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

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 ©

Production of toxins highly variable: fungal strain and species storage temperature + moisture content length and type of storage other unknown factors Therefore mold ≠ aflatoxins A. Robertson, 2006 ©

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 = 0.4-0.7 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 A. Robertson, 2006 ©

Population dynamics of A. flavus Shearer et al., 1992; McGee et al. 1996 1992 & 1993 - 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 1979-1980 and 1991-93 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 A. Robertson, 2006 ©

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 A. Robertson, 2006 ©

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 A. Robertson, 2006 ©

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 A. Robertson, 2006 ©

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 ©

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 80-100F A. Robertson, 2006 ©

Powdery olive green mold Identification 1. Aspergillus flavus Powdery olive green mold A. Robertson, 2006 ©

Identification 2. Aflatoxins Black light # BGYF particles ≠ aflatoxin = false positive The black light should no longer be used for any type of mycotoxin screening http://archive.gipsa.usda.gov/pubs/mycobook.pdf A. Robertson, 2006 ©

USDA GIPSA approved list: //151.121.3.117/techservsup/ Test kits - immunoassay strips - ELISA assays - detect +/- 20ppb (detection) USDA GIPSA approved list: //151.121.3.117/techservsup/ metheqp/testkit.htm (identification) HPLC - quantitative - list of labs www.iowagrain.org A. Robertson, 2006 ©

References Diener et al. 1987. Ann. Rev. Phytopathol. 25: 249-70 Jones et al. 1980. Plant Disease 64: 859-863 Jones et al. 1981. Phytopathology 71: 810-816 Marsh and Payne. 1984. Phytopathology 74: 1284-1289 Schindeer et al. 1967. Applied Microbiology 15: 1006-1009 Wicklow and Donahue. 1984. Trans. Br. Mycol. Soc. 82: 621-624 Aflatoxins in Corn. Pm1800. Iowa State University Extension Aflatoxin in Corn. http://aes.missouri.edu/delta/croppest/aflacorn.stm

Thank You A. Robertson, 2006 ©