plenty of moisture, it has you questioning: why are my pumpkin leaves wilting? It can be fungal infections, but when the only visible damage is stem rot, that’s most likely a squash bug or vine borer. 

Both are insects that feed on plants from the inside out, causing the leaves to keel over. 

If you do suspect a borer presence, inspect around the stem for entry holes and tiny speckles of plant debris that looks like sawdust. 

Think of the dust that forms when you drill into wood. It leaves sawdust on the ground or floor. Borers leave the same debris, only it’s the dust from the stem that’s scattered beneath their entry hole. 

Removing squash vine borers from pumpkin stems

To get rid of a squash vine borer, use a sharp knife to slit the stem open. The borer is a fat white grub about an inch in length with a brown head. 

If you’re growing miniature pumpkins on a trellis or a climbing vine that you can’t risk weakening the stem further, an alternative is to poke multiple holes in the stem with a pin. 

It’s less grotesque than slitting the stem to handle a worm, but the results is just as brutal for the grub. It’ll be stabbed to death.

If vine borers are left in the stem, they will leave the stem in the pupae stage, then burrow into the soil. When that happens, their later stages are when they bore in the vines of pumpkins causing extensive damage. 

These things can make cocoons in the soil that they use to overwinter, which is one of the reasons that it’s good idea to use crop rotation. 

These will bore into any vining plant in the cucurbit family. For pumpkins, there are no resistant varieties. 

Removing squash vine borers from vines

If you’re seeing isolated rot sites on the vine, it’s possible a squash vine borer is in the vine, feeding from the inside out. The signs of rot will be at the feeding site where the borer is. 

You can use the same slitting technique on the vine that you’d use to slit the stem open, or stab multiple holes into the vine in the hopes that you can kill the borer. 

The rot is always most intensive at the site the grub is feeding. Isolate that part of the vine and either cut it open, or go at it with a pin to kill the bug.

Preventing squash vine borers from attacking pumpkins

  • In the summer, putting a row cover over your pumpkin vines stops moths getting near the fruit to lay their eggs. 
  • An alternative is to use yellow sticky traps to catch and destroy the black and orange moth before it gets a chance to lay eggs.
  • If they do lay eggs, you’ll either see them on pumpkin leaves or around the stem base. The eggs are an oval shape with a shiny brown-copper shade. They’re usually laid in tight clusters. Remove them as soon as you see them. They hatch within a week to two weeks, which is when they bore into the stem causing rot. 
  • Applying Bacillus thuringiensis (Bt) to the base of the pumpkin stems is an organic method that kills borers. It needs to be applied to the stems before they hatch though. Once they bore into the stem, it’s too late. 
  • If they’ve burrowed into the soil, treating the soil with an insecticide that contains carbaryl can kill the larvae in the soil.


Frequently Asked Questions related to pumpkin stem rot 


Do fungal infections cause pumpkin stem rot? 

Fungal infections start in the soil rotting pumpkins from the bottom. Either the base of the fruit or the vine before it rots the stem. Pycindia causes black rot spots on the fruit. Didymella bryoniae causes similar black rot spots but with yellow halos. Crown rot is caused by a strain of fusarium. 


Does crop rotation need to be used for pumpkins? 

Pumpkins shouldn’t be planted in the same patch in succession. Squash vine borers, and multiple soil borne pathogens that infect pumpkins, pose the same threat to all cucurbit plants including cucumber, squash and watermelon. When rotating crops, plant a non-cucurbit plant every other season.