Rachel Murray-Watson, a PhD student in the Department of Plant Sciences, University of Cambridge and postgraduate member of Clare Hall, has co-authored a Royal Society paper which explores how the epidemiology of disease-resistant and disease-tolerant varieties of plants affects grower behaviour.

Rachel shares:

‘It’s intuitive that outcomes of an epidemic rely heavily on the behaviours of individuals – just look at the recent COVID-19 pandemic – but what might not be obvious is that managing plant epidemics is not different. Growers are people, and people vary in response to disease risk, or the degree to which they’re willing to cooperate with their neighbours.

There are several management strategies available to growers, each of which will have different impacts on how far the disease will spread. In this paper, we wanted to examine how growers respond to the availability of one of two crop types. The first is disease-resistant crop, which has a similar epidemiological effect as a vaccinated person. The resistant crop is less likely to get infected and then transmit infection. Therefore, if one grower uses resistant crop, other growers benefit as their fields are less likely to be infected. This is similar to the herd immunity effect for vaccines – if enough growers use sufficiently resistant crop, disease cannot spread and may die out. However, as more growers use resistant crop, the incentive for others to also use it is diminished. Why would you pay for something when you’re getting most of the protective benefit from other people’s fields?

The second is disease-tolerant crop, which is probably not as well known. Tolerant crop is interesting, as it does not have any mechanisms to prevent infection or to restrict pathogen replication. This may seem like a bad strategy, but tolerant crops actually have mechanisms that mitigate the negative effects of infection. So instead of trying to fight off the pathogen, they have protective mechanisms that mean that they do not get damaged by it, preventing yield loss. This is almost equivalent to taking paracetamol if you have a cold – you’re not fighting the cold virus, but you are reducing its damaging effects.

Tolerant crop, then, may seem like a great idea. Indeed, in this paper we found that it had great effects for individual growers. However, as it didn’t show symptoms, infected crops cannot be removed easily, causing high levels of infection in the system. For any growers using conventional crop, this was bad news as it reduced their yields.’

About the researcher

Rachel is part of the Theoretical and Computational Epidemiology Group at the Department of Plant Sciences here at the University of Cambridge. The group looks at models of plant disease with the aim of improving predictive models and implementing better control techniques.

Regarding this study, she concludes:

‘The results are important because both crop types are being promoted as eco-friendly and durable mechanisms of disease control. They do not rely on chemical controls, and tolerant crop may be more durable to pathogen evolution. However, if not enough growers use resistant crop, its full benefits will not be felt, and if tolerant crop is widespread, it could mean lower profits for those who cannot afford to use it.’

Photo: Karen Chan