Spatio-temporal dynamics of Huanglongbing in relation to vector transmission

Results from studies on the increase in HLB incidence and spread in China and Reunion Island indicate a rate of disease increase leading to a multi-year epidemic requiring 7 to 10 years for infection to approach an asymptote of 100%. In contrast, more recent studies in Brazil, Vietnam, and Florida suggest a much more rapid rate of disease increase and spread. An HLB epidemic was examined in a plantation of over 4,800 ha in South Florida where no new citrus had been introduced for 10 y and thus spread was entirely dependent on psyllid transmission. The level of psyllid infestation was unprecedented compared to previously recorded psyllid infestations. The psyllid vector was relatively newly introduced to Florida and thus lacks the biological and environmental constraints found in its native range. Consequently the HLB epidemic in Florida is undoubtedly one of the worst on record. Stochastic Markov-Chain Monte Carlo models indicated a prevalence of secondary spread with occasional primary spread from outside the plots. Interpretations of the stochastic models combined with survival analyses show spread over multiple scales from local to regional are occuring simultaneously and continually in Florida. Edge effects analyses indicate a prevalence of infections that accumulate at the transition of plantings and areas devoid of citrus such as the plantation perimeter, internal roads, canals, ponds, etc. This edge effect diminishes rapidly toward the interior of the planting and is generally well described by an inverse power function.