Density of Diseased Trees

After DATOC presented a report to the Citrus Pest and Disease Prevention Program reviewing potential exposure to HLB in Southern California, we were asked if there were differences in the apparent density of infection based on more granular geographic areas than what was presented in the report. In response, this project reviewed the locations of HLB-infected trees in terms of density by city, and compared the locations of new infections to older data.

We compared the location of HLB detections between June

2018 and April 2019 to the location of HLB detections before

June 2018 (the latter being the time period covered in the

exposure report). New HLB detections occurred broadly, but

were more common close to earlier detections. Half of the

new detections occurred within 270 m (886 ft) of previous

detections, 70% within 1 km (0.6 mi) of existing detections,

and 90% within 1.5 km (0.9 mi).

We also analyzed the geographic distribution of diseased

trees from 2012 through October 1, 2019, in cities with at

least 10 infections. For all of Southern California, 95% of

HLB-positive trees are within 213 m (698 ft) of another

positive tree (right). Compared to the prior distribution (as

reported in the exposure project), this indicates more densely

distributed discovery of HLB-positive trees.

When this distribution was broken down by city, it became 

apparent that some cities contained more densely distributed 

cases of HLB, whereas others were more diffuse (below)

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DATOC is sponsored by the Citrus Research Board and the California Citrus Pest and Disease Prevention Program.