Spatial Pattern Analysis of Citrus Greening in Shantou, China

Abstract

Assessments of individual trees for citrus greening disease were made in 1986 and again in 1988 in a citrus grove near Poutai Shantou Prefecture, China, consisting of ca. 20,000 TankanIFuzhou mandarin trees. In 1986, 16 trees displayed greening symptoms. The diseased trees were near the southern edge of the grove immediately adjacent to a small garden that contained ca. 24 trees with greening symptoms. By 1988, 14% of the trees in the Poutai grove were assessed as greening-diseased. The spatial pattern of disease was analyzed by 1) indices of dispersion, 2) ordinary runs analysis, 3) spatial autocorrelation, and 4) isopath mapping. Disease was considered to be aggre-gated based on Lloyd's-index-of-patchiness. With ordinary runs analysis, greater aggregation was found in 78% of north-south rows compared to 44% for east-west groups of trees. The greater north-south aggregation corresponded to the general grove traffic and closer spacing between trees on beds running north-south. Spatial autocorrelation indicated that clusters of diseased trees oriented north-south from one another were related. Clustering on the southern, eastern, and northern borders of the grove was verified by mapping of isopaths. Such clustering was probably indicative of ingress of psyllid vectors from the heavily infested small planting to the south, from an adjacent town to the east, and aggregation of the psyllids along a northern bordering canal, respectively.