Investigating abiotic and biotic parameters associated with gradually declining Valencia citrus trees in South Africa

Abstract

The current study determined if a combination of biotic and abiotic parameters could differentiate Citrus sinensis trees (three categories based on visual tree canopy status) with root disease-related decline symptoms in two citrus orchards. Two experimental sites, containing a range of healthy and different stages of decline trees, were randomly selected. Three decline category trees were selected with abiotic and biotic parameters associated with decline measured. Principal component analyses of 41 soil, leaf, root and trunk associated parameters ordered the category 1 to 3 trees chronologically on the factorial plan for both orchards. However, the parameters only described a low level of the variability. Only the healthy (category 1) and more diseased (category 3) trees were studied further. Soil electrical conductivity and leaf %N, were the only parameters differing significantly between the two categories in both orchards, but both parameters were significantly higher in category 3 trees in orchard 1 whereas the opposite was true for orchard 2 with significantly higher values in category 1 trees. In the more severely declining MP1 orchard, parameters differing significantly between the two categories included soil Na, cation exchange capacity and leaf Fe, B, Mo and Zn. In orchard MP2, parameters were soil %C, soil citrus nematodes, leaf size and leaf %Na and %Mg. Discriminant analysis indicated that leaf parameters were more specifically associated with decline. Leaf nutrient status can possible be an additional process to indicate decline.