COMPARISON OF LETTUCE DISEASES AND YIELD UNDER SUBSURFACE DRIP AND FURROW IRRIGATION

Subsurface drip and furrow irrigation were compared on lettuce (Lactuc a sativa) cvs. Salinas and Misty Day for yield and incidence and sever ity of three important diseases of lettuce in the Salinas Valley, CA. Experiments were conducted between 1993 and 1995 during the spring and fall seasons. The diseases examined included lettuce drop (Sclerotini a minor), downy mildew (Bremia lactucae), and corky root (Rhizomonas s uberifaciens). Replicated plots of subsurface drip and furrow irrigati on were arranged in a randomized complete-block design. All plants wer e inoculated with S. minor at the initiation of the experiment during the 1993 spring season. Plots were not inoculated for downy mildew and corky root during any season nor were the plots reinoculated with S. minor. During each season, all plots were sprinkler irrigated until th inning, and subsequently, the irrigation treatments were begun. The fu rrow plots were irrigated once per week, and the drip plots received w ater twice per week. The distribution of soil moisture at two soil dep ths (0 to 5 and 6 to 15 cm) at 5, 10, and 15 cm distance on either sid e of the bed center in two diagonal directions was significantly lower in drip-irrigated compared with furrow-irrigated plots. Plots were ev aluated for lettuce drop incidence and downy mildew incidence and seve rity at weekly intervals until harvest. Corky root severity and yield components were determined at maturity. Lettuce drop incidence and cor ky root severity were significantly lower and yields were higher in pl ots under subsurface drip irrigation compared with furrow irrigation, regardless of the cultivar, except during the 1994 fall season. Incide nce and severity of downy mildew were not significantly different betw een the two irrigation methods throughout the study. The differential microclimates created by the two irrigation treatments did not affect downy mildew infection, presumably because the mesoclimate is usually favorable in the Salinas Valley. Subsurface drip irrigation is a viabl e, long-term strategy for soilborne disease management in lettuce in t he Salinas Valley.