Identification of superior cashew trees for Northern Australian conditions

Abstract

Cashew is a highly cross-pollinated crop and trees derived from seedlings do not breed true-to-type. The most effective and quickest approach to improving this highly heterogeneous crop will be to identify superior trees, combining high yields with good quality kernels. This study was conducted to identify superior trees from amongst the trees originally derived from seedlings at Cashews Australia, Dimbulah, in Far North Queensland. Morphological characterisation and preliminary evaluation were carried out to determine the extent of variability present in the cashew population. Morphological characteristics associated with high productivity were identified.The nut yield characters were the most variable characters studied, with the number of nuts per tree showing the highest variation. Total nut weight per tree was positively correlated with the nut number per tree whilst nut number per tree was negatively correlated with the mean nut weight per tree. The lack of significant correlation between the total nut weight and the mean nut weight indicated the possibility of selecting high yielding trees with good quality nuts.From the original 135 well-buffered trees evaluated, nineteen (19) trees combined levels of total nut weight per tree, number of nuts per tree and mean nut weight per tree higher than the mean of the cashew population studied. Four trees were considered superior since they produced total nut weights of more than 6.0 kg per tree and more than 6.0 g mean nut weight. These trees were recommended as mother trees to be used for asexual propagation.A wide range of variation was observed in the nut, apple, tree, leaf and panicle characters. Testa weight, shell weight, nut weight, kernel weight, apple weight and the number of panicles per tree showed wide variation.Highly positive correlations were observed amongst the nut, shell and kernel characters, except for shelling recovery which showed negative correlations with the nut and shell characters. Apple weight was highly correlated with apple length and diameter whilst it was not significantly correlated with the total soluble solids (TSS). Apple weight, length and diameter also showed positive correlations with the nut characters, except shelling recovery, with which they were negatively correlated. Highly significant correlations were observed amongst the tree and leaf characters. The leaf characters were positively correlated with the nut, shell and kernel weights. Panicle length was highly correlated with panicle width but it was not significantly correlated with the number of panicles per tree.The total nut weight per tree was highly correlated with the tree characters (height, girth and canopy spread). Nut number per tree was negatively correlated with the nut characters (except shelling recovery) and with apple characters (except TSS) and positively correlated with tree canopy spread. Mean nut weight per tree was negatively correlated with shelling recovery and positively correlated with the apple characters (except TSS) and leaf characters.Multiple correlation analysis showed that kernel weight, nut width and tree canopy spread were the most important characters influencing the total nut weight per tree. The tree canopy spread highly influenced the number of nuts per tree whilst apple TSS and the number of panicles per tree influenced mean nut weight per tree.A wide range of variation was also observed in the qualitative characters studied. High total nut weight was highly associated with trees bearing apples with shallow cavity at the apex and early flowering season. High nut number per tree was highly associated with trees bearing apples with level apex and early flowering season. High mean nut weight was highly associated with large nut stylar scar, deep cotyledonary grooves and an oblique apple apex.The northern and western quadrants of the cashew tree canopy produced higher total nut weight, nut number, total number of shoots and number of shoots with panicles than the eastern and southern quadrants. In addition, well-buffered trees produced significantly less total nut yield than trees located in the border and poorly-buffered trees.The identification of superior trees will be useful in increasing nut production and improving the quality of nuts produced at Cashews Australia. The morphological characters associated with the nut yield components will be useful in selection work.