SCLEROTIAL SURVIVAL AND APOTHECIAL PRODUCTION BY SCLEROTINIA-SCLEROTIORUM FOLLOWING OUTBREAKS OF LETTUCE DROP

The number of sclerotia of Sclerotinia sclerotiorum gradually declined over the years following outbreaks of lettuce drop in four naturally infested fields. The rates of population decline did not differ signif icantly among the four fields tested. In two fields located in a semia rid region, 5.5 and 2% of the initial amounts of sclerotia were still viable after 7 yr. Sclerotia removed from soil samples were classified by weight into four groups: 14-40, 7-14, 3-7, and 1-3 mg per scleroti um. The frequency distribution of the four sclerotial weight groups ch anged over the years, as small sclerotia became increasingly predomina nt over large ones. This trend was significant according to a generali zed logit model. The number of apothecia produced per sclerotium showe d a significant nonlinear increase with increasing sclerotial weight. The percentage of apothecia-producing sclerotia decreased significantl y with increasing depth of burial in the soil. In addition, apothecial production was delayed with depth. Of the apothecia produced by scler otia in lettuce fields 80 days after planting, 94% were located in the top 5 cm of the soil. The highest relative frequency of carpogenicall y germinated sclerotia (24.6%) was found at a depth of 2 cm.