In botanical epidemiology, an incubation period of five days means that infections originating symptoms that can be visualized at time t have occurred at time t-5. Since not all diseases follow this rule, their time of infection cannot be derived from the time of symptom appearance. These diseases are grouped according to the plant organ in which symptoms appear: (i) young flushes/shoots; (ii) young tillers; (iii) fully developed fruits. In all these subgroups symptom expression is a function of the phenological stage of the plant organ and has little relation to the time of infection. For these diseases, studies to determine the infection period should be emphasized, rather than the trivial monitoring of disease expression. Examples are given for each subgroup and the monomolecular growth is proposed as the best model to describe the disease progress curve for the whole group.
