Phaeosphaeria leaf spot (PLS) is a foliar disease of maize caused by the ascomycete fungi Phaeospharia maydis.  PLS is widely distributed in South and Central America, Asia, and Africa and is considered a potential threat to maize production in regions where high humidity and low night-time temperatures are prevalent during the growing season.  PLS is considered a particularly important disease of maize in Brazil where yield loss as high as 60% has been recorded.  However, PLS remains a fairly minor disease of maize in most other maize growing regions. 

Pathogen

Phaeosphaeria maydis (anamorph: Phoma maydis).

Reports from Brazil suggest that three other fungal pathogens also cause symptoms similar to PLS on maize in Brazil. These are Phoma sorghina, a Phyllosticta species and a Sporormiella species. It has therefore been suggested that a number of pathogens may be involved in a PLS disease complex.

Symptoms

Immature lesions appear as small, pale green, or chlorotic spots scattered over the leaf surface. Mature lesions are necrotic, bleached, and dried with dark brown margins. Lesions are circular or oval and from 3 to 20mm in diameter. Lesions can coalesce and blight the entire leaf. Perithecia and pycnidia develop within lesions.

Confirmation

Identity of P. maydis can be confirmed by microscopic inspection of the fruiting structures and spores. Perithecia and pycnidia are spherical or globose, dark brown to black with a round ostiole from which spores can escape. Perithecia and pycnidia (74-151µm in height and 67-160µm in length) are typically immersed within lesions. Asci produced within perithecia are straight to slightly curved, hyaline, cylindrical, and contain 8 ascospores. Asci are typically 7-9 × 43-70µm. Ascospores are 3 septate with slight constriction at the septa and a swollen penultimate cell. Ascospores are hyaline, usually slightly curved and measure 3-5 × 14-18µm. Conidia produced within pycnidia are hyaline, ellipsoid, round and measure 2.4-5 × 3.2-9µm. In culture, P. maydis produces white mycelium that becomes dark with maturity and numerous pycnidia.

Why and where it occurs

PLS is most prevalent in areas of high rainfall and moderate temperatures. These conditions are particularly prevalent in high altitude tropical regions. Humidity levels above 70% and night time temperatures above 14°C strengthen disease severity.

Figure 1. Geographic distribution of Phaeosphaeria leaf spot

Host range

Unknown. Phoma sorghina is also pathogenic to sorghum.

Life cycle

P. maydis overwinters on crop debris. During subsequent growing seasons, in response to favorable climatic conditions (high rainfall and moderate temperatures), spores are rain splash and wind disseminated to freshly-planted maize where they germinate on foliar tissue. Spores produced in disease lesions initiate secondary cycles of infection during the season.

Damage

• Mechanism of damage: Loss of photosynthetic leaf area due to lesions results in less photosynthate production for grain filling. Under severe disease pressure, sugars may be diverted from the stalks for grain filling resulting in crop lodging.
• When damage is important: Yield losses are most severe when upper leaves are severely blighted by PLS during the early reproductive growth stages of the maize plant. In many maize growing regions however, conditions favorable for PLS exist only towards the end of the growing season and hence yield loss is minimal. Optimum conditions for disease development are humidity above 70% and night-time temperatures above 14°C.
• Economic importance: PLS is increasingly being considered a major disease of maize in parts of Brazil and South Africa. Yield loss in Brazil due to PLS as high as 60% has been reported. In the USA yield loss of 13% has been reported. In many maize growing regions, however, PLS remains a late season crop and is considered of minor importance.

Global distribution

PLS has been documented in the USA, India, Mexico, southern and eastern Africa, Cameroon, Brazil, Columbia, Costa Rica, and Ecuador.

Management principles

• In regions where PLS is a serious foliar disease, cultivation of resistant varieties is the most cost effective and practical means of disease management. In some regions, including the USA, many of the commercial varieties appear to be resistant to PLS.

• In the absence of host resistance, fungicides can be used to control PLS in seasons where disease severity is expected to be high. Fungicides should be applied seven weeks following planting to protect the early reproductive growth stages. Subsequent applications may be required depending on disease severity and prevailing weather conditions.

• Management of infected crop residue will reduce disease inoculum at the onset of the subsequent growing season.
• Cultivation of maize during periods that are unfavorable for disease development can also reduce crop damage.

References

Carson, M.L. 1999. Diseases of Minor Importance or Limited Occurance. In Donald G. White (ed.), Compendium of Corn Diseases. St. Paul, MN: The American Phytopathology Society.

Carson, M.L. 1999. Vulnerability of U.S. maize germ plasm to Phaeosphaeria leaf spot. Plant Disease 83: 462-64.

Carson, M.L. 2005. Yield loss potential of Phaeospharia leaf spot of maize caused by Phaeosphaeria maydis in the United States. Plant Disease 89: 986-88.

Casela, C. R. 1998. The Phaeosphaeria Leaf Spot. In C. Casela, B. Renfro and F. Krattiger (eds.), Diagnosing Maize Diseases in Latin America. ISAAA Briefs No. 9. Ithaca and Brasilia: ISAAA /EMBRAPA. Pp15-17.

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

Do Amaral, A.L., F.K. Dal Soglio, M.L. De Carli and J.F.B. Neto. 2005. Pathogenic fungi causing symptoms similar to Phaeosphaeria leaf spot of maize in Brazil. Plant Disease 89: 44-9.

Contributor: Biswanath Das