This is a problem for giant pumpkins and is also a common problem for a lot of other vine plants like cucumbers, and squash.
This powdery mildew thrives in hot humid weather and can occur in middle to late summer, at the same time the pumpkin will be getting really big. It can spread rapidly and really slow the growth of the plant, due to affecting the amount of photosynthesis the plant can do.
Prevention helps a lot
One of the main ways to prevent these diseases is to think about how you are watering your plants.
Avoiding late afternoon and evening watering is advised. Watering in the mornings allows the sun to quickly dry the leaves, where if you water at night, the water can sit on the leaves until the next morning, on a warm night this can be the perfect growing conditions for powdery mildew.
When watering try and only water the roots and vines.
It is a personal choice if you use sprays to control disease. Applying a fungicide before it becomes apparent may prevent it totally. If your pumpkin is showing signs of disease already, apply it straight away. If applied soon enough the pumpkin should recover OK.
If you do end up with a diseased plant in your garden, make sure to remove it totally from the garden. If you just turn them over in the soil, or put them in your compost pile the disease may affect next year’s effort.
Fixing the problem
There are two different Yates products that can help with this problem, Yates Greenguard Fungicide and Yates Natures way fungus spray.
Soft Stem/Stem Rot
This can happen at anytime in the giant pumpkins life cycle, but it is common to see small spots that are soft on the plant.
Fixing the problem
Brush the soft spot away gently with a toothbrush, rinse with clean water, and let dry. Use a fungicide solution and apply to the area, while trying to keep the stem as dry as possible. Apply the fungicide once a week, to help stop it reappearing.