Interplay between harvesting, planting density and ripening time affects coffee leaf rust dispersal and infection

ABSTRACT Coffee leaf rust ( CLR; Hemileia vastatrix ) is one of the principal coffee diseases and has caused devastating loss of production around the globe. This fungus disperses between plants or regions through direct contact, water splash, or local turbulent wind conditions. Some studies have proposed that coffee harvesters can also bear CLR during harvesting. Their different trajectories depend on management and coffee plant characteristics, like the planting density or the ripening synchronicity, but also on the social organization of the plantation. However, it is not clear how relevant these movements are for CLR dispersal and plot-level infection in plots with other modes of CLR dispersal and different planting densities. Here we present a qualitative computational model to explore the role of coffee ripening synchronicity and the number of workers per plot in reproducing the different movement trajectories observed in the field. We then evaluate how these trajectories modify rust dispersal and change CLR infection in plots with increasing planting densities. We found that interplant asynchronous ripening and tree scarcity generate trajectories with long steps. In our model, the number of workers does not modify the trajectories. The harvest dispersal significantly increases coffee plot infection (up to 15%) compared to scenarios where only local mechanisms for CLR dispersal (contact-mediated or splash) are present. This effect is maximal for trajectories with long steps and in plantations with medium planting densities. At larger planting densities, plants are all already infected due to the local CLR dispersal. Our results aim to spur discussion on practices that could reduce the impact of harvesting and specific trajectories, in scenarios where one can benefit from asynchronous maturation of berries and shaded plantations. Some examples of practices include reducing the long-distance movements during the same day, for instance, by avoiding harvesting at the end of the harvest season when trees ripening asynchronicity and CLR levels are higher.