Common rust is caused by the funghi Puccinia sorghi and is found in most subtropical, temperate, and highland environments with high humidity. Epidemics of common rust can cause substantial yield loss. Yield losses in excess of 50% have been recorded under severe disease pressure.

Pathogen: Puccinia sorghi

Symptoms

Circular to elongate (0.2 to 2 mm long), with dark brown pustule (uredinia) scattered over both leaf surfaces giving the leaf a rusty appearance. Pustules may emerge in circular bands due to infection that occurred in the whorl. Pustules break through the leaf epidermis and release powdery reddish-brown spores (urediospores). As pustules mature they release brownish-black spores (teliospores) which are the overwintering spores. Under severe disease pressure, leaves may turn chlorotic and senesce prematurely.

Confirmation

Symptoms of common rust are often hard to distinguish from those of Polysore rust. However, there are a number of subtle distinguishing features. Firstly, common rust can be distinguished from Polysora rust (southern rust) by the appearance of pustules on both leaf surfaces. Pustules of Polysora rust emerge predominantly on the upper leaf surfaces. Furthermore, pustules of common rust are generally elongated and red to brown in color. Pustules of Polysora rust are more orangish and circular in appearance. Although both diseases thrive in humid conditions, common rust favors cool temperatures while Polysora rust favors high temperatures (above 24°C).

Common rust and Polysora rust can also be distinguished by microscopic investigation of urediospores. Urediospores of common rust are reddish-brown, spherical to ellipsoid, measuring 21-30 x 24-33μm. Each urediospore is binucleate. Teliospores are dark brown to black in color, smooth, oblong (14-25 x 28-46μm), two celled, and attached to pale brown pedicels that are up to 80μm long.

Urediospores are clearly ovoid (20-29 x 29-40 μm) and orangish in color. They are also characterized by fewer spines compared to those of common rust. Teliospores of southern rust are not known to germinate and alternate hosts have not been identified. Major differences between the two diseases are highlighted in Table 1.

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

Why and where it occurs

Common rust is a cool weather disease and occurs worldwide in subtropical and temperate environments where the growing season is characterized by mild temperatures, high relative humidity, and high moisture. Optimum temperatures for urediospore germination and formation of infection structures are between 15 and 17°C. Common rust outbreaks are common during the ear filling period and can spread rapidly in favorable environmental conditions.

Leaves in the whorl are particularly vulnerable to rust due to high moisture content and relative humidity. Young leaves appear more susceptible than older leaves. Continuous cultivation of susceptible maize ensures that the pathogen has a continuous host and results in build up of disease inoculum.

Host range

As with most rust pathogens, P. sorghi has a complex life cycle and needs two unrelated hosts to copmlete its life cycle. The asexual stages of the life cycle are commonly complete on maize and teosinte, while the sexual stage is completed on Oxalis species (wood sorrel). Typically, the sexual stage is only completed in tropical regions from where asexual spores (urediospores) are wind disseminated to temperate regions during the growing season.

Life cycle

The life cycle of P. sorghi involves two hosts (maize/teosinte and Oxalis species) and five spore stages (teliospores, basidiospores, spermatia, aeciospores and urediniospores). The life cycle is shown in Figure 1 below. In tropical or subtropical regions urediospores can overwinter and serve as the primary source of inoculum in subsequent seasons. Urediospores are disseminated by wind over vast distances (hundreds of kilometers) and frequently spread from tropical/subtropical regions to temperate regions in spring and summer when maize is cultivated. The sexual stage of the life cycle occurs predominantly in tropical and subtropical regions. Teliospores are unable to overwinter in most temperate regions.

Urediospores germinate on leaves at temperatures ranging from 4 to 30°C, although the optimum temperature range is between 15 and 17°C. Generally, urediospores germinate within 6 hours. Infection structures penetrate the host via stomata on leaf surfaces, stems, and ear sheaths. Chlorotic spots as a result of infection can arise in as little as 24 hours in favorable climatic conditions. Optimal temperatures for pustule formation are between 15 and 20°C and in favorable weather conditions the disease cycle can be as short as 5 days.

Figure 1. Life cycle of common rust

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 (temperature of 16 to 25°C, relative humidity above 95%, and over 6 hours of leaf wetness) and susceptible hybrids are cultivated. It has been estimated that reduction in yield as high as 7% is 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. Under severe infections, sugars may be diverted from the stalks for grain filling, leading to crop lodging.
• Economic importance: Yield losses as high as 50% have been recorded due to common rust. Sweet corn varieties are particularly susceptible to common rust compared to dent maize varieties.

Figure 2. Geographic distribution of common rust

Management principles

Host resistance

• Cultivation of resistant varieties is the most cost-effective and practical means of disease management.
• Varieties with both quantitative and qualitative resistance are available. Qualitative resistance is determined by the presence of Rp genes. Several Rp genes have been documented. However, it has been reported that pathotypes have emerged in various regions worldwide that are virulent against each of the Rp genes.

Fungicides

• Application of fungicides is cost-effective if applied when levels of secondary inoculum are low (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.
• Cultivate early maturing varieties that limit secondary cycles of disease and can avoid periods of heavy disease pressure later in the season.

Prepared by: Biswanath Das

References:

CAB International. 1978. Distribution maps of Plant Diseases. Edition 4, Map 279. 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).

Harlapur, S.I. and M.C. Wali. 2003. Evaluation of fungicides for control of common rust of maize. Indian Phytopathology 56: 214-15.

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

Pemezny, K. and T. Kucharek. 2003. Rust diseases of several legumes and corn in Florida. Belle Glade, FL: IFAS Extension, University of Florida. http://edis.ifas.ufl.edu/pdffiles/VH/VH05100.pdf (16 August 2007).

Shah, D.A. and H.R. Dillard. 2006. Yield loss in sweet corn caused by Puccinia sorghi: a meta-analysis. Plant Disease 90: 1413-18.