An Assessment of the Risk Posed by Groundkeepers to the Virus Heath of Scottish Seed Potato Production

2. Scotland is renowned worldwide for the quality of the seed potatoes it produces, underpinning the whole UK potato industry and exporting to over 30 countries outside of the EU. This reputation is primarily due to low levels of virus disease present in Scottish seed potatoes enabling production of high health seed by highly specialised growers. The most important potato viruses present in Scotland are spread by aphids moving from infected to healthy plants. Scotland’s cool, maritime climate has historically kept aphid numbers, and therefore virus levels, low but factors including the changing climate and loss of effective insecticides have meant that virus levels have been increasing in recent years. Virus is managed through certification, crop inspections, and the industry. The Seed Potato Certification Scheme (SPCS) sets disease and quality tolerances, including tolerances for virus in seed potato crops, which limits the inoculum in crops and the risk of spread to other crops in the vicinity. Likewise statutory inspections of growing ware crops maintain limits on levels of virus. The industry face financial consequences if their crops do not meet tolerances required for certification and subsequently marketing; these include downgrading/no grading of seed crops, and potential destruction if ware or seed crops are above tolerances permitted. It is therefore of financial and reputational benefit for growers to take efforts to minimise the risk of virus to their crops. However, there is one potential source of virus inoculum that is neither controlled nor monitored, and whose importance is not quantified - Groundkeepers. Groundkeepers are ‘volunteer’ potato plants and therefore known hosts for potato viruses. ‘Volunteers’ are potato plants which usually grow from tubers that, at time of harvest during the previous season, were too small to be picked up by modern harvesters. Estimations of the number of tubers left on the soil surface or up to 20 cm underground varies widely throughout production areas. Some studies have reported an average tuber loss of 142 000 tubers per hectare, ranging from about 39 000 to 210 000 tubers. Consequently, volunteer potato plants are among the most important weeds in crops grown in rotation with potatoes. Incidence of groundkeepers is believed to have increased due to milder winters (with fewer ground frosts) and a loss of chemicals available to farmers. Groundkeepers are a known potential source of virus inoculum which currently sits outside the controls of both SPCS and potato producers, who often grow crops on rented ground. However, without fully understanding and quantifying the risk posed, it is impossible to justify policy changes or expensive mitigation/management processes.