Why Did My Zucchini Plant Suddenly Collapse at the Base?

A zucchini that was thriving one day and lies wilted and collapsing the next, with the trouble centred at the base of the stem, has very likely been attacked by the squash vine borer — one of the most destructive and demoralising pests a zucchini grower faces. It strikes from the inside, hidden, which is what makes it so deadly. But if you know the signs you can catch it, and sometimes even save the plant with a bit of stem surgery. Let me explain.

What the squash vine borer does

The squash vine borer is the larva of a clear-winged moth that looks almost wasp-like and flies by day. The moth lays small flat eggs on the stems near the base of the plant. When an egg hatches, the fat white grub bores straight into the stem and tunnels along inside, eating out the centre. As it destroys the stem's interior, it cuts off the flow of water and nutrients to everything above, so the plant — which looked perfectly healthy — suddenly wilts and collapses, usually starting with the leaves and runners furthest from the roots.

The telltale signs

Inspect the lower stem near the soil. The signature clue is a small hole in the stem oozing wet, sawdust-like or mushy greenish-yellow waste called frass — this is the grub's droppings being pushed out as it tunnels. You may also see the stem looking swollen, split or mushy at that point. The combination of sudden wilting and a holed, frass-oozing stem base is diagnostic of vine borer. Sometimes if you slit the stem open you will find the cream-coloured grub itself inside.

Stem surgery to save the plant

If you catch it early, you can sometimes rescue the plant. Using a sharp knife, carefully slit the stem lengthwise at the hole, find the grub inside, and dig it out, killing it. Then heap moist soil over the cut section of stem — zucchini can root along a buried stem, and the plant may recover by growing new roots above the damage. It is not always successful, but with a valued plant it is well worth trying. Check for and remove any additional grubs, since there can be more than one.

Preventing it next time

Because the borer does its damage hidden inside the stem, prevention is far better than cure, and it is all about stopping the moth from laying or the eggs from hatching successfully. Cover young plants with floating row cover during the moth's flight period to physically block egg-laying — remove it once the plant flowers so bees can pollinate. Wrapping the lower stem with foil or a barrier deters egg-laying. Inspecting stems regularly and scraping off any eggs you find helps. Some growers inject Bt into the stem or time plantings to dodge the borer's peak. Clearing away old squash debris and rotating crops reduces the overwintering pupae in the soil. With vigilance and barriers, you can keep this hidden killer from wrecking your zucchini.