Pumpkin Pests and Diseases: Vine Borers, Squash Bugs and Mildew

Sort the problems by how fast they kill

Pumpkins attract a long list of pests and diseases, and treating them all with the same urgency wastes the attention you need for the two or three that actually end the crop. Rank them by speed.

A vine collapsing overnight in midsummer is almost never disease and almost always a borer inside the stem. Powdery mildew, by contrast, takes weeks to do real damage and often arrives after the fruit have already sized up. Knowing the difference decides whether you go out with a knife or make a note and carry on.

Squash vine borer: the one that ends plants

The adult is a narrow, clear-winged day-flying moth that lays single eggs at the base of the stem around the time the vines start to run. The larva chews into the stem and eats out the center, and the first thing most gardeners see is a full-sized plant wilted flat on a sunny morning, with the soil still moist.

The signature is sawdust-like frass, wet and orange-brown, pushed out of a small hole at the base of the main stem. Once you have seen it once you will not mistake it again.

1. Check the base weekly From the time the vines begin to run, look at the bottom twelve inches (30 cm) of the main stem once a week. You are looking for the frass, not for the insect.
2. Cut the larva out Slit the stem lengthways with a sharp knife above the hole, find the fat white grub and remove it. Then mound moist soil over the cut section and the stem often heals and carries on.
3. Root the vine along its length Bury a handful of soil at every fourth or fifth node while the vines run. Each buried node roots, so a borer in the crown no longer starves the whole plant.
4. Wrap the base early A collar of aluminum foil or a stocking around the bottom six inches (15 cm) of stem stops the moth laying there. Put it on before the vines run, not after the damage.

Thick-stemmed, solid-vined squash are far less often lost to borers than hollow-stemmed field pumpkins. If borers take your plants every year, that difference in stem type is worth more than any spray.

Squash bugs and cucumber beetles

These two do their worst in different ways. Squash bugs suck sap in growing numbers until leaves go bronze and die back and the plant runs out of leaf before the fruit finish. Cucumber beetles chew a little, but the real damage is the bacterial wilt they carry, which no treatment will reverse.

To confirm bacterial wilt, cut a wilted stem and press the two cut ends together, then draw them apart slowly. A fine sticky thread between them is the bacterium and the plant will not recover.

Powdery mildew and the other leaf diseases

White floury patches on the upper leaf surface in the second half of summer are normal on pumpkins and rarely a disaster, because by then the fruit are usually made. What it costs you is the last few weeks of sizing and the sugar that goes into flavor.

Keep the leaves that shade the fruit. A pumpkin exposed to full sun after its canopy collapses can scald on the shoulder, and that scald becomes the soft spot in storage.

1. Space for airflow Give each plant its full spread. A crowded patch holds humidity under the leaves and mildew arrives one to two weeks earlier for exactly the same management.
2. Water the soil, not the leaves Water at the crown early in the day. Overhead watering in the evening leaves foliage wet all night, which suits every leaf disease you do not want.
3. Remove the worst leaves only Take off leaves that are more than half covered and carry them off the plot. Stripping healthy foliage to open the plant up costs more than the mildew does.
4. Rotate the ground Leave two or three years before squash follows squash. Ground that grew cucumbers or zucchini counts as the same slot for soil-borne disease.

Fruit rotting on the underside

A pumpkin sitting directly on damp soil for weeks rots where it touches, and you often find it only when you lift the fruit at harvest and the bottom comes away. It is entirely preventable and costs nothing to prevent.

1. Get the fruit off the ground Once a fruit is the size of a fist, slide a tile, a board or a thick pad of straw under it. Anything that breaks the contact with wet soil will do.
2. Turn it once, gently Turn each fruit a quarter turn once during its growth so it colors evenly and no side stays permanently damp. Do not twist it against the stem.
3. Clear spent flowers The dried flower on the end of a young fruit holds moisture and gray mold starts there. Flick them off once they are dry and brown.

Blossom-end rot on squash is far less common than on tomatoes and is usually inconsistent watering rather than a soil problem. Even out the watering before you reach for anything else.

What to record so next year is easier

Pest pressure repeats in the same place at close to the same time each year, which makes a short written record more useful here than in almost any other crop. Dates and positions, nothing elaborate.

1. The date you first saw frass Note the day you find the first borer damage. Your foil collars want to go on two weeks before that date next season.
2. The date of the first white patch Record when mildew first appears. If it is arriving before the fruit have sized, your spacing is too tight.
3. Which plant went first Note the position of the plant that failed first. The sheltered end of the patch and the end nearest last year's squash are the usual answers.
4. Which fruit rotted in store Note whether stored fruit that failed had a ground contact mark. That single note usually ends the problem for good.