## Huanglongbing in Southern CA

February 12, 2020

We continue to monitor the dynamics of the emerging epidemic in southern California. We recently re-fitted the set of growth models previously used to describe the HLB cases. In the most recent exercise, a segmented linear model remained the best choice to explain the cumulative total number of HLB-positive trees detected, when data through the end of 2019 were included in the analysis. Before mid-2017, the number of confirmed cases increased at a rate of one per month (although no HLB+ trees were found in 2013 and 2014). From mid-2017 until the end of 2019 the rate was 58 per month, or roughly 10 per week.

We also looked at the growth in the area under HLB quarantine over the same period. Similar to the data for number of cases, the increase of area under quarantine was best described by a segmented linear model. In the period from 2012 to mid-2017, the HLB quarantine area grew at a rate of 1.7 square miles per month. From mid-2017 to the end of 2019, the corresponding rate was just over 25 square miles per month.

Either in terms of numbers of trees or quarantined area, the apparent epidemic is increasing linearly, but at a much faster rate since mid-2017. A couple of inter-comparisons are worth highlighting. In terms of the numbers of cases, the apparent epidemic is increasing 58 times faster since mid-2017 than the period before. In comparison, when expressed in terms of area, the current rate of increase is 15 times faster than during the early period. The difference in the rate of increase in cases versus area under quarantine reflects the detection of dense clusters of infected trees inside existing quarantine areas, which increase the detection rate but do not expand the quarantine boundary. Another way to look at this is to compare the ratios of the rates in the two periods.

In the pre-mid 2017 period the ratio of tree detection to area increase was roughly 1:3 -- i.e. new detections occurred at a rate of about 1 tree for every 3 square miles of new quarantine area. In the more recent period, post-mid-2017, the corresponding figure is just over 2:1 -- i.e. 2 new trees for every new square mile added to the quarantine area.

It is important to note that some of these increases are undoubtedly due to changes and improvements in programmatic activities. For example, lab capacity has increased (allowing more samples to be tested), technical improvements have been made to the test used to detect the pathogen, and the number of samples taken from trees at locations where an HLB+ tree has been detected quadrupled in 2017.

July 1, 2019

Analyzing data for agricultural diseases is often a complicated matter. That’s why the Citrus Pest and Disease Prevention Program (CPDPP) works closely with DATOC to interpret how the appearance of Huanglongbing-diseased trees correlates to program activities that strive to manage HLB in Southern California.

DATOC delivers responsive analyses to help answer direct questions from the CPDPP, but the team also undertakes investigative analyses, such as tracking how trends in weekly detections of HLB are affected differentially by disease development or management activities.

See The HLB Epidemic for more information.