There are various diseases that can affect your lawn, but the good news is that by choosing a suitable lawn variety for your conditions and keeping it well maintained and healthy you will greatly reduce the chance of diseases.
However there is still a chance of disease, and so knowing what symptoms to look for and the optimum conditions for their development will help you to control them before disaster strikes.
Many fungal diseases thrive in warm, humid conditions, others set in during cool, damp conditions; by reducing humidity and improving drainage you will therefore reduce fungal growth. One way to achieve this is by avoiding evening watering.