Comparison of three fungicide spray advisories for lettuce downy mildew

Lettuce growers in coastal California have relied mainly on protective fung icide sprays to control downy mildew. Thus, timing of sprays before infecti on is critical for optimal results. A leaf-wetness-driven, infection-based advisory system, previously developed, did not always perform satisfactoril y. In this study, the advisory system was modified by incorporating a patho gen survival component (system 1) or both survival and sporulation componen ts (system 2). These systems were then evaluated in commercial lettuce fiel ds in coastal California during 1996-1998. Three or four treatments were ca rried out in each field: (i) no spray; (ii) sprays as scheduled by the grow ers; (iii) sprays following modified system 1; and (iv) sprays following th e original advisory system (1996) or modified system 2 (1998). Downy mildew incidence was evaluated every 2 to 9 days. In fields with drip irrigation, the number of fungicide applications was reduced by one or two regardless of the advisory system used compared to the grower's calendar-based schedul e, although one unnecessary spray was recommended in 1996 at Soledad and 19 97 at Salinas. Under all three systems, disease levels were low (incidence < 25% and about 1 lesion per plant) for fields with drip irrigation, but no t for fields with sprinklers (incidence up to 100% and 5 to 10 lesions per plant). For the first time, we established that survival and sporulation co mponents are not needed for a lettuce downy mildew forecasting system. Inst ead, a threshold with a shorter period of morning leaf wetness and high tem peratures were found to have potential for improving forecasting efficiency .