7

Huanglongbing: Devastating Disease

of Citrus

Greg McCollumandElizabeth Baldwin
USDA-ARS Horticultural Research Laboratory, Ft. Pierce,
FL, USA

ABSTRACT

Citrus greening or Huanglongbing (HLB) disease is devastating the citrus indus-
try worldwide, from Asia to the Americas. The only major citrus regions free of
this disease are the Mediterranean and Australia/New Zealand. To date there is
no cure and no commercially implementable resistance for HLB, and once in-
fected, an orchard often becomes unsustainable due to poor yield. Meanwhile,
fruit and juice products become unmarketable due to problems of flavor, size re-
duction, and poor color. HLB is associated with three species of Gram-negative
alpha proteobacters, CandidatusLiberibacter asiaticus (CLas),Candidatus
Liberibacter americanus (CLam), andCandidatusLiberibacter africanus (CLaf).
CLas and CLam are vectored by the Asian citrus psyllid (Diaphorina citri)and
CLaf is vectored by the African citrus psyllid (Trioza erytreae). Recommended
management of HLB in the orchard involves identification and removal of CLas-
infected trees and aggressive psyllid control. Many citrus growers are attempt-
ing to maintain production of infected trees by applying additional fertilizers,
foliar sprays of micronutrients, and compounds that affect plant defense re-
sponses, along with special attention to irrigation. Other therapeutic strategies
being evaluated include thermal and antimicrobial treatments, but these have
yet to be applied on a large scale due to practicality of implementation and/or
regulatory hurdles. Some tolerant rootstock and scion varieties have been
identified and are finding their way into growers’ hands. These varieties have
the potential to slow the rate of HLB progression in CLas-infected trees. Citrus
juice processors are managing flavor issues by sorting out symptomatic fruit,
blending HLB-affected juice with healthy juice, adding citrus aroma to mask off-
flavor and debittering juice. The cure for this disease will likely be a transgenic

Horticultural Reviews, Volume 44, First Edition. Edited by Jules Janick.
© 2017 Wiley-Blackwell. Published 2017 by John Wiley & Sons, Inc.

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