Social impacts of subsidising crop control – a Plant Pathology highlight.
13th February 2023
Human behaviour has a strong but understudied influence on how crop epidemics play out. Individual growers have to navigate a complex web of disease management options, each of which will have different strengths and weaknesses depending on the prevailing conditions. Importantly, growers will also have to consider the actions of their fellow growers. The decisions made by one could have a strong impact on the disease outcomes of others – if one grower controls for disease, their efforts could be futile if neighbouring fields are left uncontrolled. A good analogy for this can be found in vaccines for human diseases – if the vaccine does not confer 100% protection, an individual may still become infected if they encounter an unvaccinated individual.
Previous papers studying the interaction between human behaviour and plant diseases have limited growers’ options to just two disease control mechanisms. Additionally, they do not quantify the “population-level” benefit, and can introduce subsidy schemes without consideration for the associated costs of the subsidy provider. In our paper, we wanted to address each of these assumptions in turn, and build a modelling framework that was sufficiently flexible to allow adaptation to other disease systems.
Tomato yellow leaf curl virus symptoms on left. From bugwood, by Central Science Laboratory, Harpenden, UK
We used Tomato Yellow Leaf Curl Virus (TYLCV) as a case study. TYLCV can have devastating results on crop yield, with records of up to 100% loss in uncontrolled fields. Importantly, there are TYLCV-resistant and -tolerant tomato varieties available to growers. Resistant varieties can protect from infection, but if they are not completely resistant growers risk losing a lot of yield. However, by limiting viral replication post-infection, they are less likely to act as a source of inoculum and thus can protect neighbouring fields. Tolerant varieties, by contrast, don’t protect from infection, but can limit yield loss. They can therefore disadvantage their neighbours by allowing virus to build up in the field.
We also included the perspective of a “social planner”, whose goal was to provide crop subsidies for improved and tolerant crop to maximise the yield of all growers in the system whilst also minimising the cost of the subsidies to themselves. We found that providing subsidies for resistant crop benefited the most growers, largely because of the protective effects of resistant crop. However, because of this protection, there was never a scenario where all growers used resistant crop; some growers always used the unimproved crop, as they benefited enough from the presence of some resistant crop in the system. It was rarely beneficial to subsidise a tolerant crop, mainly because it provided such strong benefits to individual users that the social planner ended up subsidising everyone, increasing their own costs.
The study can help explain why, when disease management options are available, they rarely see universal adoption. It can also help us understand how those providing control options can best utilise, deploy or incentivise them at minimum costs to themselves.
Effect of the relative loss due to infection on the choice of unimproved, tolerant or resistant crop and on the profits of each strategy at equilibrium. Figure 4 from the study.
Rachel E. Murray-Watson andNik J. Cunniffe published this study in Plant Pathology:
