Gibberella and Fusarium ear rot occur widely throughout maize growing regions of the world. They are of particular concern as the causal pathogens produce mycotoxins that are harmful to humans and livestock. The causal pathogens of Gibberella and Fusarium ear rot also cause stalk and seedling blights of maize.

Pathogen

• Gibberella ear rot (also known as red rot) is caused by Gibberella zeae (Anamorph: Fusarium graminearum).
• Fusarium ear rot is caused predominantly by Fusarium verticilioides syn. Fusarium moniliforme (Teleopmorph: Gibberella fujikuroi). F. proliferatum (teleomorph: G. fujikuroi mating population D) and F. subglutinans (teleomorph: G. fujikuroi mating population E) have also been associated with Fusarium ear rot.

Symptoms

Gibberella ear rot. Ear infection is initially characterized by the presence of white mycelium on the ear tips that gradually moves towards the base of the ear. The mycelium turns a distinctive reddish-pink color in infected kernels. Early infection can result in entire ears being colonized by reddish-pink mycelium, extensive kernel rotting, and husks adhering tightly to ears. Bluish-black perithecia may also form on the husks.

Fusarium ear rot. Fusarium ear rot is characterized by cottony mycelium growth that typically occurs on a few kernels or is limited to certain parts of the ear, unlike Gibberella ear rot. Mycelium is generally white, pale pink or pale lavender. Infected kernels typically display white streaking (also known as ‘starburst’ symptoms) on the pericarp and often germinate on the cob. Typically, infection occurs close to ear tips and is commonly associated with damage and injury caused by ear borers. Under severe infestation, the entire ear appears withered and is characterized by mycelium growth between kernels.

Confirmation

Symptoms and microscopic examination of fungal spores and fruiting structures can be used to confirm pathogen identity.

Gibberella zeae. Conidia of the asexual stage of Gibberella zeae (Fusarium graminearum) are hyaline, slightly curved, tapering at both ends, and 3 to 5 septate.  Condia measure 4-6 × 10-30µm.  Perithecia can form within infected husks and stalks (Gibberella stalk rot) late in the season.  Perithecia of G. zeae are bluish-black and contain 8 ascospores.  Ascospores are hyaline, up to 3 septate, slightly curved, and taper at both ends.  Ascospores measure 3-5 × 10-30µm.  The fungus frequently overwinters as perithecia and ascospores only mature at the onset of the subsequent season when they are the primary source of disease inoculum.

Fusarium verticillioides. The perfect stage of F. verticillioides is rarely seen in contrast to that of G. zeae.  During the imperfect (asexual) stage both macroconidia and microconidia are produced.  Macroconidia are hyaline, slightly curved and taper at both ends.  They are 3 to 5 septate and measure 2-5 × 15-60µm. Microconidia are produced prolifically and are borne in chains. They are single-celled and measure 2-3 × 5-12µm.

Why and where it occurs

Gibberella ear rot is more prevalent in cool and humid maize growing regions. The disease is favored by cool, wet weather immediately following silking. Gibberella ear rot can be particularly serious when water collects between husks and kernels at the base of the plant following heavy rainfall. Gibberella ear rot is typically more prevalent where infected crop debris is allowed to overwinter.

Figure 1. Countries where Gibberella zeae has been documented.

Fusarium ear rot is one of the most common ear rots of maize. Fusarium ear rot is more prevalent where dry and hot weather occurs during flowering. The disease is commonly associated with injury to ears caused by borers.

Figure 2. Countries where Fusarium verticillioides (teleomorph: Gibberella fujikuroi) has been documented.

Host range

Gibberella zeae infects a range of other cereals including wheat, barley, oat and rye. It is also known to infect species of Lycopersicon, Pisum, Trifolium and Solanum in addition to carnations and other ornamentals.

Fusarium verticillioides infects a range of cultivated crops including sorghum, sugarcane, wheat, cotton, banana, pineapple and tomato.

Life cycle

Gibberella zeae. Ascospores and conidia are wind and rain splash disseminated from overwintering perithecia and stalk lesions and infect the ear through silks. The fungus advances towards the ear base during grainfilling, giving rise to characteristic symptoms. The fungus overwinters as perithecia, which are produced on husks.

Fusarium verticillioides. Fusarium verticillioides and other Fusarium species that cause Fusarium ear rot overwinter in infected crop debris. Mycelium in infected crop debris produce macroconidia and microconidia that are wind and rain splash disseminated, infecting ears through silks and colonizing kernels. F. verticillioides can also infect maize plants systemically in which case ears may be infected through the ear shank. Insects such as the European corn borer have also been reported to act as vector and transfer F. verticillioides spores between plants or cause plant injury that enable the fungi to infect the plant.

Damage

• Mechanism of damage: Damage is caused by direct loss of yield due to ear rot and by the production of mycotoxins that are harmful to humans and livestock. Storing grain at high moisture levels and temperatures leads to storage rot and production of mycotoxins.

• When damage is important: Damage is most critical when mycotoxins are produced at levels that are toxic and unsuitable for human and livestock consumption. Consumption of infected maize can lead to diseases known as mycotoxicoses.

  Mycotoxins produced by Gibberella zeae include deoxynivalenol, zearalenone and zearalenol. Deoxynivalenol belongs to the Trichothecene family of mycotoxins that are protein synthesis inhibitors and potent immunosuppressants. Symptoms of deoxynivalenol toxicosis in livestock include reduced weight gain, vomiting, and reduced feeding. Chronic symptoms include diarrhea, lethargy, intestinal hemorrhage, and increased susceptibility to other diseases. The Unites States Food and Drug Administration recommends that deoxynivalenol levels do not exceed 1 ppm (parts per million) for human consumption.

  Fusarium verticillioides and F. proliferatum produce mycotoxins known as fumonisins. However, not all isolates of the pathogen produce fumonisins. Fumonisins cause equine leukoencephalomalacia (‘blind staggers’) in horses and pulmonary edema in swine. Maize contaminated with fumonisin for human consumption has been associated with esophagal cancer in parts of Africa and China. Commercial kits are available to detect fumonisin levels in grain.

• Economic importance: Contamination of grain with mycotoxins can render stored grain unsuitable for human and livestock consumption which can result in serious economic losses.

Management principles

Host resistance

• Cultivation of varieties that are resistant to ear rots is the most practical and cost-effective means of ear rot management.
• Varieties with tight husks appear to be more vulnerable to Gibberella ear rot.

Chemical control

• As ear rot typically develops late in the season and in storage, use of fungicide is not appropriate.
• Where the crop is infected systemically with Fusarium species, application of fungicide early in the season can limit ear infection.
• Management of insect pests (ear borers) will reduce infection of the ear through injury sites caused by insect feeding.

Proper storage

• Storing grain at low moisture (below 15%) reduces the incidence of Gibberella zeae and Fusarium species in storage. 

Cultural control

• Management of infected crop debris will reduce the amount of disease inoculum in subsequent season.

References

Adejumo, T.O., U. Hettwer and P. Karlovsky. 2007. Occurrence of Fusarium species and trichothecenes in Nigerian maize. International Journal of Food Microbiology 116: 350-7.

CAB International. 1998. Gibberella zeae. Distribution Maps of Plant Diseases. Edition 1 (October), Map 763. Wallingrford, UK: CAB International.

CAB International. 1977. Gibberella fujikuroi. Distribution Maps of Plant Diseases. Edition 5 (October), Map 102. Wallingford, UK: CAB International.

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

Cullen, D., R.W. Caldwell and E.B. Smalley. 1983. Susceptibility of maize to Gibberella zeae ear rot: relationship to host genotype, pathogen virulence, and zearalenone contamination. Plant Disease 67:89-91.

Payne, G.A. 1999a. Ear and Kernel Rots. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 44-7.

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