PCC Nov/Dec 2019

A
Reduction of contaminated samples (%)
50
44.9% P value = 0.0033
40
38.6%
39.9%
36.7%
30
20.4%
20
10
0
2008 2009 2010 2011 2008 - 2011
(Doster et al. (2014), Plant Disease 98:948-956)
(4 years average)
B
Reduction of contaminated samples (%)
100
80
60
40
20
0
NO SAMPLES
23.6%
85.4%
58.1%
54.6%
2008 2009 2010 2011 2009-2011
(3 years average)
Figure 2
Reduction of aflatoxin contamination (left, main crop; right, reshakes) during the years of experimental use permit
(2008 to 2011) after application of AF36 strain in commercial orchards.
Continued from Page 4
the threshold, the consignments are
rejected and must ...either be reconditioned
(re-sorted) or destroyed.
Below is a historical summary
how this technology developed
to help our California nut crop
industries and the fig industry.
In the first eight years we focused
on cultural practices that affect the
predisposition of the pistachio crop
to aflatoxin contamination and to
also find out whether contaminated
nuts show special characteristics
that can be used to sort out these
nuts at the processing plant.
Below is a list of the findings
from those studies led by Drs.
Doster and Michailides:
a. We confirmed that early split
nuts (ES) contained large amounts
of aflatoxins and we named
these nuts the “Achilles Heel”
for aflatoxin contamination.
b. ES by themselves explained 84
percent of the aflatoxin contamination
in the samples we analyzed.
c. When the ES were combined
with the navel orangeworm (NOW)
damaged nuts explained 99 percent
of the aflatoxin contamination
in the samples we analyzed.
d. We reduced the incidence of ES
by providing the trees with sufficient
water during early season (May)
when it is the critical time for the
full size development of nut shell
(water stress of trees during May
leads to higher incidence of ES).
e. We compared the incidence of ES
on Kerman under the influence of
four rootstocks: UCB1 and Pioneer
Gold I resulted in significantly lower
ES incidence than Pistacia atlantica
and Pioneer Gold II rootstocks.
In my first Aflatoxin Elimination
Committee (AEC) Meeting in 1991, in
Peoria, Illinois, I learned for the first
time that some strains of Aspergillus
flavus do not produce aflatoxins and
some USDA researchers reported that
a very large portion of the A. flavus
population consisted of strains that
do not produce any aflatoxin, called
atoxigenic strains. They started using
these atoxigenics as candidates for
biological control of aflatoxigenic fungi.
It was also discovered that the proportion
of strains that produced aflatoxin
included strains that produced variable
amounts of aflatoxins. USDA researchers
started first working with atoxigenic
strains to be used as biocontrol agents
to reduce aflatoxins in the various
crops. One of this strains was initially
selected in Arizona from samples taken
from cotton fields, and the Cotton
Research Council of Arizona that
supported financially this research were
able to get AF36 registered for use in
cotton and corn in 2008. Meanwhile
starting in 2002, we discovered the
same strain was the most commonly
encountered strain among the other
atoxigenic strains in pistachio, almond,
and fig orchards in California, and
immediately included this strain in
our aflatoxin biocontrol studies. With
multiyear support of USDA funding
and funding by the pistachio and
fig industries in California, we were
able to show that this strain is among
the most common atoxigenic strains
occurring in California nut crop and
fig orchards, and indeed it can be found
at much higher incidence in comparison
with all other atoxigenic strains.
For instance, during these studies 15
different groups of atoxigenic strains
were determined, and each one was at
a rate of less than one percent, while
the AF36 atoxigenic strain (Figure 1,
see page 4) was found in an average
of five to eight percent depending on
the field and the type of the crop, and
in some instances up to 12 percent of
populations of the atoxigenic strains.
Initially, the studies were confined in
small experiments in replicated micro-plots
where we showed that when
the AF36 was applied once preseason, it
Continued on Page 8
6 Progressive Crop Consultant November / December 2019