Scion and stock diameter size effect on growth and fruit production of Sclerocarya birrea (marula) trees

Abstract

Sclerocarya birrea (marula) is a highly valued fruit tree in southern Africa because of its products which have gained both regional and international markets. However, its fruits have been harvested from the wild with a few provenances being managed on farmland. Grafted marula trees have shown wide variations in scion and rootstock growth (for example, stem diameters), but effects of these variations on tree survival, growth and fruit production have not been evaluated. The objectives of this study were to (1) assess variations in growth of scions and their stocks and (2) evaluate the effect of scion and stock growth differences on overall grafted tree growth, survival, flower and fruit production on farmland, six to eight years after planting. Over 70% of the grafted trees had similar scion and stock diameters, while 24% had thicker (i.e, slow growing) scions than their stocks. Trees with similar scion and stock growth rate had a high flowering percentage (> 40%), but there was also significant fruit abortion during the three years of fruit production. Grafted trees with thicker scions than their stocks produced significantly more (P < 0.05) fruits than the rest. Such trees were also significantly (P < 0.0126, N = 50) taller than the rest. There was a low survival for grafted trees with either thick or thin scions relative to the stock diameters. It can be concluded that unequal growth rate of scion and stock in grafted marula trees has a negative effect on early tree survival and dwarfing characteristics, but this may not reduce fruit yield in the early years. Therefore, scion and stock selection is critical to reducegrowth variations which have negative effects on grafted tree growth and fruit productivity.