Aspergillus ear rot is caused by several Aspergillus species. Although crop damage due to Aspergillus ear rot is minimal in most years, the disease is of importance as a number of Aspergillus species (A. flavus and A. parasiticus) produce potent mycotoxins (aflatoxins) that are toxic to birds and mammals.

Pathogens

• Aspergillus flavus and A. parasiticus (produce aflatoxins).
• Aspergillus niger A. glaucus, A. ochraceus, A. toxicarius, A. oryzae (other Aspergillus species involved in the ear rot complex).

Symptoms

Aspergillus ear rot is characterized by the production of powdery masses of spores on and between kernels. Spores produced by A. niger are black in color, while those produced by other Aspergillus species are typically yellow-green and turn dark green to brown as colonies mature. Typically only a few kernels (usually injured kernels) of the ear are infected and infection usually occurs close to the ear tip. Occasionally, injured kernels may not show signs of infection but will be dull and discolored.

Confirmation

Generally ear rots caused by Aspergillus species can be differentiated from those caused by other species (Penicillium, Gibberella, etc.) by the yellow tint of the powdery masses.

Aspergillus flavus

Hyphae are hyaline and septate. Colorless conidiophores originate from the hyphae and terminate in a vesicle. Vesicles are globose and are characteristic of Aspergillus species. Flask shaped phialides cover the surface of the vesicle radially or only at the upper surface (columnar). Phialides may be attached directly to the vesicle or via a supporting cell (metula). Conidia form in radial chains on the phialides. Conidia are globose, smooth to slightly rough, and 2-7µm in diameter. A. flavus may also form sclerotia which are 400-700µm in diameter, globose, and dark red to black in color.

A. flavus can be detected in corn as it produces compounds that fluoresce under black light. This method provides a qualitative assessment but is not useful to quantify precise levels of infection.

Incidence and factors favoring disease severity

Aspergillus ear rot is more prevalent in hot and dry years. Drought and high temperatures increase disease severity. Unlike many other pathogenic fungi and bacteria, A. flavus is thermotolerant and is able to survive high temperatures. Stressed plants (drought or nitrogen deficiency) and cobs damaged by insect feeding are more susceptible to Aspergillus ear rot. A. flavus can be a serious storage rot, particularly when grain is stored at high moisture levels.

Host range

A. flavus is known to infect a variety of stored grains, nuts and seeds including groundnut, Brazil nut, beans, rice, sorghum, wheat and cucurbits. A. flavus occurs worldwide as a saprophyte in soil and decaying organic matter.

Life cycle

A. flavus overwinters as a saprophyte in the soil and on decaying organic matter. Hot and dry weather greatly favor the proliferation of the fungi. Spores (conidia) are wind and insect disseminated to silks through which the kernel is infected.

Aflatoxins

Aflatoxins are toxic, carcinogenic compounds produced by A. flavus and A. parasiticus. Aflatoxins are toxic to birds and mammals and therefore maize grains affected by Aspergillus ear rot may not be suitable for human and livestock consumption. Livestock fed with aflatoxin-contaminated feed may lose weight and suffer from liver damage, hemorrhages, and increased susceptibility to opportunistic diseases.

Meat from livestock fed with aflatoxin-contaminated feed is not contaminated, as aflatoxins are metabolized and excreted. However, aflatoxins can accumulate in milk from cows fed with aflatoxin-contaminated feed.

Damage

• Mechanism of damage: Damage is caused by the contamination of grain with aflotoxins produced by various Aspergillus species.
• When damage is important:Damage is most critical when aflatoxin levels in contaminated grain exceed threshold levels. The Unites States Food and Drug Administration have set a threshold level of 0.5 parts per billion (ppb) for aflatoxin M1 in milk.
• Economic importance:Direct yield loss due to Aspergillus ear rot is rare. However, extensive losses can be incurred if aflatoxin levels in stored grain exceed threshold limits for human and livestock feed.

Global distribution

A. flavus is distributed worldwide but is more prevalent in tropical soils.

Management principles

Host resistance

• Cultivation of resistant varieties offer the most practical and cost-effective means of disease management.
• Resistance to Aspergillus ear rot is considered to be quantitatively inherited with additive gene action.

Fungicide application

• Treatment of stored, infected maize with ammonia is known to reduce aflatoxin contamination.
• Foliar application of fungicide is not appropriate as Aspergillus ear rot typically occurs close to crop harvest and in storage.

Cultural means

• Reducing crop stress by irrigation and appropriate fertilization can reduce the incidence of A. flavus.
• Management of insect pests, which damage the ears creating infection sites and which disseminate the fungi, can reduce disease incidence.
• Storage of maize at low moisture levels will reduce storage rots.

References

CIMMYT. 2004. Maize Diseases: A guide for Field Identification. 4th Edition. Mexico, D.F.: CIMMYT.

Naidoo, G., A.M. Forbes, C. Paul, D.G. White and T.R. Rocheford. 2002. Resistance of Aspergillus Ear Rot and Aflatoxin Accumulation in Maize F1 Hybrids. Crop Science 42: 360-4.

Onions, A.H.S. 1966. Aspergillus flavus. IMI Descriptions of Fungi and Bacteria, 1966 (No.10) Sheet 91. Surrey, UK: CABI Bioscience.

Payne, G.A. 1999a. Aspergillus Ear Rot. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 44

Payne, G.A. 1999b. Mycotoxins and Mycotoxicoses. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 47-9.

Contributor: Biswanath Das