If conditions are right a plant can get sick just like people. This video describes the steps used to determine what can cause plants to get sick, and profiles four main types of pathogens that affect plants: fungi, bacteria, viruses and nematodes. The cause of a plant’s illness can be based on the environmental conditions and various pathogens that might be present. A pathogen is a type of organism that can cause a disease. It is up to a Plant Diagnostician to determine the cause of a plant’s illness.