This is an ill-defined, much misused term that is sometimes taken to mean horizontal resistance. Strictly speaking, tolerance means that, if two different plants are equally diseased, the tolerant one will suffer less of a yield loss. However, to demonstrate tolerance, it must first be shown that those two plants have equal yields when disease-free. A term to be avoided whenever possible. See also: Field resistance. Tollocan

The name of a Mexican, short-day potato which has an exceptionally high level of horizontal resistance to blight. Tomato

See: Lycopersicon esculentum. Toxicity

The degree of poisonousness of a substance such as a pesticide. Toxicity is usually measured in terms of the LD50, which stands for the lethal dose required to kill 50% of a population, usually of insects or laboratory rats. The LD50 is normally expressed as milligrams of poison per kilogram of body weight, but other ratios are possible.

For the purposes of labelling, the USA recognises four categories of toxicity. Category One involve substances with an acute oral LD50 of 0-50 mg/kg and these are very dangerous. Category Two are moderately toxic substances with LD50 of 50-500 mg/kg. Category Three substances have LD50 of 500-5000 mg/kg and are only mildly toxic. Category Four have an LD50 greater than 5000 mg/kg. However, these categories involve oral ingestion only, and they take no account of inhalation and skin penetration, or of hormone mimics that can damage the development of young children and foetuses. Toxicology

The scientific study of toxins. See also: Toxicity. Toxin

A poisonous substance. The term is sometimes restricted to toxins produced by a living organism but, it the context of crop science, it can also be applied to pesticides and other synthetic chemicals.

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