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Simon A. Mng’omba and Elsa S. du Toit

Grafting has been a common propagation method in many fruit trees to achieve fruiting precocity. Its success has depended on proper alignment of parenchymatous tissues of both scions and their respective rootstocks. Although grafting has been practiced for a long time, the right or ideal length of a diagonal cut surface that promotes proper alignment of vascular bundles of both graft partners to ensure fast graft-take and eventually graft success have not been investigated. An experiment was carried out on mango, avocado, and peach trees with an objective of establishing suitable or correct length of a diagonal cut surface when grafting (splice method) to improve graft success. Diagonal cut surface lengths applied to both scions and rootstocks included 5 mm, 10 mm, 20 mm, 40 mm, 60 mm, 80 mm, or 100 mm with at least three grafters treated as a random component. A significant difference (P < 0.0001) was found among the diagonal cut surface lengths and over 80% of graft success was obtained with 40- to 100-mm diagonal cut surface lengths, especially for mango. Significant differences (P < 0.0001) were also obtained in shoot length and stem diameter size (thickness) for all tree crops studied and also for number of leaves except for avocado plants. It is concluded that improved graft success can be achieved with a diagonal cut surface length of 40 mm long with the need for a few grafting strips.

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Simon A. Mng'omba, Elsa S. du Toit, Festus K. Akinnifesi, and Helena M. Venter

Compatibility of scion and stock combinations in Uapaca kirkiana fruit trees has not been evaluated despite noticeable growth irregularities. The objectives of this study were to determine graft compatibility of scion/stock combinations and possible causes of graft incompatibility. Scion, stock, and graft union diameters were measured. Stem sections comprising the graft unions were immersed in formalin acetic acid and then washed in sterile water. These were transversely dissected across the unions and examined under using light microscope. There were considerable growth disorders at the unions, which included significant overgrowth of stocks and unions and constricted unions. There were cracks in the bark across the union in many graft combinations. Anatomic and histological studies showed accumulation of phenol deposits and necrotic tissues, and there was no continuity of vascular tissues above the union. There were also differences in proliferation of callus tissues among grafted partners. Continuity in wood and bark tissues below the unions supported growth of partially compatible partners, whereas isolated parenchymatous tissues at the union supported growth of incompatible partners. There were many necrotic tissues and unfilled areas above the union. Accumulation of phenolic and necrotic cell deposits, poor or a high level of callusing at the union, and possibly specific incompatibility reactions were implicated as the causes of graft incompatibility in U. kirkiana trees.

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Festus K. Akinnifesi, Simon A. Mng'omba, Gudeta Sileshi, Thomson G. Chilanga, Jarret Mhango, Oluyede C. Ajayi, Sebastian Chakeredza, Betserai I. Nyoka, and France M.T. Gondwe

One of the limitations of Uapaca kirkiana on-farm cultivation is the long juvenile phase to reach a stable fruiting stage. Marcots and grafts have been identified as feasible and reliable propagation methods for precocious fruiting, but the effects of different propagule types on tree growth and fruit yield have not been evaluated. There is limited knowledge on development and growth forms for trees derived from different propagules. Grafts and marcots were compared with saplings to assess the variability in 1) field growth and fruiting of U. kirkiana; and 2) dry matter allocation pattern and tree development models among trees derived from different propagules. Tree development models were used to examine differences among trees from different propagules. The results show that number of branches and fruit load significantly differed between vegetative propagules and saplings, whereas tree height, root collar diameter, crown spread, and fruit size and weight were similar 8 years after establishment. The results suggest differences in intraspecific scaling relationships between height and diameter among propagule sources. Saplings showed a significantly better fit (r 2 = 0.891; P < 0.0001) to the scaling relationship than grafts (r 2 = 0.724; P = 0.002) and marcots (r 2 = 0.533; P = 0.018). After 3 years, marcots and grafts started producing fruits. Fruit load was greatest in marcots despite some fruit abortions; thus, marcots had greater fruit yield, early growth, and development and better dry matter allocation.