Pests, poor soil quality, nutrient deficiencies and age can cause the leaves of a pumpkin to turn yellow. Mature vines may have yellow leaves soon after pumpkins ripen.
Pumpkin vines should be treated for squash bugs and squash leaf borers if leaves are turning yellow and then browning along the outer edge. Squash bugs can be eliminated using a commercial pesticide applied right before dusk to prevent harming beneficial insects.
Pumpkin leaves begin to yellow when the soil does not contain enough nitrogen. Treating the soil with additives boosts nitrogen levels. Using a drip system or spraying water directly on the base of the plant ensures the plant has access to necessary moisture.
Rotating crops prevents problems, such as fusarium wilt, that may cause pumpkin leaves to yellow prematurely. Diseases that are related to poor soil caused by lack of rotation also include phytophthora and rhizoctonia. These issues can be prevented by starting seedlings in a different area of the garden each year and treating soil for nutrient deficiencies before planting.
Mature pumpkin vines that have produced full-sized, healthy pumpkins will naturally start to decay as the season progresses. If the vine is also yellow and pumpkins are ripe, the best choice is to remove the pumpkins and allow the vine to shrivel.