Successful control of apple scab with hydrated lime

Summary in the original language of the document

In an attempt to control scab, a 12-year-old apple orchard was treated with suspensions of 5 g/l hydrated
lime (Ca(OH)2) using overhead irrigation for 15 min. No other fungicides were used. In treatment A, irrigation
was started when infection according to Mills had taken place. In treatments B and C, treatment was delayed
by 6 h and 20 h, respectively. Irrigation was initiated when the scab warning system signaled even very low
infection risk, and a total of 62 treatments were applied. At harvest, the fruits of ‘Golden Delicious’ and
‘Jonagold’ varieties of untreated controls were scabbed, while those of ‘Elstar’, ‘Fiesta’, and ‘Kent’ had no
visible scab symptoms. During both conventional and ULO storage for 7.5 months, all control fruits
developed scab symptoms. All fruits from A, B, and C treatments were free of scab at harvest and after
storage. All control trees had scabbed leaves at harvest time; treated trees were free of scab on their upper
leaf surfaces, while up to 15% of the leaves had at least one scab lesion on their lower surfaces. This was
attributed to poor wetting of these surfaces by sprinkler irrigation, while the fruits and upper leaf surfaces
were fully covered with hydrated lime suspensions. At harvest, leaves and fruits were covered with whitish
calcium carbonate residues which could be removed by a brief dip in 20 g/l aqueous citric acid. There were
no signs of phytotoxicity on either leaves or fruits.