Cassava (Manihot esculenta Crantz) is affected by more than 40 diseases induced by viruses, bacteria, fungi and phytomonas. Even though a 100% disease loss is rare in most cassava production systems, disease outbreaks occur and pathological problems of cassava are in many cases responsible for the reduced yields. The most important pathological problems of cassava are those that affect the sanitary quality of stems (commercial source of planting material), soil-borne pathogens capable of inducing root rots and microbial root inhabitants that induce post-harvest deterioration via root injuries at harvest. Based on world average losses, root rot pathogens causing both pre- and/or post-harvest deterioration are the most important.