Neutralised phosphonic acid sprays applied to cauliflowers in the field within 3 weeks of harvest reduced postharvest development of downy mildew. Two applications of 2.4 kg a.i./ha, 21 and 7 d before harvest, reduced the incidence of curds affected by downy mildew from 92% in untreated plots to 8%. The max. phosphonate residue in curds at harvest was 12 micro g/g, while control curds contained no detectable phosphonate. No effects of phosphonic acid on crop appearance or maturity were observed, and while a small (8%) reduction in curd weight was significant (P = 0.039), there was no yield trend with increasing phosphonic acid rate.