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

Abstract

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.

Modelling interference between vectors of non-persistently transmitted plant viruses to identify effective control strategies

“…Further, growers can be reluctant to remove diseased plants as soon as symptoms are identified, since infected plants may continue to produce a marketable yield (Sisterson and Stenger, 2013). Finally, the degree of coordination among farmers concerning the decision of roguing is likely to affect the success in slowing disease spread (Laranjeira et al, 2020). Our results unsurprisingly suggest that the incidence of the disease decreases with the effort put into roguing.…”

Epidemiological and ecological consequences of virus manipulation of host and vector in plant virus transmission

“…Including these complexities would be an interesting extension for our future work, particularly by linking our treatment of vector preference with other recent work addressing competition between populations of vectors which colonise a particular host population vs. those that only pause briefly on the host population as part of larger-scale migratory behaviour [ 69 ]. This would also allow us to begin to address the effects of cooperation–or otherwise–in managing vector densities for epidemics spreading within a community of growers [ 70 ]. Since the outcome for any one grower will depend, at least in part, on the controls adopted by other growers, accounting for grower decision making via a game theoretic framework [ 71 ] could be a particularly interesting extension of the work presented here.…”

Modelling interference between vectors of non-persistently transmitted plant viruses to identify effective control strategies

“…Further, growers can be reluctant to remove diseased plants as soon as symptoms are identified, since infected plants may continue to produce a marketable yield [ 47 ]. Finally, the degree of coordination among farmers concerning the decision of roguing is likely to affect the success in slowing disease spread [ 71 ]. Our results unsurprisingly suggest that the incidence of the disease decreases with the effort put into roguing.…”