Why Is Sunburn On My Ugu Plants (Pumpkin Leaves) - And What Is The Best Solution?

If it was planted in sac bags, then no proper ventilation.

Sometimes, it is the weather. The heat is too much and that is what causes the sickness to it. Keep watering the ones remaining, they will pick up when rain comes and weather change.

Sunburn to me, is due to the high temperature causing it. 

One other reasons is, most people apply to much fertilizer to their ugu plants, which ends up burning the plants. If you are experiencing this, then it means there is too much of some fertilizer nutrient in the soil.

Only apply chicken dung and see wonder. The chicken droppings must not be warm.

As a matter of fact, decomposer manure should be used on the leaves and not directly in contact to the roots.