Why are my young pumpkins rotting when the vines look healthy?

A common cause is incomplete pollination. A young pumpkin can start growing a little, then yellow, soften, and stop if it was not pollinated well enough. Healthy-looking vines do not rule that out.

If the next fruit does the same, check whether the female flowers are getting pollen. If the damage looks more like spreading, mushy lesions rather than a fruit simply shriveling, disease or other growing conditions could also be involved.

Try hand-pollinating a newly opened female flower in the morning. The female flower has a tiny pumpkin-shaped swelling behind it. Take pollen from a freshly opened male flower and gently transfer it to the center of the female flower.

Do it while the flowers are open, and try again on another female flower if you have one. It is a practical way to check whether pollination is the issue.

Not every early fruit is going to make it, and a vine may abort some young pumpkins, especially if it is already supporting other fruit. I would remove the ones that have clearly failed and keep an eye on the next few flowers.

If several fruits keep rotting despite good pollination, look more closely at where the rot begins and whether the fruit is touching persistently wet soil. The description alone is not enough to pin it on one disease.