- HUANGLONGBING: DEVASTATING DISEASE OF CITRUS 343
susceptible to HLB than are current varieties, is the only long-term solu-
tion. As indicated earlier, all important commercial citrus types are sus-
ceptible to HLB, however there do appear to be differences amongst the
types in their rate of decline following CLas infection, that is, their sus-
ceptibility HLB. Some evidence indicates lower susceptibility to HLB in
limes (Schwarz et al. 1973; de Lange et al. 1985; Shokrollah et al. 2009),
pummelos (Schwarz et al. 1973; Koizumi et al. 1997), lemons (Schwarz
et al. 1973; Cheema et al. 1982; Nariani 1982), some mandarin types
(e.g., “Ladu” and “Som Pan” in Thailand; Koizumi et al. 1997), and var-
ious non-cultivated citrus or related species.
In an orchard survey of various common citrus types planted in mixed
blocks in commercial orchards, it was found that “Temple” tangor was
consistently less frequently infected with CLas, and when infected,
had lower titers and less severe HLB symptoms than other cultivars
included in the survey (Stover and McCollum 2011). In the same study,
Murcott tangor and Minneola tangelo had the highest CLas titers and
most severe HLB symptoms. These data support the contention that all
citrus types are susceptible to becoming infected with CLas, but that
there is variation in the incidence of infection, eventual CLas titer, and
development of HLB symptoms.
Citrus reticulata, C. aurantium, C. grandis, C. madurensis,andMur-
raya paniculatawere assessed for tolerance by grafting and vector trans-
mission tests towardCa.L. asiaticus-Terengganu isolate (Ahmad et al.
2011). Seedlings ofM. paniculata,andC. grandistested negative for
CLas, showed no foliar HLB symptoms and tested CLas-negative 6
months after inoculation.C. aurantiumwas tolerant of CLas infection;
trees tested CLas-positive but exhibited only mild leaf mottling and
green vein. Meanwhile,C. reticulataandC. madurensiswere very sus-
ceptible to CLas infection with inoculated seedlings developing severe
HLB symptoms within 6 months after inoculation.
Poncirus trifoliataappears to be less susceptible to HLB than other
cultivated citrus scion varieties (Folimonova et al. 2009; Stover et al.
2010; Albrecht and Bowman 2011, 2012; Albrecht et al. 2012). Although
P. trifoliatahybrid rootstocks are apparently less susceptible to HLB
than are non-Poncirushybrids, in the orchard, citrus rootstocks do not
produce leaves on which ACP will feed. If the resistance ofPoncirus
is based on inefficient transmission of CLas by ACP, then it brings into
question whetherPonciruswill have impact on HLB in the scion. How-
ever, one study did show resistance following graft inoculation (Stover
et al. 2015a). Heretofore, the use ofP. trifoliataas a parent in scion breed-
ing has gone nowhere due to the resulting unacceptable fruit quality.
Although some success has been achieved toward development of new