Gray leaf spot (GLS) is a serious foliar disease of maize in many temperate and tropical highland regions of the world. GLS is caused by fungal species in the genera Cercospora. The disease is considered the most important foliar disease of maize in the USA and is considered one of the principal constraints to maize production in the mid-altitude maize growing regions of sub-Saharan Africa. GLS is also emerging as a major constraint to maize production in China. Yield losses in excess of 70% have been caused by GLS. 

Pathogen

Cercospora zeae-maydis
Cercospora zeina

Initially, C. zeae-maydis was considered the sole causal agent of GLS.  However, recent molecular investigations show that GLS is caused by two genetically distinct but morphologically indistinguishable Cercospora species.  Initially they were considered ‘sibling species’ (C. zeae-maydis Group I and C. zeae-maydis Group II). In 2006, C. zeae-maydis Group II was classed as a separate species, Cercospora zeina, while C. zeae-maydis Group I retained the name Cercospora zeae-maydis.

Cercospora sorghi var. maydis has also been associated with GLS lesions but has not been shown to infect maize. It is suspected to be an opportunistic saprophyte.

Symptoms

GLS can be easily identified by characteristic rectangular, tan-colored lesions that are contained within leaf margins, as the fungi is not able to penetrate sclerenchyma tissue in the leaf veins. As lesions mature they assume a graying cast due to sporulation of the fungi. Lesions are typically 2-4 × 10-60mm in size and usually develop on the lower leaves, gradually spreading upwards on the plant during the season. Under severe disease pressure, entire leaves can be blighted and lesions can develop on cob sheaths. Symptoms of GLS caused by Cercospora zeae-maydis and C. zeina are indistinguishable.

Confirmation

The causal agent of GLS can be confirmed by microscopic examination of conidiophores and conidia and characteristics of cultural growth on artificial medium.  C. zeae-maydis and C. zeina are indistinguishable based on morphological characteristics of the conidia and conidiophore.  Conidia are hyaline, obclavate, slightly curved and contain 3 to 10 septa.  Conidia are 5-10 × 30-100µm in size and are borne on conidiophores that emerge in fascicles from the leaf stomata.  Conidiophores are 3 to 8 septate, olivaceous to brown, and 4-8 × 70-130µm in dimension. Generally conidia dimensions are considered more reliable for taxonomic purposes as conidiophore morphology can vary widely according to light, temperature and humidity.

Both C. zeina and C. zeae-maydis grow slowly on agar medium.  On V8 juice agar C. zeae-maydis has been reported to grow faster (8-12mm per weeks) than C. zeina (4-5mm per week).  Both species produce either black, compact, dome-like cultures that sporulate profusely or white, cottony, sterile growth.  C. zeae-maydis has also been reported to produce cercosporin in culture, indicated by the presence of a pink pigment, while C. zeina has not been reported to produce cercosporin in culture. 

Molecular techniques (species specific PCR diagnostics) offer the most reliable means of distinguishing C. zeina from C. zeae-maydis.

Why and where it occurs 

GLS occurs throughout maize growing regions of the world where the growing season is characterized by high humidity and temperatures between 22 and 30°C.  Overcast, cloudy days favor disease severity.  Favorable conditions in the early part of the season favor disease severity as inoculum levels are able to increase.  The crop is most vulnerable to GLS following full canopy development which results in high relative humidity within the crop canopy. 

GLS is endemic in regions where maize is cultivated intensively.  High levels of nitrogen fertilisation have been associated with increased disease severity.  As the pathogen is host specific and only able to survive between seasons in infected crop debris, the disease tends to be more prevalent in regions where reduced tillage techniques have been adopted.

Figure 1. Global distribution of GLS

Host range 

Both C. zeae-maydis and C. zeina are only known to infect maize.  C. sorghi var. maydis has not been shown to be pathogenic on either maize or sorghum. 

Life cycle 

C. zeae-maydis and C. zeina have similar life cycles.  The fungi are host specific and overwinter as mycelium and stromata in infected maize plant debris left on the soil surface.  At the onset of the following growing season, fungi in infected crop debris begin sporulating.  Conidia are then wind and rain splash disseminated to freshly-planted maize fields.  Conidia infect leaves through the stomata and colonize leaf tissue, giving rise to characteristic symptoms. Secondary cycles of disease are initiated by conidia produced within lesions. Under favorable climatic conditions, disease progress can be rapid during the grain filling stages of maize growth. 

Damage 

• Mechanism of damage: Yield loss is caused by loss of photosynthetic leaf area available for grain filling.  Under severe disease pressure, sugars are diverted from the stalks for grain filling which predisposes plants to lodging.
  
• When damage is important: Damage is most important when leaves above the ear are severely infected as the upper leaves of the maize plant are responsible for the majority of photosynthate production for grain filling.  Under severe infection, leaves may be completely blighted and killed up to 30 days prior to crop maturity. Crop lodging is a particular problem where maize is machine harvested. 
• Economic importance: GLS causes significant yield loss in the USA, southern and eastern Africa, and China. Complete crop loss has been recorded in the USA where maize is machine harvested, while yield losses as high as 70% have been documented in Africa.  

Global distribution

GLS is documented in many temperate and tropical regions of the world.  C. zeae-maydis is documented in North and South America and China.  C. zeae-maydis has not been documented in Africa. C. zeina has been documented in Africa, Brazil and the eastern USA.

Management principles 

Host resistance 

• Cultivation of resistant varieties is the most cost-effective and practical means of disease management.

• Resistant hybrids are available in most regions where GLS is endemic.

Fungicides

• In the absence of host resistance, fungicides can be used to provide effective control of GLS.Fungicide use is particularly profitable for seed production.

• Various triazole and benzimidazole fungicides have been shown to provide most effective control of GLS.

• Fungicide application should begin when lesions are first observed on basal leaves.  A secondary application may be necessary 30 days after the first application depending on prevailing climatic conditions.

Cultural control

• Management of infected crop residue following harvest can reduce inoculum levels in subsequent seasons.

• In some regions early planting is recommended to avoid periods of high disease pressure.

References

Beckman, P.M. and G.A. Payne. 1983. Cultural techniques and conditions influencing growth and sporulation of Cercospora zeae-maydis and lesion development in corn. Phytopathology 73: 286-9.

Carson, M.L. 1999. Gray leaf spot. In Donald G. White (ed.), Compendium of Corn Diseases. St. Paul, MN: The American Phytopathology Society. Pp. 22-3. 

Carson, M.L. and M.M. Goodman. 2006. Pathogenecity, aggressiveness and virulence of three species of Cercospora associated with gray leaf spot of maize. Maydica 51: 89-92.

Crous, P.W., J.Z. Groenewald, M. Groenewald, P. Caldwell, U. Braun and T.C. Harrington. 2006. Species of Cercospora associated with gray leaf spot of maize. Studies in mycology 55: 189-97.

de Nazareno, N.R.X., P.E. Lipps and L.V. Madden. 1993. Effect of levels of corn residue on epidemiology of gray leaf spot in Ohio. Plant Disease 77: 67-70.

Latterell, F.M. and A.E. Rossi. 1983. Gray leaf spot of corn: A disease on the move. Plant Disease Reporter 67: 842-7.

Wang, J., M. Levy and L.D. Dunkle. 1998. Sibling species of Cercospora associated with gray leaf spot of maize. Phytopathology 88: 1269-75.

Ward, J.M.J., M.D. Laing and F.H.J. Rijkenberg. 1997. Frequency and timing of fungicide applications for the control of gray leaf spot of maize. Plant Disease 81: 41-8.

Ward, J.M.J., E.L. Stromberg, D.C. Nowell and F.W. Nutter, Jr. 1999. Gray leaf spot: A disease of global importance in maize production. Plant Disease 83: 884-95.

Contributor: Biswanath Das