342 G. McCOLLUM AND E. BALDWIN
fruit quality. However, the use of antibiotics as a therapy for citrus
infected with CLas has not become commercially adopted for a num-
ber of reasons. First of all, there are regulatory hurdles, then there is the
issue of horticultural application methods (trunk injection is so far most
successful, for duration of effect) and pathological efficacy (bacterio-
static or bactericidal characteristic of particular molecules and potential
for phytotoxicity).
Recently, there has been renewed interest in the potential use of
antibiotics to control HLB (Zhang et al. 2010, 2011, 2012, 2014). Zhang
et al. (2011) found that a combination of penicillin and streptomycin
was effective in reducing CLas titer and provided a therapeutically
effective level of control for a much longer period of time than when
administering either antibiotic separately. Puttamuk et al. (2014) treated
HLB-affected pummelos with streptomycin, ampicillin, penicillin, and
Bacicure reducing CLas titer by four logs, but did not eliminate the
pathogen. Zhang et al. (2014) screened 31 antibiotics for phytotoxi-
city and efficacy against CLas. Ampicillin, Carbenicillin, Penicillin,
Cefalexin, Rifampicin, and Sulfadimethoxine were effective in reduc-
ing CLas, indicated by reduced incidence of infection and titers and
with only limited phytotoxicity.
In the United States, streptomycin, oxytetracycline, and kasugamycin
are the only crop approved antibiotics. These are used to control fire-
blight, and currently there is an emergency use permit for control of
canker with streptomycin in Florida grapefruit. These products are
currently in trials to determine efficacy against HLB, (data not yet
available). Fireblight is a topical pathogen and spray applications of
antibiotics come into immediate contact with the pathogen when leaves
are wetted. In contrast, CLas is phloem-limited making delivery of
antibiotics more complex than for topical pathogens. Trunk injections
of antibiotics have shown positive results (Puttamuk et al. 2014), how-
ever, the economics and logistics of trunk injections may make such
treatments non-viable. In Florida, the Citrus Research and Development
Foundation is currently attempting to receive Section 18, Emergency
Exemption for two formulations of oxy-tetracycline and one formu-
lation of streptomycin based on the belief that these treatments may
provide effective therapy for HLB-affected trees and perhaps preventive
benefits for those trees not yet symptomatic (CRDF 2015).
- Citrus Types/Cultivars with Reduced Susceptibility to HLB.The
susceptibility of citrus to HLB when ACP and CLas are present, inabil-
ity to eradicate ACP, and the absence of effective therapeutics to min-
imize HLB symptoms, means that new citrus varieties, that are less