- HUANGLONGBING: DEVASTATING DISEASE OF CITRUS 335
moderate and severe symptoms had greater loss of photosynthetic effi-
ciency and increased unregulated and regulated heat dissipation.
E. Nutrients
The similarity of some HLB symptoms to nutrient deficiencies has
prompted a number of investigations to compare the nutritional status
of CLas-infected and non-infected plants. It has been reported in several
publications (da Grac ̧a 1991) that HLB-infected trees are consistently
deficient in B, Ca, Mg, Mn, and Zn, and in a grove setting, these nutri-
ent deficient trees are more likely to be CLas-infected than the nutrient
sufficient trees. Nwugo et al. (2013) reported significant reductions in
concentrations of Ca, Mg, Fe, Zn, Mn, and Cu in leaves of grapefruit
plants in response to CLas infection, particularly in symptomatic plants.
However, Shen et al. (2013) reported no differences in citrus leaf nutri-
ent content between CLas-positive and CLas-negative trees. Meanwhile,
Masaoka et al. (2011) reported that CLas-infected leaf samples had on
average, approximately one-half the concentrations of Fe and Zn than
what were found in non-infected plants. Mn was occasionally, but not
consistently, lower in CLas-infected plants than in non-infected plants.
InC. unshiu, concentrations of Fe and Zn were reduced before chlorosis
appeared. These results suggested that the concentrations of Fe and Zn,
but not other elements are reduced by CLas infection. Koen and Lan-
genegger (1970) found that concentrations of K were higher, while Ca
and Mg were lower in an unnamed citrus species infected with CLaf
(causal organism of the African form of HLB). Aubert (1979) (cited in
da Grac ̧a 1991) reportedly showed that CLas-infected plants contained
lower concentrations of Ca, Mn, and Zn than did non-infected plants.
Nevertheless, apparent decreases in nutrients found in CLas-infected
leaves, when expressed on a dry weight basis, are confounded by the
increase in dry weight that results from the excessive amount of starch
found in HLB-symptomatic leaves. If dry weight is adjusted for starch
concentration (Spann and Schuman 2009) or expressed on a leaf area
basis (Cimo et al. 2013) nutrient contents of CLas-infected leaves do not`
differ significantly from non-infected leaves. Using Micro-XRF imag-
ing, it was revealed that Zn concentration in the phloem of veins in
healthy leaves was more than 10 times higher than that in HLB-affected
leaves (Tian et al. 2014). No significant differences in other nutrients
were observed between CLas-infected and non-infected samples. These
authors suggest that reduced phloem transport of Zn is an important
factor contributing to HLB-induced Zn deficiency in grapefruit.