The Influence of Toxins in Disease Symptom Initiation in Plants: A Review

The concept that plant pathogens cause disease by producing toxic substances dates back about a century. Evidence for its general validity, however, has accumulated only recently. The difficulty has been that most plant disease symptoms are the result of a complex interplay of several factors. Some pathogens, in addition to producing one or more toxins, may also excrete enzymes which degrade cell walls, causing tissue disorganization; they may destroy plant hormones, causing abnormal growth of the host; or they may physically block the water-conducting vessels of the host by their prolific growth or by production of viscous polysaccharides. Because of this complexity, proof of the role of a toxin as one of several interacting factors causing a disease symptom is often difficult to obtain, even though the presence of toxic substances in cultures of most plant pathogens is easily demonstrated. Despite these problems, our growing knowledge of the biochemistry of symptom causation clearly establishes the role of toxins as a dominant one in most plant diseases