Yield-density relations for apple trees on a dwarfing and a semi-dwarfing rootstock

In a planting system trial with Golden Delicious on M IX and M II, started in 1962, lowdensity plots are compared with high-density plots. The low-density plots — 1100 trees per ha on M IX and 660 trees per ha on M II — came into bearing slowly and yields leveled off at about 40 tons per ha. For the high-density plots — 3300 and 2260 trees per ha for M IX and M II, respectively — yields quickly increased to about 75 tons per ha in 1967 and 1968, but thereafter yields declined, associated with more intensive prun­ ing and fruit thinning to restore tree vigour and fruit quality. Yield per unit growth was initially somewhat higher but finally much lower in the high-density plots, indicating that fruiting suffered more than growth from competition between trees. Yield per unit growth was much higher on M IX than on M II, an advantage of decisive importance at high density. Fruit size was slightly better on M IX, and at high density the problems in maintaining fruit size were not as serious as on M II. Growth control in the high-density M II plot became difficult and tree thinning may be advisa­ ble. Under the conditions of the trial the optimum density for Golden Delicious on M IX probably lies above 3000 trees per ha, on M II below 2000 trees per ha. Notions regar­ ding rootstock effects on yield per unit growth, fruit quality, etc. may be misleading if they are not based on comparisons under equal competitive stress.