CS Young1, JMLI Davies2, JM Whipps3, SP Budge3, LC Hiron2, JA Smithl and M Watling2

lADAS Wolverhampton, Woodthorne, Wergs Road, Wolverhampton WV6 8TO, UK; 2ADAS Terrington, Terrington St. Clement, Kings Lynn, Norfolk PE34 4PW, UK; 3Horticulture Research International, Wellesbourne, Warwick CV 35 9EF, UK

Background and objectives
Sclerotinia sclerotiorum of field-grown lettuce can be found at a low incidence on most farms but high losses can occur, leading to total crop loss in some plantings. The objectives of this field study were to determine the relationships between cropping practice, soil type, sclerotial position and environmental conditions on apothecial production and infection of lettuce by S. ;sclerotiorum.

Materials and methods
In 1996 and 1997, from April to August, five sequential crops of lettuce were planted monthly on a naturally infected site and a disease-free site, in a Cheshire peat soil (both sites), an East Anglia clay loam (disease-free site) and in a very fine silty loam soil (naturally infected site). At the start of each planting, conditioned sclerotia were retrieved and buried at a depth of 2 ;cm in predetermined positions under plastic grids. The grids were placed either under or between lettuce plants to investigate the effects of scierotial position. Each of these inoculum position treatments was also applied to plots of single (commercial) and double-spaced lettuce to investigate the effects of plant spacing. Meteorological stations were set up to measure rainfall, air and soil temperatures, soil moisture, soil moisture tension, surface wetness, wind speed and solar radiation during the growing seasons.

Results and conclusions
Apothecia were observed at least 3-4 ;weeks after sclerotia were buried in both years. In 1996, there were more apothecia and diseased plants earlier in the season in Cheshire, with more apothecia later in the season in East Anglia. The maximum sclerotial germination observed from any treatment in 1996 and 1997 was, respectively, 100% and 52% in Cheshire, and 44% and 54% in East Anglia. There was considerable variation in the timing and quantity of apothecial development between plantings and particularly between sites, in both years. In 1997, fewer apothecia were produced in Cheshire than in 1996, probably owing to rotting of sclerotia. However, there were higher numbers of apothecia in East Anglia in 1997, possibly owing to higher than average rainfall during the growing season. There was no consistent effect of lettuce spacing or sclerotial position on the number of scierotia germinated, the number of apothecia produced, or the distribution of these over time. Only at the infected site, in East Anglia, in the last two plantings in 1997 were there significantly more sclerotia (mean of 3.6 times more) germinated among single-spaced lettuce than double-spaced lettuce. There was no clear relationship between the appearance of apothecia and the development of disease. Disease was seen from nil to 4 ;weeks after and, in a few cases preceding, apothecial production in the grids. Disease was not related to the presence of apothecia directly beneath plants, which suggests that ascospores from other sources may be responsible for infection. The differences in the production of apothecia and disease development cannot be explained entirely by environmental conditions. The sclerotia at each site may be acclimatized to their own soils and may have different seasonal responses. Qualitative examination of the meteorological data revealed that the first appearance of apothecia was usually preceded, 7-12 days, by a rise in soil tension followed by an increase in soil moisture. Apothecia did not usually develop unless temperatures were between 10 and 20C. Laboratory and glasshouse studies under controlled conditions are underway to confirm field observations of the key factors controlling apothecial production and lettuce infection.