Powdery mildew, a fungus, is one of the most common plant diseases. It thrives in humid conditions, so you should not feel like it is your "fault".
Downy mildew is another fungal disease that takes hold in humid and wet conditions.
Cases of blight are becoming more and more common and it is seen more in the north than it used to be.
Fungal rust is quite common in roses and some other berries and ornamental plants.
Black spot is one of the most common diseases seen in roses.
Maintenance is the best prevention for mosaic viruses.
Damping off disease is not uncommon, but fortunately it is preventable with good watering and management practices.
Management is the key to overcoming verticillium wilt
For some diseases, like fusarium wilt, your best option is to plant resistant varieties of plants.