Penicillium ear rot is most commonly caused by Penicillium oxalicum, although other Penicillium species are also involved in the disease complex.  The disease is common worldwide wherever maize is grown though incidence is higher in warmer climates. Penicillium oxalicum is known to infect greenhouse crops in addition to water buffalo. Certain Penicillium species involved in Penicillium ear rot produce mycotoxins (ochratoxins) that are carcinogenic and acutely toxic to mammals.

Pathogen

• Penicillium oxalicum (most common).
• Other Penicillium species associated with Penicillium ear rot: P. chrysogenum, P. glaucum, P.cyclopium, P.funiculosum.
• P. oxalicum is being investigated as a biocontrol agent of Fusarium and Verticillium wilt of tomato.

Symptoms

Penicillium ear rot is characterized by a distinct light blue-green powdery mold that grows between kernels and on the ear surface. Infected kernels are typically bleached and streaked. Infection of stored grain results in blue-green discoloration of the embryo.  Penicillium ear rot can be distinguished from Diplodia (Stenocarpella) and Gibberella ear rot which form a whitish-grey and reddish-pink mold respectively.

Confirmation

Penicillium ear rot can be distinguished from other ear rots by its distinctive symptoms. Growth of Penicillium species on culture is filamentous, rapid, flat, and cottony in texture. Colonies are initially cottony-white but become blue-green as colonies mature and sporulate.

Microscopic examination of Penicillium species reveals septate, hyaline hyphae up to 5µm in diameter. Conidia are borne on long conidiophores that typically branch in a ‘broomlike’ manner. Conidia are single-celled, round (resembling glass beads), up to 5µm in diameter, usually green in color, and form in chains at the tips of conidiophores.

Why and where it occurs

Penicillium ear rot predominantly occurs on ears that have been damaged mechanically or by insect injury.  Penicillium ear rot is particularly common in fields that are infested with insect borers that attack the stem. The disease can also arise where maize is stored at high moisture levels, particularly if ear rot was observed prior to harvest. Humidity above 80% after grain fill leads to increased disease severity.

Host range

Penicillium species are widely distributed and occur in soils and decaying vegetation. P. oxalicum is recognized as an important pathogen to cucurbits in greenhouses in addition to maize. P. oxalicum has also been reported to cause genital infection of water buffalo.

Life cycle

Conidia are predominantly wind and rain splash disseminated and infect ears through wounds. The ear is colonized and secondary sporulation occurs, giving rise to the characteristic green-blue color of the mold. The fungi overwinter in the soil and on alternate hosts.

Damage

• Mechanism of damage: Damage is caused by rotting of kernels leading to direct yield loss. Rotting of stored grain can lead to post-harvest losses. Contamination of grain by ear rots can also result in lower market prices.
• When damage is important: Ears are most vulnerable to infection following mechanical damage or injury caused by insect damage. Infection of the cob at early reproductive stage can lead to complete ear rot. Grain stored at high moisture levels are also vulnerable to infection, which can result in extensive post harvest loss. Damage to kernels is particularly important if grain is to be used as seed in subsequent seasons. Some Penicillium species are known to produce the mycotoxin ochratoxin, which is considered carcinogenic and acutely toxic to mammalian kidneys. Ochratoxin in livestock feed is very important in poultry and pig production. Ochratoxins have been identified as the principal causal agent of porcine nephropathy in pigs.
• Economic importance: Economic losses due to stalk rots arise from direct loss of kernels, reduction in market prices due to contamination, and post-harvest storage losses. Losses depend on climatic conditions, levels of insect damage, and storage conditions. Where maize is intended for livestock, grain may be completely unsuitable due to contamination with ochratoxin.

Global distribution

Penicillium species are documented worldwide and are amongst the most common fungal species in soils and decaying vegetation. P. oxalicum is documented worldwide but is more common in warmer regions.

Management principles

Host resistance

• Where available, resistant varieties should be cultivated as the most cost-effective and practical means of disease management.
• Varieties with poor sheath coverage (exposed kernels) are more prone to infection.

Cultural control

• Harvest maize as soon as possible and dry grain to below 13% moisture levels where facilities exist.
• Shelled maize should be stored below 13% moisture while unshelled ears should be stored at below 18% moisture.
• Store clean maize by eliminating infected ears to prevent storage rots.
• Control insect pests to reduce damage to ears. Minimize mechanical damage to ears.

References

CIMMYT. 2004. Maize Disease: A guide for field identification. Mexico, D.F.: CIMMYT.

Kozakiewicz, Z. 1992. Penicillium oxalicum. IMI Descriptions of Fungi and Bacteria, 1992 (no. 111) Sheet 1107. Surrey, UK: CABI Bioscience.

Larena, P., P. Sabuquillo, P. Melgarejo and A. de Cal. 2003. Biocontrol of fusarium and verticillium wilt of tomato by Penicillium oxalicum under greenhouse and field conditions. Journal of Phytopathology 151: 507-12.

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

Payne, G.A. 1999. Penicillium Ear Rot and Blue Eye. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 46.

Tenuta, A. 2006. Identifying Corn Ear Molds. Ontario Ministry of Agriculture, Food and Rural Affairs. http://www.omafra.gov.on.ca/english/crops/field/news/croppest/2006/17cpo06a1.htm (29 August 2007).

Contributor: Biswanath Das