Polysora rust (Southern rust) is a major disease of maize in tropical and subtropical regions worldwide.  Unlike common rust, Polysora rust is most severe in warm growing conditions.  In some seasons characterised by warm growing conditions, Polysora rust can become a major constraint to maize production in temperate regions.  Yield losses in excess of 45% have been recorded due to Polysora rust.

Pathogen: Puccinia polysora

Symptoms

Symptoms of Polysora rust are very similar to common rust (P. sorghi).  However the two can be distinguished, as pustules of Polysora rust occur predominantly on the upper leaf surface, whereas pustules of common rust occur abundantly on both leaf surfaces.  Pustules (uredinia) are orangish in color, circular (0.2 to 2 mm long), raised, and most prominent on the leaf, but also occur on the stems and sheaths.  When the pustules rupture, urediospores are released, resulting in the secondary cycles of the disease, giving the leaves a rusty appearance. Towards plant maturity, pustules become dark brown to black and release teliospores.  Pustules that release teliospores are known as telia.  Under severe disease pressure, leaves can turn chlorotic and senesce prematurely. Rupturing of pustules results in the release of powdery brown spores.

Confirmation

Identity of P. polysora is most effectively confirmed by microscopic analysis of spores, although spores are quite similar to those produced by P. sorghi. Urediospores of Polysora rust are clearly ovoid (20-29 × 29-40µm) and golden in color, whereas urediospores of common rust are reddish brown, spherical to ellipsoid, and measure 21-30 × 24-33µm. Urediospores of P. polysora are also characterised by fewer spines compared to those of P. sorghi. Also, teliospores of P. sorghi infect various Oxalis (wood sorrel) species, while teliospores of P. polysora are not known to germinate. P. polysora is not known to have an alternative host on which the sexual stage of the life cycle is completed.

Common rust can be distinguished from Polysora rust by the appearance of pustules on both leaf surfaces.  Pustules of Polysora rust emerge predominantly on the lower leaf surface. Pustules of Polysora rust also remain intact for longer than those of common rust. Key differences between common and Polysora rust are highlighted in Table 1.

Table 1. Key differences between common and Polysora rusts of maize.

Why and where it occurs

Polysora rust occurs throughout the tropical and subtropical regions of the world including Africa, Southeast Asia, Australia, Central and South America, and southern regions of the United States.

Figure 1. Geographic distribution of Polysora rust

Polysora rust is favored by warm and humid conditions, with the optimal temperature for urediospore germination and infection from 24 to 28°C. Periods of extended dew or high humidity favor Polysora rust. In temperate regions, Polysora rust is more prevalent towards the end of the growing season when temperatures rise. Late planted and late maturing maize is therefore more vulnerable.

Urediospores are the primary source of inoculum both at the start and during the growing season. Continuous maize cultivation and presence of infected crop debris at the onset of subsequent seasons will result in large levels of initial disease inoculum. Urediospores are wind dispersed.

Host range

An alternate host for P. polysora has not been identified.  Urediospores of P. polysora are known to infect maize, plume grasses, gama grasses and teosinthe.

Life cycle

Urediospores are the sole source of both primary and secondary disease inoculum. At the onset of the growing season, urediospores from overwintering infected corn debris are disseminated by wind and rain splash to freshly planted maize.  They germinate and infect maize plants through the stomata. Characteristic pustules develop on the upper leaf surface and rupture to release urediospores that serve as the source of secondary disease inoculum, which is again disseminated by wind and rain splash.  Under favorable disease conditions, Polysora rust can spread rapidly. Spores remain viable even when disseminated vast distances in excess of hundreds of kilometres.

Damage

• Mechanism of damage: Damage is caused by loss of photosynthetic leaf area, chlorosis, and premature leaf senescence leading to incomplete grain filling and poor yields.  Under severe disease pressure, sugars are diverted from the stalks for grain filling leading to plant lodging.
• When damage is important: Damage is most critical when environmental factors favor disease development (temperatures above 24°C, high relative humidity and leaf wetness) and when susceptible hybrids are cultivated. It has been estimated that reductions in yield as high as 8% are possible for each 10% of total leaf area infected. Damage is most serious in early infected plants and when disease spreads to leaves above the ear which contribute most to grain filling. Late planted maize is particularly vulnerable.
• Economic importance: Polysora rust is an important disease in many tropical regions where maize is cultivated continuously.  Yield losses in excess of 45% have been recorded.

Management principles 

Resistant varieties offer the most efficient and cost-effective means of disease management. Eleven genes offering qualitative resistance to Polysora rust have been documented and many modern hybrids offer adequate levels of resistance. Some land races in east and west Africa are also known to offer quantitative resistance to Polysora rust.

Fungicides should be applied while there is a limited amount of secondary inoculum (i.e. when pustules are first observed on leaves).

• Scouting for rust should be conducted 2 weeks prior to tasseling.
• If wet weather is not expected, rust is unlikely to increase in severity.
• Cultivating early maturing varieties can limit secondary cycles of the disease.
• Planting early can avoid periods of heavy disease pressure later in the season.
• Management of infected crop residue will reduce disease inoculum levels at the start of the subsequent season.

References 

CAB International. 1970. Distribution maps of plant diseases. Edition 5. Map 237. Wallingford, UK: CAB International

Dolezal, B., A. Sayers and S. Butzen. 2007. Common and Southern Rust in Corn. Pioneer Agronomy Resources. https://www.pioneer.com/growingpoint/agronomy/crop_insight/common_rust.jsp.  (16 August 2007).

Hernandez, J.R., M. Yasem de Romero, C.G. Diaz and J.C. Ramallo. 2002. First report of Puccinia polysora on corn in Argentina. Plant Disease 86:187.

Pataky, J.K. 1999. Rusts. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 35-8.

Pernezny, K. and T. Kucharek.  Rust diseases of several legumes and corn in Florida. IFAS Extension, University of Florida. http://edis.ifas.ufl.edu/pdffiles/VH/VH05100.pdf (16 August 2007).

Pinho, R.G.V., MA.P. Ramalho, H.P.Silva, I.C. Resende and G. Pozar, G. 1999. Damage caused by southern and tropical rusts of corn. Fitopatologia Brasileira 24:400-409.

Contributor: Biswanath Das