1Horticulture Research International, Wellesboume, Warwick, UK; 2ADAS Wolverhampton, Woodthome, Wergs Road, Wolverhampton, UK; 3ADAS Terrington, Terrington St Clement, Kings Lynn, Norfolk, UK.

Background and objectives
Sclerotinia sclerotiorum is a commercially important disease of field-grown lettuce in the UK, with over 50% crop loss occurring in some seasons favourable to the disease. Laboratory and field studies are being carried out to gain information on factors influencing carpogenic germination of sclerotia and ascospore release by S. ;sclerotiorum and subsequent infection of lettuce at two different sites in the UK. Many previous laboratory studies have examined factors affecting carpogenic germination by sclerotia of S. ;sclerotiorum but contradictory results were frequently found, implying that strain differences in S. ;sclerotiorum exist and that other factors such as soil type may be important. This paper reports on the influence of temperature and duration of sclerotia conditioning, soil type and soil water potential, on carpogenic germination of three isolates of S. ;sclerotiorum. Isolates were obtained from fields in Cheshire and Suffolk and from a glasshouse in Sussex. A peat soil from Cheshire and a sandy loam from Suffolk were used throughout.

Results and conclusions
Without a period of cold-treatment conditioning, less than 10% of sclerotia of either the Cheshire or Suffolk isolates germinated; conditioning sclerotia of these two isolates increased germination up to 80% and 50%, respectively. In contrast, nonconditioned sclerotia of the glasshouse isolate, consistently germinated at levels approaching cold-treated sclerotia of this strain (65% and 75%, respectively). The influence of temperature and duration of sclerotial conditioning was similar for each strain. All three cold-treatment temperatures (0, 4 and 12C) increased carpogenic germination. Sclerotia incubated at OC germinated at an average rate of 40% and those incubated at 4 and 12C germinated at 70%. The duration of conditioning further affected optimum conditioning conditions. At 0 and 4C, over 70% of total sclerotial germination potential was reached after 4 ;weeks. However, when sclerotia were conditioned at 12C, less than 30% of the total germination potential was achieved; a further 4 ;weeks was required to attain optimum rates of carpogenic germination. The results suggest that although low soil temperatures (<1OC) are not required to overcome constitutive sclerotial dormancy, the higher the soil temperature, the longer the period of conditioning needed. The soil type in which sclerotia were germinated strongly affected carpogenic germination rates. The peaty Cheshire soil consistently supported the highest levels of apothecial production for all three isolates of S. ;sclerotiorum (80, 70 an 50% for the Cheshire, glasshouse and Suffolk isolates, respectively). In the Suffolk soil, germination was generally poorer (50, 40 and 15% for the glasshouse, Suffolk and Cheshire isolates, respectively). Not only was there a significant soil effect but there was also an indication of strain specificity to soil of its own origin, as seen by the response of the Cheshire isolate. The three Sclerotinia isolates behaved similarly when sclerotia were germinated in the two different soils at a range of soil water potentials between -1 and -10 ;kPa. Although both soil types induced a similar response of carpogenic germination (the optimum water potential for both soils was -10 ;kPa), the Cheshire soil allowed germination of sclerotia to occur over a significantly broader range of soil water potentials (down to -500 ;kPa) than the Suffolk soil (down to -200 ;kPa). The variation found between strain behaviour in different soils in this study reflects the conflicting results reported by previous workers. Isolates from disparate origins appear to behave differently in response to environmental factors and may have adapted to their own particular environment. Any future disease forecasting models may have to take into account regional variations of pathogen response to specific conditions.