Every gardener has put in plants with hopes for wonderful flowers, fruits, or vegetables, only to have those hopes dashed as the plants get sick and die. Plants that die are considered diseased. Many things can cause plants to become diseased, including living agents, other factors (nonliving), or a combination of the two. This chapter focuses only on living agents—fungi, bacteria, viruses, nematodes, and parasitic plants. Nonliving factors, such as nutrient deficiencies, lack of water, temperature stress, and these problems in combination as they relate to specific types of plants, are discussed elsewhere.
Pscheidt, Jay W. and Hartman, John R., "Plant Diseases" (2011). Agriculture and Natural Resources Publications. Paper 121.