1. aflatoxin growth and identification
Science of
aflatoxin growth and identification
Alison Robertson
Extension Field Crops Pathologist
A. Robertson, 2006 ©

2. Poisonous substance produced by the fungi
What is aflatoxin?
Poisonous substance produced by the fungi
Aspergillus flavus and A. parasiticus.
- once produced, they are VERY stable
A. Robertson, 2006 ©

3. Production of toxins highly variable:
fungal strain and species
storage temperature + moisture content
length and type of storage
other unknown factors
Therefore mold ≠ aflatoxins
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4. Life Cycle of Aspergillus flavus
Wind and insects
Usually thought of as a storage pathogen
BUT
fungal contaminations starts in the field
Survives as
conidia and sclerotia
in soil and crop debris
Overwintering inoculum = conidia (last 1 year) and sclerotia (last 3 years)
Sclerotia = mm in diameter
Conidia = 100x smaller
Sclerotia form naturally in standing corn and can be dispersed into field soils during harvest
Conidia are produced directly from the Sclerotia 48-72h at 32C/96F
Dispersed in wind or insects
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5. Population dynamics of A. flavus
Shearer et al., 1992; McGee et al. 1996
1992 & A. flavus recovered at greater frequencies in CC and no till
Populations in soil
significantly greater in
July vs June, Aug & Sept
Measured populations of Af once a month from June to Sept in corn crop residue, corn leaf tissue, soil and air
and
Signif. Higher popln detected in soil in July vs J, A and S (Figure 1)
Note: theses populations are 50xless that those detected in epidemic year of 1988 by Shearer et al.
Tillage and rotation treatments  Af detected in all treatments
 cc – Aflavus recovered at greater frequencies
 2/3 years: tilled = lower popln of Af
However – no corresponding popln changes in soil, air or leaves of same plots (probably because poplns so low)
In addition to Nashua – also examined 40 commercial fields – populations increased signif in July
Populations measured
once a month
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6. Aspergillus : Disease cycle
1. Infection
Through the silks:
Yellow/brown = germination and colonization
Pollination = changes in physiology and structure of silk
 A. flavus continues growth as a saprophyte
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7. Enhanced by damage by birds/insects
2. Colonization
Enhanced by damage by birds/insects
Physical damage allows further spread
Broken pericarp allows invasion
Moisture content drops rapidly <35%
 A. flavus competes successfully with other MOs (e.g. Fusarium spp.)
 grows best at 17-20% grain moisture
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8. 3. Colonization and aflatoxin production
High max, min and ambient temp (esp. July and Aug) –
particularly night – more important than moisture
Very low rainfall
Stressed plants = altered nutritional status of developing kernels
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9. Optimum conditions for Aspergillus growth and aflatoxin production
Temperature
13%
30%
45F
120F
Aspergillus growth
Aflatoxin production
17%
55F
104F
? %
Moisture
So, the fungus can grow
at higher and lower temperatures & moistures and not produce aflatoxin
A. Robertson, 2006 ©

10. Managing Aspergillus and aflatoxin
Early planting (April v May)
Reduce plant stress
Harvest early
Avoid damage during harvest
Dry grain a.s.a.p. to 13% moisture
(inhibits growth at any temperature)
6. Cool grain a.s.a.p. to <45F
(very slow growth <55F)
7. Ensure storage facilities are clean
Difficult  SPORADIC outbreaks.
Look ahead to 2006 – very low soil moisture in SE Iowa; 2/3 chance of drought (La Nina)
– try to avoid late season drought stress, Research 1/3 less aflatoxin in April planted corn)
Plant regionally adapted hybrids, balanced fertility with adequate nitrogen, following weed and insect mgt recommendations
Significant association of high aflatoxin levels with delayed harvest; Moisture = 23-26% considered optimum,
within 24h after harvest – 13% if going into storage. <13% fungal growth and aflatoxin production virtually stops.
grows best at F
A. Robertson, 2006 ©

11. Powdery olive green mold
Identification
1. Aspergillus flavus
Powdery olive green mold
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12. Identification 2. Aflatoxins Black light # BGYF particles ≠ aflatoxin
= false positive
The black light should no longer be used for any type of mycotoxin screening
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13. USDA GIPSA approved list: //151.121.3.117/techservsup/
Test kits
- immunoassay strips
- ELISA assays
- detect +/- 20ppb
(detection)
USDA GIPSA approved list:
// /techservsup/
metheqp/testkit.htm
(identification)
HPLC
- quantitative
- list of labs
A. Robertson, 2006 ©

14. References Diener et al. 1987. Ann. Rev. Phytopathol. 25: 249-70
Jones et al Plant Disease 64:
Jones et al Phytopathology 71:
Marsh and Payne Phytopathology 74:
Schindeer et al Applied Microbiology 15:
Wicklow and Donahue Trans. Br. Mycol. Soc. 82:
Aflatoxins in Corn. Pm Iowa State University Extension
Aflatoxin in Corn.

15. Thank You
A. Robertson, 2006 ©