Submitted to: Australian Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/5/1996
Publication Date: N/A
Citation: Interpretive Summary: This work reports the first occurrence of citrus blight in New South Wales (NSW) Australia. Blight is a serious disease of citrus of unknown cause that can kill trees and has no known control. Because the cause of blight is unknown, there is no direct means to detect/confirm blight. The work that was carried out included several indirect tests including impaired water uptake by the tree, zinc accumulation in the wood, microscopic plugging of the wood cells, and a serological assay to confirm that blight was the cause of the tree decline observed. All of these tests were positive for blight and in combination considered conclusive. Occasionally a bacterium is associated with blighted trees in Florida, but was absent in the blighted grove in NSW. In Florida where blight is prevalent, blighted trees often occur in groups and natural root grafts that occur among the trees is suspected to transfer the blight organism from tree to tree. Statistical analyses were conducted to examine the relationship among the position blighted trees, one to another. We found that blighted trees in NSW were not clustered together at any time during the epidemic which indicated that root graft transmissions were not the cause of blight spread in NSW. This result also raises questions of the validity of these conclusions for Florida.
Technical Abstract: Citrus blight was identified in two orchards at Lake Wyangan in the Riverina area of New South Wales on the basis of field symptomatology, impaired water uptake, zinc accumulation in trunk bark, plugging of xylem cells and positive serological assays (dot blot and Western dot) for the 12kD blight-associated protein. Xylella fastidiosa was not detected. Spatial analyses indicated a lack of association of adjacent blighted tree at 5, 10, and 15 percent blight incidence. Spatial and spatio-temporal analyses confirmed this lack of adjacency and also indicated little or no associations among blighted trees over greater distances or over time. Therefore, root-graft transmission of blight was not a likely contributor to spatial processes of the blight epidemic in the orchard studied in New South Wales.