MECHANISMS OF SUBSURFACE DRIP IRRIGATION-MEDIATED SUPPRESSION OF LETTUCE DROP CAUSED BY SCLEROTINIA MINOR

Subsurface drip irrigation and associated mandatory minimum tillage pr actices significantly reduced the incidence of lettuce drop (Sclerotin ia mir?or) and the severity of corky root on lettuce compared with fur row irrigation and conventional tillage. Three possible mechanisms for the drip irrigation-mediated disease suppression were examined in thi s study: qualitative and quantitative differences in the soil microflo ra under furrow and subsurface drip irrigation; their antagonism and p otential biocontrol effects on S. minor; and the physical distribution of soil moisture and temperature relative to the two irrigation metho ds. To determine if the suppressive effects under subsurface drip irri gation were related to changes in soil microflora, soils were assayed for actinomycetes, bacteria, and fungi during the spring and fall seas ons. The effects of the irrigation methods on microbial populations we re nearly identical during both seasons. In the spring season, the tot al number of fungal colonies recovered on potato dextrose agar amended with rose Bengal generally was greater in soils under drip irrigation than under furrow irrigation, but no such differences were observed d uring the fall. Numbers of actinomycetes and bacteria were not signifi cantly different between irrigation methods during either season. No i nteraction between sampling time and irrigation methods was observed f or any of the microbial populations during both seasons. Thus, the sig nificant effect of sampling time observed for actinomycete and bacteri al populations during the spring was most likely not caused by the irr igation treatments, There were also no qualitative differences in the three groups of soil microflora between the irrigation treatments. Eve n though some fungal, actinomycete, and bacterial isolates suppressed mycelial growth of S. minor in in vitro assays, the isolates came from both subsurface drip-and furrow-irrigated soils. In in planta assays, selected isolates failed to reduce the incidence of drop in lettuce p lants. The soil moisture under subsurface drip irrigation was signific antly lower at all depths and distances from the bed center after an i rrigation event than under furrow irrigation. The soil temperature, in contrast, was significantly higher at both 5 and 15 cm depths under d rip irrigation than under furrow irrigation. The suppression of lettuc e drop under subsurface drip irrigation compared with furrow irrigatio n is attributed to differential moisture and temperature effects rathe r than to changes in the soil microflora or their inhibitory effects o n S. minor.