Downy mildew is caused by a fungus that usually shows up on pumpkins in late August or September. Downy mildew causes yellow spots on leaves that can coalesce and turn brown as the fungus grows and takes over more of the plant. You may see fuzzy spores on the undersides of leaves. The spores are spread by wind, so they can easily infect other pumpkins, squash or melons growing in your garden. Purdue University Extension suggests using contact fungicides containing chlorothalonil and maneb at the earliest sign of infection.
Caused by the bacteria Erwinia tracheiphila, bacterial wilt is spread by the cucumber beetle. While pumpkins are only occasionally infected, the bacteria can be devastating to a crop is exposed. Bacterial wilt clogs the plant's vascular system so water and nutrients can't get where they need to go. The vines will wilt and eventually turn brown and die. Controlling the cucumber beetle is the best way to prevent the spread of bacterial wilt. Use pesticides containing carbaryl, methoxychlor or diazinon. If a plant does become infected, pull it up but leave it in the garden and allow it to dry out. Untangling the plant from its healthy neighbors may damage them.
Warm, wet weather favors the spread of alternaria blight. Caused by the fungus Alternaria cucumerina, it causes targetlike brown-to-black spots on leaves and, occasionally, soft spots on pumpkins as well. Eventual defoliation and death of the plant will occur. The fungus over winters in pumpkin debris from previous crops so be sure to clear your garden of refuse each year. Use fungicides thriam and captan to control spread of the fungus.
Spider mites, squash bugs or whiteflies can all cause brown or yellowed leaves. Spider mites will leave a fine webbing on plants along with spotted yellow leaves. Whiteflies hide on the undersides of leaves and you'll see them fly up in clouds when you touch the plant. They turn leaves yellow with spots of sooty mold. Squash bugs are small flattened, beetlelike insects that leave yellow spots on leaves. These spots will eventually turn brown and leaves will begin to wilt. Insects can be controlled by insecticidal soaps or pesticides but these methods can also harm bees which are critical to pollination. Spider mites and whiteflies can be controlled by regularly washing plants with water. Gently spray undersides of leaves with the hose daily and you'll be able to keep them in check. Squash bugs can be trapped at night under boards. In the morning turn the boards over and kill the bugs. You can also pick young bugs and egg masses off your plants to prevent the population from increasing.