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Spatiotemporal dynamics and modelling support the case for area‐wide management of citrus greasy spot in a Brazilian smallholder farming region

Laranjeira, Francisco F., Silva, Suely X. B., Murray‐Watson, Rachel E., Soares, Ana C. F., Santos‐Filho, Hermes P., Cunniffe, Nik J.
Plant pathology 2020 v.69 no.3 pp. 467-483
Citrus, climatic factors, defoliation, disease control, disease incidence, groves, growers, humidity, inoculum, leaves, models, small-scale farming, surveys, Brazil
Citrus greasy spot (CGS), caused by Zasmidium citri, induces premature defoliation and yield loss in Citrus spp. The epidemiology of CGS is well understood in high humidity areas, but remains unaddressed in Brazil, despite differing climatic conditions and disease management practices. The spatiotemporal dynamics of CGS were characterized in the Recôncavo of Bahia (Brazil) at four hierarchical levels (quadrant, plant, grove, and region). A survey conducted in 19 municipalities found the disease to be present throughout the region with an incidence of 100% in groves and plants, and higher than 70% on leaves. Index of dispersion (D) values suggest the spatial pattern of units with symptoms lies between random and regular. This was confirmed by the parameters of the binary power law for plants and their quadrants (log[A] < 0 and b < 1). No consistent differences were observed in the disease incidence at different plant heights. We introduce a compartmental model synthesizing CGS epidemiology. The collected data allow such a model to be parameterized, albeit with some ambiguity over the proportion of new infections that result from inoculum produced within the grove versus external sources of infection. By extending the model to include two populations of growers—those who control and those who do not—coupled by airborne inoculum, we investigate likely performance of cultural controls accessible to citrus growers in northeastern Brazil. The results show that control via removal of fallen leaves can be very effective. However, successful control is likely to require area‐wide strategies in which a large proportion of growers actively manage disease.