My pumpkin vine is very healthy, but the pumpkins grow to about the size of a golf ball and die. I water them and they are in good soil. Why is this happening?
Seems very unusual; photos or a plant description (colour, appearance etc.) would indeed be helpful in this case - providing such would help draw attention to your question/ get more exact answers.– Harry DavidSep 8, 2016 at 11:38
Do you keep the male flowers? Do you have pollinating insects? Possibly do you water too much, or the pumpkins don't have enough sun.– Giacomo CatenazziSep 8, 2016 at 13:44
I have the exact same problem with my kabotchan pumpkins in Portugal. I have no idea why this is happening :(– mmalmeidaSep 18, 2016 at 20:50
What do the vines, themselves, look like? Are they turning brown and shriveling or rotting? The biggest problem I have with pumpkins and squash are squash vine borers. The moth lays eggs on a stem, the larvae burrow into the vines and eat out the middle of the vines, killing them. You'll see a small hole right at the base/border of a branch of the vine, with what looks like sawdust (from the larvae burrowing in). If you catch it early enough, you can rinse off the eggs before they hatch.
If they burrow in, and haven't killed off that part of the plant entirely, you can take a sharp box-cutter or utility knife, slice open the vine, lengthwise, get those grubs out, then close up the slit by wrapping masking tape around the vine like a bandage. The plant will recover just fine, and the tape doesn't hurt it at all. Lacking photos or more details, I suggest this only because it happens to often to my plants.
If you aren't hand-pollinating, a possible answer is that the pumpkins weren't pollinated well enough. If the female flowers don't receive adequate pollination they plant may try to put out a fruit but it may abort partway through the process.
Plant stress is also a possibility from pests, inadequate watering, or poor soil.
A few links that may help
It's possibly an early case of severe BER. There may be a problem with calcium availability, pH, your watering frequency, nutrient composition, temperature, and/or any of some other factors.
Calcium unavailability is implicated in BER, but the presence of BER doesn't mean the soil is necessarily calcium-deficient. It may just be in unavailable forms.
I would recommend spraying the plants with calcium nitrate. Not only does it have calcium, but nitrates are very helpful for plants before and soon after they set fruit. Spray the fruit, too. I've never tried it on pumpkins (to date), but I have tried it on zucchini, other cucurbits, tomatoes, peppers, etc., and it has been very helpful for them in a drought year. Nitrates can come from rain; so, if there hasn't been much rain then your plants might not have as many as they normally would; this can lead to a less vigorous start on your harvest.
You might also consider making some calcium acetate and giving it to the plants (this won't have the nitrate benefit, though). It's a readily available water-soluble form of calcium that plants can use. You can create it by dissolving calcium carbonate in vinegar (make sure the final solution uses up the excess vinegar, and apply it as soon as the sun goes down). While you probably could do a foliar spray, I don't know how much to dilute in water for it to be safe; so, it might be safer in the soil until you figure out the proportions.
Calcium acetate is kind of hard to find commercially for some unknown reason (except maybe for lawns).
Warning: Large amounts of vinegar given to plants will act as a herbicide in the sun. Small amounts given after the sun goes down shouldn't be a big deal. Calcium acetate is less acidic than vinegar.