Asymptomatic spread of huanglongbing and implications for disease control

Abstract

Huanglongbing (HLB) is a bacterial infection of citrus trees transmitted by the Asian citrus psyllid Diaphorina citri. Mitigation of HLB has focused on spraying of insecticides to reduce the psyllid population and removal of trees when they first show symptoms of the disease. These interventions have been only marginally effective, because symptoms of HLB do not appear on leaves for months to years after initial infection. Limited knowledge about disease spread during the asymptomatic phase is exemplified by the heretofore unknown length of time from initial infection of newly developing cluster of young leaves, called flush, by adult psyllids until the flush become infectious. We present experimental evidence showing that young flush become infectious within 15 d after receiving an inoculum of Candidatus Liberibacter asiaticus (bacteria). Using this critical fact, we specify a microsimulation model of asymptomatic disease spread and intensity in a grove of citrus trees. We apply a range of psyllid introduction scenarios to show that entire groves can become infected with up to 12,000 psyllids per tree in less than 1 y, before most of the trees show any symptoms. We also show that intervention strategies that reduce the psyllid population by 75% during the flushing periods can delay infection of a full grove, and thereby reduce the amount of insecticide used throughout a year. This result implies that psyllid surveillance and control, using a variety of recently available technologies, should be used from the initial detection of invasion and throughout the asymptomatic period.

Keywords: asymptomatic huanglongbing; control strategies; latent period; transmission model.

Fig. 1.

All panels are for invasion scenario iii. No control (A), elimination of 75% of adult psyllids and nymphs on days 16 and 30 of each flushing period (B), and elimination of 75% of adult psyllids on days 16 and 30 of each flushing period (C). Within each panel, the fraction of psyllids infected (Left) and the fraction of flush infected (Right) are shown. The green segments on the vertical axis (i.e., days since initial invasion of the grove) correspond to flushing periods. Migration of psyllids between trees accounts for the spread through the entire grove. Coordinates in the x–y plane correspond to locations of trees. In the simulation, we are not assuming trees are evenly spaced (Microsimulation Modeling of Transmission).

Fig. 2.

All panels are for adult psyllid counts on a single tree located at (11,11) under invasion scenario iii. For all panels, black lines represent the total adult psyllids, blue lines represent healthy adult psyllids, and red lines represent infected adult psyllids. No control (A), elimination of 75% of adult psyllids and nymphs on days 16 and 30 of each flushing period (B), and elimination of 75% of adult psyllids on days 16 and 30 of each flushing period (C) are shown. The green segments on the horizontal axis (i.e., days since initial invasion of the grove) correspond to flushing periods.