A Few Bad Potatoes: Modelling Economic Aspects of Disease Control in Seed Potatoes

Background: Production and export of seed potatoes is of immense value to the UK economy, yet threatened by multiple disease threats. For example, potato blackleg caused by the Pectobacterium atrosepticum has recently increased to levels not seen for 20 years. Seed growers face trade-offs between economic pressures and requirements for biosecurity, which leads to different management decisions that might not be optimal from the point of view of disease control. Disease management practises can impact on neighbours, with farms in close geographic proximity or through the use of shared equipment. Seed potatoes cannot be marketed in Scotland unless they have been registered and certified under the Seed Potato Classification Scheme (SPCS), administered by SASA (Science and Advice for Scottish Agriculture), who is the Certifying Authority in Scotland. In order for a seed crop to be certified, it must be visually inspected at least twice during the growing season by official Plant Heath Inspectors for disease and other faults such as trueness to type and the presence of groundkeepers. A crop is certified if it meets the requirements for the grade. Control of blackleg in certified seed is achieved through strict tolerances for symptoms of the disease in the growing crop and harvested tubers. Plants with symptoms of the disease can be rogued from a crop, although the value of roguing a crop with blackleg to minimise disease in the progeny crop is not known. Disease development in the subsequent crop is not always directly related to either inspection findings or bacterial loading in the seed crop due to the important influence of the weather conditions during the growing season.
Aims and objectives: i) To develop three models that address build-up of blackleg in a single farm, single season and across seasons, and in trading across the industry, and ii) produce decision support information for growers to better manage their crops, and an evaluation of the strengths and weaknesses of the potato crop inspections process.
Methods and Approach: While focussing on blackleg disease, the project will construct a range of models for diseases including potato blackleg, tuber blemish diseases and potato cyst nematodes, using the plant pathogen modelling framework developed by Kleczkowski. Following the feasibility studies carried out under Kleczkowski, Davey and Toth’s supervision (including the 2016 project funded by the BSPP), we will concentrate on three bioeconomic models addressing increasingly complex problems in seed potatoes production. Model 1) will address the build-up of blackleg within a single season. Farmers are informed of potential blackleg infestation at the first and second inspection. Based on this information, they make a decision on whether to harvest the crop early if the expected infestation at the end of the season would have caused downgrading of their production. We will use the bioeconomic modelling to capture the farmer behaviour and understand what factors drive the decision. We will subsequently look at the role of the third inspection in the decision-making process, as well as attempt to find the optimal timing of the first and second inspections. Model 2) will extend the single-season model and address the build-up of blackleg over seed potato generations. The farmers would be able to adopt different strategies, investing either in an increased density or in an extended number of generations. We would use the data from the SPUDs database on all 4,000+ seed crops grown annually in Scotland, including disease prevalence, locations, origin and heritage which are collected and maintained by SASA, to parameterise the model. We will then explore the dependence of an optimal number of generations on the disease and economic parameters. Model 3) will examine the interacting decisions of farmers using a game-theoretical approach on a heterogeneous contact network and include an epidemiological model to quantify the potential impact of risky decision-making by a small proportion of farmers over the whole landscape. The model will be partly parameterised using SASA’s data on seed potatoes and will result in a theoretical framework that can be used by policy makers.