PCC Nov/Dec 2019

Biocontrol of Aflatoxin
Contamination in Nut
Crops is Working!
Figure 1
The biological agent, Aspergillus flavus
strain AF36, producing large sclerotia
(black, spherical structures in the plate).
All photos courtesy of Themis Michailides.
It was in 1991, when I received a
call by the President of the former
California Pistachio Commission,
Karen Reinecke, asking if there
was a way to get involved as a
technical member of the newly then
established Aflatoxin Elimination
Workgroup. The goal of this Workgroup
was to evaluate proposals submitted
by the United States Department of
Agriculture (USDA) and University researchers
to the USDA Special Fund to
participate toward research to eliminate
the problem of aflatoxin contamination
by the year 2000. We are now in the
second half of 2019 and we still have aflatoxin
contamination problems in both
pistachio and almond, particularly in
years when navel orangeworm damage
is high in these crops.
Our first proposal was submitted
in 1992 to the USDA/Aflatoxin
Elimination Technical Committee. It
was funded and research started after
hiring postdoc associate Dr. Mark
Doster, a University of California (UC)
Davis graduate with postdoc research
at the University of Cornell. Mark an
4 Progressive Crop Consultant November / December 2019
By THEMIS MICHAILIDES | Professor and Plant Pathologist, UC Davis
MARK DOSTER | Plant Pathologist, UC Davis (retired)
RAMON JAIME | Professor and Plant Pathologist, UC Davis
RYAN PUCKETT | Professor and Plant Pathologist, UC Davis
JOHN LAKE | Professor and Plant Pathologist, UC Davis
VICTOR GABRI | Visiting Agronomist, University of San Juan, Argentina
expert in fungal pathology and practical
plant pathology was immersed
quickly in this research. We were
also supported then by the California
pistachio industry which supplemented
funding to intensify the research to
reduce aflatoxins and find solutions for
the growers. At the same time other
researchers from UC Davis focused
on aflatoxin research to reduce it in
almond and walnut. Later on, we expanded
our aflatoxin management research
and included almonds and figs.
Aflatoxins are toxic compounds produced
mainly by certain molds called
Aspergillus flavus and A. parasiticus
when these molds grow on various
susceptible crops. These molds produce
toxins which are considered very harmful
when they are consumed with food
products. These toxins produced are
called aflatoxins which are considered
as the most potent naturally-produced
carcinogenic compounds causing liver
cancer and in acute situations deaths.
Strains of Aspergillus flavus that do not
produce toxin are called atoxigenic, and
they act as biological control agents.
When they are applied on the orchard
floor displace the toxin-producing mold
strains, and reduce the potential for aflatoxin
contamination in various crops.
There are four major aflatoxin types:
the B1 and B2 produced by both the
above mentioned fungi and G1 and
G2 produced only by A. parasiticus.
B1 is the most toxic among the four
aflatoxin types. Because of this high
toxicity these compounds are regulated
strictly by various governments, and in
fact, the B1 is regulated separately. For
instance, in the USA the tolerance for
B1 is 10 ppb (parts per billion) and for
all the aflatoxins (total) is 15 ppb. The
European Union (EU) has even stricter
tolerances, i.e., 8 ppb for B1 and 10 ppb
for total aflatoxins. One can judge the
seriousness of aflatoxin contamination
not only from the very strict tolerances
but also from the losses and additional
costs associated with the re-sorting
and losses associated with the dumping
the contaminated product. It should
be noted that when shipments exceed
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