Adjuvant effects on disease severity caused by the bioherbicide P. papaveracea on opium poppy were evaluated. Tween 20, Tween 80, Triton X-100, Tactic, CelGard, and Keltrol inhibited appressorium formation but not conidial germination on detached leaves. The disease severity varied from 11 to 83% necrosis in field experiments involving eight adjuvants at various concentrations plus 1 × 10⁶ conidia ml⁻¹ or minus pathogen. The three best-performing adjuvants when combined with pathogen, Tactic (1%, v/v), Bond (1%, v/v), and Tween 20 (1%, v/v), were included along with Tween 20 (0.001%, v/v) in field experiments in 1998. Tween 20 (1%, v/v) plus pathogen (1 × 10⁶ conidia ml⁻¹) caused the most severe disease, averaging 68% necrosis within 2 wk of treatment. Overall, plots treated with adjuvant plus P. papaveracea had a 22% reduction in capsule weight per plot as compared to plots treated with the adjuvant alone. Tactic (1%, v/v), Silwet-L77 (0.1%, v/v), Tween 20 (1%, v/v), and Tween 20 (0.001%, v/v) were included in field experiments in 1999. The treatment with Tween 20 (1%, v/v) plus pathogen (2 × 10⁶ conidia ml⁻¹) caused severe disease, averaging 56% necrosis within 2 wk of treatment. In 1999 plots treated with adjuvant plus pathogen averaged a 27% reduction in capsule weight as compared with plots treated with the adjuvants alone. The inclusion of Tween 20 (1%, v/v) with P. papaveracea conidia greatly enhanced efficacy on opium poppy.

Nomenclature: Opium poppy, Papaver somniferum L. PAPSO; Pleospora papaveracea Sacc.