- HUANGLONGBING: DEVASTATING DISEASE OF CITRUS 341
and soil. Some growers are reducing the pH of irrigation water by the
addition of acid, and soil by the use of acidifying fertilizers and appli-
cation of sulfur to keep carbonates low. However, there have not yet
been enough studies on the acidification of irrigation water and soils, to
provide clear evidence that these practices are improving productivity
of citrus trees affected by HLB.
- Thermal Therapy.Hoffman et al. (2013) described the use of heat
treatments to “cure” HLB in citrus. Holding potted citrus trees con-
tinuously at 40–42◦C for a minimum of 48 h resulted in decreases in
CLas titer and plants that survived showed “healthy vigorous growth.”
In some cases, continuous exposure to 40–42◦C for 48 h was sufficient to
reduce CLas titer even to non-detectable levels. The authors suggested
that thermal therapy may be useful for the control of HLB in nursery and
greenhouse settings, where the whole plant can be heated. To date, an
economically significant effect of heat treatment on HLB-symptomatic
trees in the field has not been demonstrated, although trees are often
reported to respond with vigorous vegetative growth. However, the
belief that thermal therapy can suppress or eliminate CLas, and as a con-
sequence, maintain productivity of trees has spurred growers to exper-
iment with various methods of applying thermal therapy to citrus trees
in the orchard (Obrien 2013).
Treatment is typically achieved by covering individual trees with a
plastic tent (Ehsani et al. 2013). Elevated temperatures are obtained
within the tent as a consequence of passive solar heating. Tempera-
tures obtained using tents are highly variable and a function of ambi-
ent temperature along with the amount of radiant energy reaching the
tent. During periods of cloudy and rainy weather, temperatures within
the tents are considerably lower compared to sunny days. In addi-
tion, during the night temperatures within the tents return to ambient.
Recently, covering trees with a shroud and injecting steam has been
investigated (Al-Jumaili and Ehsani 2015, https://www.youtube.com/
watch?v=8GIS1Ao-QVk). CLas is known to move systemically into cit-
rus roots (Tatineni et al. 2008; Johnson et al. 2014), and currently it is
not possible to elevate root temperatures in the orchard. This is a seri-
ous drawback in orchard thermal therapy, and probably why results are
inconsistent. - Antimicrobial Compounds.da Grac ̧a (1991) reviewed the literature
on the use of antibiotic therapy of citrus trees. In a number of trials in
a number of countries, streptomycin, tetracycline, and penicillin have
been shown to reduce CLas titer and foliar HLB symptoms and improve