1 Central Science Laboratory, Sand Hutton, York YO4 1LZ, UK; 2ADAS Cardiff, St Agnes Road, Gabalfa, Cardiff CF4 4YH, UK

Background and objectives
Potatoes are susceptible to over 30 bacterial, fungal and virus diseases and 12 common disorders, all of which have economic significance in the UK. The incidence and severity of these diseases and disorders are influenced by a wide variety of interacting factors throughout the life of the crop and during storage. National survey data between 1993 and 1997 were analysed to identify key factors influencing the incidence of internal diseases and blemishes. If these disorders are severe, tubers may be rejected by the processing industry.

Materials and methods
ADAS and CSL have jointly undertaken a survey of potato diseases in England and Wales since 1993 [1]. Disease incidence and severity in 80 crops were monitored both in the field and in store each year. Crops were selected according to the area of potatoes grown in each county and were restricted to four main cultivars: Estima, Cara, Maris Piper and Record. Crop husbandry data, storage conditions and chemical inputs were also recorded. The data collected were stored and analysed using an INFORMIX relational database.

Results and conclusions
Internal rust spot (IRS) was the most frequently encountered internal blemish throughout the period of the survey. In 1997, IRS, spraing, black leg and tuber blight reached their highest recorded incidence of 3.5, 0.3, 0.3 and 0.2% tubers affected, respectively. Gangrene was highest in 1994, with 0.3% tubers affected, and dry rot in 1995, with 1.0% tubers affected. Analysis of the relationship between tuber damage and disease levels showed that, with the exception of dry rot, the incidence of internal blemishes was higher in damaged samples. A damaged sample was defined as a sample where at least 5% of the tubers assessed had growth cracks or mechanical damage. The levels of IRS increased proportionately with the per cent tubers damaged per sample both at harvest and after a period in store. Disease incidence was also related to cultivar. Between 1993 and 1997, harvested tubers of 'Estima' showed the greatest incidence of black leg with 0.3% tubers affected, which is consistent with its NIAB rating [2], and the highest incidence of gangrene with 0.2% tubers affected. 'Maris Piper' showed the highest incidence of IRS with 3.0% tubers affected. Dry rot incidence was highest on 'Record' (0.6%), and spraing was highest on 'Cara' (0.2%). After a period in store, dry rot and gangrene increased, while levels of black leg and spraing decreased slightly. Levels of IRS increased during storage every year, except 1996. Conditions during storage influenced disease/blemish levels in tubers. Samples held in refrigerated stores showed a lower incidence of IRS than samples held in forced air or ambient stores. Generally, this was true whether or not the samples were damaged. In contrast, in 1995, IRS was lowest on samples that relied on natural convection. Spraing was only evident in tubers in stores where forced ventilation was used and only in samples that were also damaged. The effect of type of ventilation on gangrene and dry rot was variable throughout the survey. Black leg was not recorded in samples from refrigerated stores.

A number of factors have therefore been identified which influence incidence of tuber diseases/blemishes at harvest and during storage. Future work will continue to monitor the annual incidence of the diseases and will examine the influence of a wide range of factors including seed class, seed source and chemical inputs. Geographical information systems will be used to examine the influence of climatic factors and soil type on incidence of symptoms at harvest.

1. Hardwick NV, Turner JA, 1996. Proceedings of the 13th Triennial Conference of the European Association for Potato Research, pp. 631-632.
2. Anonymous, 1996. NIAB Farmers Leaflet No. 3.