Adjustment of soil-surface pH and comparison with conventional fungicide treatments for control of lettuce drop (Sclerotinia minor)

The use of soil-surface applications of finely powdered calcium hydroxide (Ca(OH)₂) to inhibit Sclerotinia minor sclerotial germination and infection at the collar region of lettuce plants is described. In the laboratory, a pH > 8·0 reduced sclerotial germination of the three S. minor isolates tested. In the glasshouse, surface applications of 2–10 t Ca(OH)₂ ha^?1 raised the pH of the top 1–2 cm of a duplex sandy loam soil above 8·5 for at least 8 weeks without affecting soil pH within the transplant root zone. There was a linear relationship between the rate of Ca(OH)₂ applied and disease control, with complete disease suppression at 10 t Ca(OH)₂ ha^?1. In field trials on two soil types (duplex sandy loam, pH 6·0; and red ferrosol, pH 6·9), a rate of 2·5 t Ca(OH)₂ ha^?1, maintained soil-surface pH above 8·5 for 1–3 weeks and provided up to 58% reduction in lettuce drop. Application of polyvinyl alcohol (a soil-conditioning polymer) over the Ca(OH)₂ layer appeared to reduce Ca(OH)₂ loss by wind, but did not improve retention of raised soil-surface pH or disease suppression. Ca(OH)₂ treatment gave similar disease control to the industry standard treatment of a procymidone-based fungicide seedling drench. A combined treatment of Ca(OH)₂ and fungicide drench gave greater control than either individual treatment, and equivalent control to fungicide drench and three procymidone foliar sprays, offering integrated management options. The use of soil-surface-applied Ca(OH)₂ with fungicides, rotation and drip irrigation offers an opportunity for enhanced and sustainable control of lettuce drop.