Relationship of Leaf Necrosis and Defoliation to Phosphorus and Potassium Concentrations In Selected Tissue and to Certain Fruit Quality Parameters of Pecan

Pecan [Carya illinoinensis (Wangenh.) K. Koch] fruit enlarge to full size during the first 70-100 days after pollination followed by rapid cotyledon development the last 30-45 days before ripening (shuck dehiscence).  The primary storage carbohydrate in the seed is oil, accounting for about 70% of the kernel’s weight.  This places an enormous late-season demand for carbohydrate translocation to the developing fruit followed by oil synthesis.  Potassium (K) decreases rapidly in the leaves and accumulates in the fruit beginning about 75 days before ripening, while rapid phosphorus (P) movement to the fruit begins about 30 days before shuck split.  Potassium allocation patterns appear related to carbohydrate transport during the rapid size increase and continue during cotyledon development.  Phosphorus allocation appears related to oil synthesis and subsequent P storage in the seed.  Trees in an orchard near Cleveland, OK developed necrotic areas in the leaves during the latter part of the growing season.  Leaf samples collected during July indicated that P (0.114% DW) and K (0.62% DW) were low, but other elements were within accepted sufficiency ranges.  Twenty trees were rated for necrotic symptoms on 1 Oct. 2009 using a 1 (none) to 5 (severe necrosis and partial defoliation) scale.  Fruit (shuck and nut) were harvested after shuck split (1 Oct.) and 20-fruit samples were analyzed for weight/nut (pericarp plus cotyledons), kernel weight, and graded using the commercial 1 to 4 scale where 1 is best and 4 is a reject.  Leaflet pairs (25/tree) were collected from the middle leaf on fruit bearing shoots 1 October.  Leaves, shucks and kernels were analyzed for P and K concentration.  Necrosis rating was negatively correlated with weight/nut, weight/kernel, and P concentration in the kernel, shuck and leaf.  The rating was also negatively related to K concentration in the kernel and leaf.  Leaf, kernel and shuck P concentration were positively correlated with weight/nut, weight/kernel, and the percentage of number 1 (best) kernels.  Leaf K was weakly correlated to weight/nut and weight/kernel.  Leaf P concentration was a good predictor of shuck and kernel P concentration, but leaf K was moderately correlated with shuck K and not related to kernel K concentration.  Results suggest that P and K were limiting, but P appeared to have a greater impact on measured parameters than K and leaf P concentration was more closely related to leaf necrosis and defoliation than leaf K.