Relationship between plant virus concentration and infectivity: a 'growth curve' model

A logistic ('growth curve') model is formulated and applied to the relationship between numbers of infections (local lesions) produced by a virus on inoculated plants and the concentration of the virus in the inoculum. This model has the advantages of being simple, data-based and therefore not founded on limiting postulates, and of being applicable to a wide range of infection-dilution series, including those obtained from multicomponent viruses. Here, it is applied to common tobacco mosaic virus. Examples of infection-dilution series taken from the literature are fitted more closely and more objectively than they were fitted by the original authors. A limiting number for lesions is estimated by minimizing chi 2 values for differences between observed lesion numbers and those calculated from the logistic equation. The complete fitting procedure can be programmed on a hand-held calculator.