Stenocarpella (Diplodia) ear rot is caused by Stenocarpella species and commonly found in hot, humid maize growing regions. The causal agents of Stenocarpella ear rot also cause Stenocarpella stalk rot and Macrospora leaf stripe. Stenocarpella species can produce various mycotoxins that are harmful to birds.

Pathogen 

Stenocarpella maydis (syn. Diplodia maydis)
Stenocarpella macrospora (syn. Diplodia macrospora)

Symptoms

Early infection of ears (just after flowering) results in development of husks that are bleached (straw colored) and dry although the maize plant remains green and healthy. Ears are also shrunken, bleached, and chaffed, with gray-white cottony mycelium growth between and over the kernels. Infected kernels are typically dull grey to brown. Symptoms of Stenocarpella ear rot usually first appear at the base of the ear, before the entire ear is colonized by the fungi. Infected ears may be totally rotted and light in weight. Late in the season, abundant, black, miniature pycnidia form on kernel and ear tissues. Some pathogen isolates are known to cause premature germination of kernels.

Confirmation

Stenocarpella ear rot is characterized by the formation of abundant, miniature, black pycnidia on kernels and ear tissue late in the season. For accurate confirmation of pathogen identity, microscopic analysis is recommended. Pycnidia of both S. maydis and S. macrospora are flask-shaped (spherical with circular, protruding papillate ostiole) and multicellular. Pycnidia are 150-450µm in diameter, while the ostiole is typically 30-40µm in diameter. 

Pycnidia contain pale brown, straight to slightly curved conidia with rounded ends.  Conidia of S. macrospora are 0-3 septate measuring 7-12 × 44-82µm. Conidia of S. maydis are 1-2 septate and measure 5-8 × 15-34µm.  Occasionally, pycnidia contain colorless, narrow spores that measure 1-2 × 25-35µm. A sexual stage for these fungi has not been recorded.

Incidence and factors favoring disease severity

Wet weather immediately following silking increases disease severity. The disease is also more prevalent where maize follows maize and crop rotation is not used. Additionally, the disease is more prevalent where ears are damaged due to insect injury (e.g. stalk borer feeding). Generally Stenocarpella ear rot is predominantly a problem in the field, although it can emerge as a problem post-harvest if grain is stored at high moisture content (above 20%).

Host range

Both S. maydis and S. macrospora infect maize as the primary host. However, S. maydis has also been reported to infect bamboo.

Life cycle

The causal fungi overwinter as pycnidia in infected maize stalk debris. During wet weather, conidia are produced and are rain splash disseminated to silks. The fungi then grow down the silks and infect the ear. Generally, ears are most vulnerable to infection in the three week period following silking. Infection can also occur through the base of the ear or through ear injuries caused by birds and insects. If infection occurs early (following flowering), the fungi colonizes the ear, giving rise to pycnidia late in the season.

Damage

• Mechanism of damage: Direct yield loss is caused by rotting of ears and kernels leading to reduced weight and nutritional content. Infection of kernels at the blister stage can result in reduced kernel size and grain filling.
• When damage is important: Damage is most critical if infection occurs early (immediately following flowering), as the entire ear may rot and kernels may fail to develop fully. S. maydis produces the mycotoxin diplodiatoxin and S. macrospora produces the mycotoxin diplodial, which are both harmful to birds. Livestock may refuse grain that is severely affected by Stenocarpella ear rot.
• Economic importance: In some cases up to 80% of ears can be affected by Stenocarpella ear rot, leading to considerable yield loss. Infected ears can weigh up to 35% less than healthy ears.

Global distribution

The global distribution of S. maydis and S. macrospora are shown in Figures 1 and 2 respectively.

Figure 1. Geographic distribution of S. Maydis

Source: EPPO (European and Mediterranean Plant Protection Organisation).

Figure 3. Global Distribution of S. macrospore. Source: EPPO

Management principles

Host resistance 

• Cultivation of resistant varieties where available offers the most practical and cost‑effective means of disease management.

Cultural control

• Crop rotation is effective at reducing disease severity as the causal pathogens are predominantly pathogens of maize.
• Management of infected crop debris (stalks, ears) following harvest will reduce the amount of overwintering disease inoculum.
• Storing grain at below 15% moisture content will reduce storage rot caused by the Stenocarpella species.

References

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

Dorrance, A.E., K.H. Hinkelman and H.L. Warren. 1998. Diallel analysis of Diplodia ear rot resistance in maize. Plant Disease 82: 699-703.

Flett, B.C., N.W. McLaren and F.C. Wehner. 2001. Incidence of Stenocarpella maydis ear rot of corn under crop rotation systems. Plant Disease 85: 92-4.

Lipps, P.E. and D.R. Mills. Diplodia Ear Rot of Corn. Ohio State University Extension Fact Sheet AC-0046-01. http://ohioline.osu.edu/ac-fact/0046.html (31 August 2007).

OEPP/EPPO. 2006. Data sheets on quarantine pests No.67: Stenocarpella macrospora and Stenocarpella maydis. http://www.eppo.org/QUARANTINE/fungi/Stenocarpella_macrospora/
DIPDSP_ds.pdf (31 August 2007).

Olatinwo, R., K. Cardwell, A. Menkir, M. Deadman and A. Julian. 2004. Inheritance of Resistance to Stenocarpella (Earle) Ear Rot of Maize in the Mid-Altitude Zone of Nigeria. European Journal of Plant Pathology 105: 535-43.

Payne, G.A. 1999. Diplodia Ear Rot. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 44-5.

Rao, S.K., P.N. Achar and M.F. Rahman. 2003. Biochemical changes induced in liver and serum of diplodiatoxin (Stenocarpella maydis) treated male and female rats. Drug and Chemical Toxicology 26: 231-43.

Vincelli, P. 1997. Ear Rot of Corn Caused by Stenocarpella maydis (=Diplodia maydis). University of Kentucky Cooperative Extension Service. Data Sheet PPA-43.  http://www.ca.uky.edu/agc/pubs/ppa/ppa43/ppa43.pdf (31 August 2007).

Contributor: Biswanath Das