Aspergillus spp. (Corn/Maize)

Scientific Name Aspergillus flavus Link:Fr [anam.], Aspergillus parasiticus Speare [anam.]
Synonyms Aspergillus fasciculatum Batista & H. Maia, Aspergillus flavus f. magnasporus (Sakag. & G. Yamada) Nehira Aspergillus flavus var. wehmeri (Costantin & Lucet) Blochwitz, Aspergillus humus E.V. Abbott, Aspergillus luteus (Tiegh.) C.W. Dodge, Aspergillus toxicarius Murak., Aspergillus wehmeri Costantin & Lucet
Common Names English: Aspergillus ear rot, storage rot of maize
Description A. flavus and A. parasiticus are closely related species which are rarely differentiated. They are ascomycetous fungi of the order Eurotiales. A. flavus produces the mycotoxins aflatoxin B1 and B2, A. parasiticus aflatoxin G1, G2, and M1. Conidial heads are radiate, with both metulae and phialides. Conidia are globose to subglobose, echinulate, usually 3 - 6 µm in diameter. Sclerotia are dark-red to black, and 380 - 700 µm diameter. Their role as inoculum sources for A. flavus infection of corn is not clear. Large variations in strains of A. flavus have been reported. A. flavus consists of two morphological groups, S and L, differentiated by the formation of small or large sclerotia, respectively; typically S strain isolates characterized by numerous small sclerotia (diameter <400 µm) produce larger amounts of mycotoxins than L strain isolates. The formation of sclerotia and aflatoxin by S strain isolates is reduced in the presence of atoxigenic L strain isolates.

Biology

Damage

Typically, only some kernels on an ear are infected having masses of yellow green spores (mold) on and between kernels. Heavily colonized kernels are discolored and rotten. However, the fungi often are present in kernels without visible sporulation. They produce aflatoxin mycotoxins which, when ingested put the health of humans and animals at risk. Kernel infection and aflatoxin production is often associated with a bright greenish yellow fluorescence of kernels when irradiated with UV light at 365 nm. This may be used for diagnostic purposes; however, BYGF may also occur without aflatoxin production.

Lifecycle

Fungal colonization often proceeds from senescing silks, the glumes to the kernel surfaces, first ear tips, later the ear base. Mycelial spread is quick at high temperatures and often forms a clustered distribution. Environmental and edaphic factors influence infection, development and the spread of A. flavus and aflatoxin production in the field, at harvest and in storage. Low moisture content of grains, low temperature and low rel. humidity generally result in low mycotoxin contamination.

Cause

Insects like the corn earworm Helicoverpa zea and the European corn borer Ostrinia nubilalis contaminated with A. flavus may be inoculum sources and increase the risk of mycotoxin contamination when they feed on corn plants. Non-aflatoxin producing Aspergillus species that may also occur on corn are A. toxicarius (Murakami) and A. oryzae (Ahlburg) Cohn.

Occurrence

Agricultural Importance

A. flavus is widespread in soils, crop residues and air throughout the world and predominates on corn. The frequency of aflatoxin-producing strains in the soil varies (40 - 80 %) and is often higher than from plant tissue. In cooler seasons the number of propagules decreases considerably; however, soil temperatures above 30 °C increase soil populations of A. flavus within 7 days which is associated with an increase in the number of conidia in the air.

Control

Integrated Crop Management

Although some sources of genetic resistance seem to be available, differences in susceptibility among commercial cultivars are not sufficient for the control of the disease. Aflatoxin contamination is influenced by the crop management system. Early planting, use of well-adapted cultivars, moderate number of plants per unit of area, adequate irrigation in order to avoid drought stress and the control of ear insects often results in low aflatoxin levels. A. flavus has been isolated more frequently from corn shelled immediately after harvest than from corn dried on the cob. For storage, the water content of kernels should be <18 %. Competition from atoxigenic L strain isolates of A. flavus to inhibit or reduce aflatoxin contamination of corn has been tested successfully. Atoxigenic strains may interfere with the contamination process both by physically excluding the toxigenic strain during infection and by competing for nutrients required for aflatoxin biosynthesis. Ammoniation of aflatoxin-contaminated corn is described to reduce the mycotoxin contamination of kernels by more than 90 %.

Chemical Control

Aspergillus species are relatively weak plant pathogens. Accordingly, not the visible plant damages in the field but the potential contamination of harvested plant parts with aflatoxins are the most relevant hazard. To prevent the entrance of aflatoxins into the human and animal food chain is therefore the aim of most strategies which all try to minimize plant stress and to optimize (after harvest) storage conditions. During storage, propionic acid has been reported to suppress fungal growth as well as aflatoxin formation.

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