HS1122
An Iodine-Based Starch Test to Assist in Selecting
Leaves for HLB Testing 1
Ed Etxeberria, Pedro Gonzalez*, William Dawson and Timothy Spann2
1. This document is HS1122, one of a series of the Horticultural Sciences Department, UF/IFAS Extension. Original publication date December 2007.
Reviewed February 2011. Visit the EDIS website at http://edis.ifas.ufl.edu.
2. Ed Etxeberria, professor, CREC-Lake Alfred; Pedro Gonzalez, CREC-Lake Alfred; W.O. Dawson, eminent scholar, Plant Pathology Department, Citrus REC,
Lake Alfred; and Timothy Spann, assistant professor, Citrus REC, Lake Alfred, Department of Hor ticultural Sciences, UF/IFAS Ex tension, Gainesville,
32611.
*All photos by Pedro Gonzalez unless otherwise noted.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services
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national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office.
U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County
Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.
Diagnosing huanglongbing (HLB or citrus greening
disease) can be difficult under field conditions when relying
solely on visual symptoms. The best diagnostic symptom
of HLB is the blotchy mottle pattern on leaves (Figure 1A).
However, it can be difficult to distinguish blotchy mottle
caused by HLB infection from similar symptoms caused by
girdling of the branches and other physiological disorders
or diseases. For example, deficiencies of micronutrients
such as zinc, manganese and iron (Figure 1B-D) can
be mistaken for HLB. Currently, the only definitive test
for HLB is polymerase chain reaction analysis, or PCR,
analysis, a DNA-based test. PCR analysis, however, is time
consuming and expensive, and is not suitable for large
numbers of samples. Thus, a rapid, simple field diagnostic
test that could be used to pre-screen samples intended for
PCR analysis would be beneficial.
Anatomical studies conducted in the 1960s, found “massive
accumulation” of starch in leaf samples collected from
HLB-infected sweet orange trees. More recent studies have
quantified starch accumulation in HLB-infected leaves at
six times more than healthy leaves. Starch readily reacts
with iodine, resulting in a very dark grey to black stain.
Recently, a number of researchers from Vietnam and Japan
have been working to adapt this starch/iodine reaction
into a diagnostic tool for HLB, and they report up to 90%
agreement between PCR analysis and starch tests with io-
dine. IFAS has not performed a similar correlation analysis,
although studies are ongoing. An IFAS-developed version
of this test, how to perform it, the required materials, its
potential benefits, its limitations, and how to interpret the
results is presented here.
Figure 1. Citrus leaves showing HLB symptoms (A), manganese (B),
zinc (C) and iron (D) deficiency symptoms. Credits: Tim Spann (A) and
Tom Obreza (B, C, and D), University of Florida.
