PCC Nov/Dec 2019

Continued from Page 6
persisted well until the next application
in the following year and displaced the
toxigenic A. flavus strains at a rate of
90 to 95 percent displacement. These
results, the fact that this strain was
native to California orchards, and it
was the most common atoxigenic strain
and the toxicological data developed
by the USDA in Arizona were sufficient
to submit to the Environmental
Protection Agency (EPA) to request
an experimental use permit (EUP) to
test the strain commercially on a larger
acreage without the need to follow
crop destruct requirements as needed
with application of experimental
compounds. The EUP was approved
in 2008 and 3,000 acres of pistachios
were treated with the commercial
product of Aspergillus flavus AF36
strain produced in the Cotton Council
of Arizona facility in Phoenix, Arizona.
Also, 3,000 acres of pistachio close to
the AF36-treated orchards were used
as untreated controls. All this acreage
for the EUP was provided by the former
Paramount Farming Company (now
Wonderful Orchards Company). At
harvest the treated and the untreated
fields were sampled separately and the
samples, specifically called “library
samples”, were analyzed for aflatoxins.
For four years we showed a significant
reduction of aflatoxins (Figure 2A, see
Page 6). The average of this reduction
for the four years of the EUP was close
to 40 percent. When library samples of
reshakes were analyzed for aflatoxins,
this reduction in one year reached to
85 percent (Figure 2B, see page 6).
These commercial efficacy data were
sufficient to obtain registration of
AF36 for use in pistachio in the states
of California, Arizona, Texas, and
New Mexico. After the registration
of AF36 on pistachio, the Almond
Board of California and the California
Fig Institute (the latter has funded
research during the early stages of our
aflatoxin research) became interested
in completing any additional research
so that the AF36’s registration is
expanded to include almonds and figs.
It took five years of additional research
(funded by the Production Research
and Food Safety and Quality
Committees of the Almond
Board of California) to
provide the USEPA and the
California Department of
Pesticide Regulations the
additional data for the registration
of AF36 for use on
almond and figs. Meanwhile
because the manufacturer
of the commercial product
changed the initial wheat
carrier of the AF36 strain
to sorghum (Figure 3) the
product was registered as
AF36 Prevail® in January 2017
and included all, pistachio,
almond, and fig. Although the
pistachio industry adapted the
use of AF36 widely and from
75,000 acres treated in 2012 reached to
up to 200,000 acres in 2018, the almond
industry was a little hesitant in widely
adopting this new technology, despite
the fact that there was good efficacy
in reducing aflatoxin contamination
in pistachio orchards. We expect to
see better efficacy when all pistachio,
almond and fig orchards are treated
on an areawide basis because the
spores of the biocontrol can spread
from field to field easily with even
slight windy conditions and dust.
Description of the Biocontrol
Product
Initially the AF36 product used sterilized
wheat as the carrier. Sterilized
wheat seed was inoculated and incubated
under certain favorable conditions
for the strain to invade and grow
in the entire wheat seed. More recently
and after additional studies in Arizona
it was determined that sorghum, which
has a lower cost was as good carrier as
the wheat (even better under conditions
in cotton and corn fields) and the
manufacturer replaced the wheat with
sorghum and at the same time changed
the method of inoculation. The sorghum
seed now is coated with a mix of
a polymer and spores of AF36 instead
of waiting for the seed to be colonized
by the atoxigenic mold. This was
done in order to satisfy the increased
demand for tons and tons of inoculum
since the rate is 10 lbs per acre. Studies
Figure 3
The sorghum career of the AF36 Prevail® commercial
product.
in California under orchard conditions
indicated that the sporulation on the
sorghum seed is delayed, although by
the end of a week both sorghum and
wheat showed similar sporulation rates.
The time of production of spores and
the rate of sporulation are very critical
for the successful use of this biocontrol
agent. It is a numbers game: we want
to overload the soil with atoxigenic
spores and displace the spores of the
toxigenic molds. That is the way this
biological control approach works in
the field (see challenges at the end of
this article relevant to sporulation).
Proper Way for Ground Application:
For detailed information please
read the label of the product:
Michailides emphasized that as far
as we know up to now there are
four critical application factors that
need to be taken into account for
AF36 Prevail to be successful in the
orchard: a) the timing of application;
b) the rate (amount) per acre; c) the
proper placement on the orchard
floor; and d) the proper irrigation
before and after the application.
Continued on Page 10
8 Progressive Crop Consultant November / December 2019