1Plant Pathology Department, University of California, Riverside, CA 92521, USA; 2Cooperative Extension, University of California, 624 W. Foster Road, Ste A, Santa Maria, CA 93455, USA

Background and objectives
Downy mildew of lettuce is caused by Bremia lactucae Regel. In the semi-arid climate of California, disease intensities and crop losses show large seasonal and annual variations, depending to a large extent on weather factors. The disease is not economically important in the desert areas of the Imperial or Palo Verde valleys near the Mexican border because of low humidity and extremely low rainfall. But in the coastal regions from the Santa Maria area north to the Salinas Valley and sometimes on the west side of the San Joaquin Valley, the disease reduces quality and yield of the lettuce pack. In previous years single-gene resistant cultivars provided excellent control for many years, but strains of B. lactucae have developed that overcome this resistance. Therefore there is a continuous need for testing new protective or systemic fungicides for control of downy mildew of lettuce.

Materials and methods
Three trials were conducted during 1997 on commercial iceberg lettuce farms in the coastal Santa Maria area of Santa Barbara County, approximately halfway between San Francisco and Los Angeles. Fogs and heavy dews occur frequently and rainfall in the spring and fall enhance development of the downy mildew disease of lettuce. The cultivar Legacy was used in all trials. Fungicides were applied four times in each trial on a 10-day schedule, starting shortly after the lettuce transplants were planted in the field. Fungicides were applied in 598 I water/ha using a CO2 sprayer set at 35 p.s.i. using a four-nozzle boom. Plots were 7.6 m long with two rows of lettuce per bed. Fungicides included maneb, mancozeb, banol, dimethomorph, mefenoxam, azoxystrobin, aliette, acibenzolar-S-methyl and copper hydroxide. Disease ratings were made approximately 10 days after the last application and rated on a scale of 0-10, with 10 being severe mildew.

Results and conclusions
Acibenzolar-S-methyl provided excellent control of downy mildew of lettuce in all three trials. Propamocarb/mancozeb was effective for control in the third trial and provided moderate control in the second trial. Azoxystrobin did not provide commercial control of downy mildew in any of the trials.

1. Fletcher JT, 1976. Annals of Applied Biology 84, 294-298.
2. Scheftini TM, Legg EJ, Michelmore RW, 1991. Phytopathology 81, 64-70.