Yuma Agricultural Centre, University of Arizona, Yuma, AZ 85364, USA

Background and objectives

Sclerotinia minor and S. ;sclerotiorum are both causal agents of leaf drop of head lettuce in the state of Arizona in the USA. During the winter months of late December through early March, Sclerotinia leaf drop can cause significant economic losses of this crop. Head lettuce in Arizona is grown on raised beds, with two rows of lettuce on each bed spaced 30 ;cm apart. After the harvest of over 20,000 ;ha of lettuce, crop residue, including sclerotia of both pathogens, is incorporated into the soil by disking, followed by planting of wheat on nonbedded soil or melons, cotton, corn or safflower on newly constructed beds. To develop a successful management programme for Sclerotinia lettuce drop, we need to know the fate of the sclerotia of S. ;minor and S. ;sclerotiorum in soil between crops of lettuce. In earlier research, low soil moisture was said to reduce the survival of sclerotia of S. minor [1]. Field trials were conducted in 1996 and 1997 in Arizona to study potential changes in the survival of sclerotia of S. ;minor and S. ;sclerotiorum with respect to their depth in soil, soil moisture and duration of time in the field in the absence of a lettuce crop.

Materials and Methods
Field studies during July and August of 1996 and 1997 were established at the Yuma Agricultural Centre in a dry silty clay loam soil typically used for lettuce production. Raised beds 60 ;cm wide, 17 ;cm high and 110 ;cm between bed centres were established. Small packets constructed of nylon mesh screen material each were filled with 15 sclerotia of S. ;minor or S. ;sclerotiorum, then placed on the soil surface or buried at a depth of 5 or 10 ;cm in the middle of the raised beds or in the furrows. Water was applied by furrow irrigation after placement of sclerotia in plots and normally every 2 ;weeks thereafter. An additional treatment consisted of sclerotia placement in raised beds that received no irrigation. At 2, 4, 6 and 8 ;weeks after initiation of each trial, sclerotia were collected, surface-sterilized and plated on potato dextrose agar to determine their viability. Soil temperature and moisture were monitored for the duration of each study.

Results and conclusions
At the surface of non-irrigated soil, the percentage of viable sclerotia of S. minor but not S. sclerotiorum was significantly higher than that amount recorded at a depth of 10 cm. The percentage of sclerotia of S. minor at the soil surface or at a depth of 5 or 10 ;cm that germinated in the 1996 study was 57, 59 and 5%, respectively, and 80, 33 and 16%, respectively, in 1997. For S. ;sclerotiorum, germination of sclerotia in non-irrigated soil at a depth of 0, 5 and 10 ;cm in 1996 was 16, 7 and 5%, respectively, and 93, 93 and 82%, respectively, in 1997. The percentage of viable sclerotia for S. minor and S. ;sclerotiorum in the 1996 and 1997 studies was consistently and significantly higher in non-irrigated soil than in irrigated furrows. For S. ;minor, the percentage of sclerotia that were viable in non-irrigated beds, irrigated beds and irrigated furrows during 1996 was 40, 4 and 0%, respectively, and 43, 52 and 14%, respectively, during 1997. For S. ;sclerotiorum, the percentage of viable sclerotia in non-irrigated beds, irrigated beds and irrigated furrows in 1996 was 9, 1 and 0%, respectively, and 89, 84 and 46%, respectively, in 1997. In both years, there was no significant difference in the viability of sclerotia of S. ;minor or S. ;sclerotiorum in irrigated and non-irrigated soil at 2, 4, 6 and 8 ;weeks following placement in the field. This data suggests that one or more flood irrigations could significantly reduce and perhaps virtually eliminate viable sclerotia of S. ;minor and S. ;sclerotiorum from lettuce production fields in Arizona.

1. Adams PB, 1987. Plant Disease 71, 170-174.