Assessing the Mismatch Between Incubation and Latent Periods for Vector-Borne Diseases: The Case of Sharka.

The relative durations of the incubation period (the time between inoculation and symptom expression) and of the latent period (the time between inoculation and infectiousness of the host) are poorly documented for plant diseases. However, the extent of asynchrony between the ends of these two periods (i.e., their mismatch) can be a key determinant of the epidemic dynamics for many diseases and consequently it is of primary interest in the design of disease management strategies. In order to assess this mismatch, an experimental approach was developed and applied using sharka, a severe disease caused by Plum pox virus (PPV, genus Potyvirus, family Potyviridae) affecting trees belonging to the genus Prunus. Leaves of infected young peach trees were used individually as viral sources in aphid-mediated transmission tests carried out at different time points postinoculation in order to bracket symptom onset. By fitting a nonlinear logistic model to the obtained transmission rates, we demonstrated that the first symptoms appear on leaves 1 day before they rapidly become infectious. In addition, among symptomatic leaves, symptom intensity and transmission rate are positively correlated. These results strengthen the conclusion that, under our experimental conditions, incubation and latent periods of PPV infection are almost synchronous.