Field trials of copper fungicides for the control of potato blight III. Low‐copper fungicides

In trials at Beaford, north Devon, and at Galmpton, south Devon, in 1945. proprietary preparations of copper oxychloride, cuprous oxide, and cuprammonium, used at low‐copper dosage (0.063 % Cu), were compared with Bordeaux mixture at the same and at higher dosages for the control of potato blight under two commonly recurring conditions of disease incidence, respectively early and late, which were closely defined by foliage decay curves. All the copper fungicides were tested with and without added adhesive of caseinate‐rosin soap type, and two sulphur fungicides: lime sulphur (1 in 50) and Dithane 14 (1 in 200) with zinc sulphate and lime, were also included. The trials were made on four‐times replicated plots at both centres; special precautions were taken to avoid confusion of the observations by concurrent mineral deficiency and virus disorders; and the treatments were compared by three criteria: (1) direct foliage protection, (2) spray retention and (3) indirect effect on yield. The number of spray applications was reduced to a minimum to avoid masking the superiority of the better treatments, and timed as nearly as possible for maximum effect in relation to weather conditions. Thus two applications were given at Galmpton and three at Beaford. None of the proprietary copper fungicides used at low‐copper dosage, with or without added adhesive, equalled plain (4.5.40) Bordeaux mixture in spray retention or direct protective effect; and none gave any significantly better protection than Bordeaux mixture at correspondingly reduced copper dosage (1.11/2.40). With the few well‐timed and thorough sprayings given, however, the 1% Bordeaux mixture itself gave almost sufficient protection for the production of a full crop, and the trials indicated horticultural conditions under which low‐copper fungicides can be used with success, while pointing to the important factor of safety secured by using 1 % Bordeaux mixture in routine agricultural practice. Both of the sulphur fungicides were less effective than i % Bordeaux mixture.