Submitted to: Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: 5/4/2012
Publication Date: 8/3/2012
Citation: Yokomi, R.K., Sisterson, M.S. 2012. Estimation of incidence and spatial temporal distribution of Citrus Stubborn disease. Phytopathology. 102:S4:141.
Technical Abstract: Citrus stubborn disease (CSD) is caused by Spiroplasma citri, a culturable wall-less prokaryote. The pathogen is graft-transmissible and vectored by the beet leafhopper (BLH). The objective of this study was to determine incidence and spread of S. citri in two sweet orange citrus groves in the San Joaquin Valley (SJV). The Ducor plot, with 20-y old trees, was near foothills not known to have large overwintering populations of the BLH; whereas the Huron plot, with 10-y-old trees, is adjacent to foothills with overwintering BLH populations. Each plot had 3 replications consisting of trees in 16 rows x 16 trees, totaling 768 trees. DNA was extracted and pooled from 3 fruit columellae per tree and tested by quantitative Polymerase Chain Reaction assay for presence of S. citri DNA sequences. Spatial and temporal distribution over a 3-year period showed a slow annual spread of S. citri regardless of initial disease incidence. Disease incidence in Ducor went from 20.1 to 22.3% (2.2% increase), whereas disease incidence in Huron went from 3.1 to 3.9% (0.8% increase). Infected trees were distributed randomly in both plots, suggesting only primary spread was occurring. D-vac samples taken in and around the plots showed BLHs were absent from citrus foliage but present on weed hosts (e.g. Russian thistle, mustard). These data confirm grower observations that S. citri spread in citrus is low and originate from sources outside of the citrus grove. Therefore, disease management by weed control and prudent replacement of severely affected trees may be effective.