Geraniums are one of the most versatile and widely used flowering plants in the floriculture industry. The diseases discussed in this bulletin are primarily those that affect the florist, garden, or zonal geranium, Pelargonium x hortorum. 20 specific diseases are addressed, organized by type of disease, i.e. bacterial, fungal, viral, and non-parasitic. Each disease is described in terms of importance, symptoms, the pathogen that causes the disease, environmental relations, and control. Use of fungicides, and soil treatments are discussed in separate sections. 33 black and white photographs and 5 tables supplement the text.