The best way to prevent diseases is keeping the hibiscus as healthy as possible through proper fertilization and best care.
What occurs regularly when hibiscus are over watered is root rot or collar rot and shows with wilting leaves or a soft, mushy spots at the stem near the soil.
Both is caused by several different types of fungi.
The same goes for the so called tip dying where either the tips of the plant or even branches shrivel and die, sometimes the whole plant is effected.
Here as well is the phytophtora or other fungi working.
The fungi block the passage of nutrition from the roots to the stem and when cutting the stem of an attacked plant one can clearly see the brown ring either underneath the cambium or right in the middle of the stem.
It mainly occurs in our colder climate in winter time when the plants get less light and are easily out of balance.
One can try and prevent the fungi by using regularly a fungicide during the winter months.