THE EFFECT OF CALCIUM HYDROXIDE PRE-PLANT SOIL TREATMENT ON APPLE REPLANT DISEASE

Apple replant disease is a debilitating soil problem affecting most orchards when they are replanted. Symptoms normally affect the entire orchard and include slow, uneven growth and poor tree performance. After extensive glasshouse pot studies into apple replant disease, in 1999 a field trial was established to examine the performance of trees with potential treatments identified from the pot experiments. Over the first 5 years of crop life the cumulative yields fell into one of two categories, between 170 and 190 tonnes per hectare due to pre-plant soil treatment with calcium hydroxide, Dazomet or methyl bromide and between 120 and 140 tonnes per hectare for the untreated, nematicide and MAP fertilizer. After the first 10 years of crop life the cumulative yield of fruit due to apple replant disease was reduced from 450 to 325 tonnes of fruit per hectare. At this point in the crop life the impact of replant disease on annual yield had not disappeared although its impact was reduced. To confirm the potential of calcium hydroxide, an environmentally friendly treatment, as an environmentally friendly alternative to soil fumigation a second field trial at a different orchard was established in 2006. By the conclusion of the first three seasons of growth it was found that the highest rate of lime used, 10 t/ha stimulated tree growth to the same extent as Dazomet supporting the first trial results. All rates of application had an effect and a linear relationship between rate of application and tree growth was detected. This finding for pre-plant soil treatment with calcium hydroxide potentially provides an environmentally friendly method of ensuring successful and profitable apple orchard replacement without the use of toxic soil sterilants.