Control of contamination of potatoes with air‐borne Erwinia carotovora by foliar application of copper oxychloride

SUMMARY In field experiments in 1983 and 1984, micropropagated plants of cv. Désirée planted in soil tested and found free of erwinias became contaminated with airborne erwinias, mostly E. carotovora pv. carotovora, in September when rain was more abundant, but not earlier. Control of contamination of progeny tubers harvested in September and October was achieved by spraying plants with copper oxychloride (‘Cuprokylt’; 5 g litre‐1) twice weekly from June or July but not from August nor by weekly or fortnightly treatments from June. Bactericidal activity of copper in the effective spray regimes was not expressed on the leaves or in the soil and contamination was similar in the sprayed and unsprayed treatments. In contrast, multiplication of erwinias in senescent leaves on the soil surface (leaf debris) was inhibited in the sprayed treatment but not in the unsprayed treatment and population numbers of c. 103 cells g‐1 fresh weight were detected. It is suggested that fewer erwinias would be washed from leaf debris by rain in the sprayed than in the unsprayed treatment to contaminate the progeny tubers.