Rust

Rusts can affect all turfgrass species and are commonly observed in home lawns, sod production, and sports turf. While they rarely cause turfgrass death, they can significantly inhibit growth and create an unsightly orange covering on the leaves. This covering can also stain shoes and clothes. Grasses that are growing under suboptimal conditions, such as those caused by heat, drought, nutrient deficiencies, and compacted soils, are more susceptible to rust fungi infestation.

 

Causal agents

Rust diseases on turfgrasses are caused by multiple fungal species, including various species of Puccinia, Uromyces, and Physopella.

 

Primary Hosts

All turfgrass species.

 

Disease Cycle

In year-round growing regions, rust fungi overwinter on infected turfgrass foliage. Favorable conditions trigger new infections and the formation of uredinia, producing urediniospores. Wind disperses these spores over long distances, serving as primary inoculum in colder regions. Urediniospores infect susceptible hosts, generating new uredinia and countless spores. Rust fungi also produce teliospores in fall, which overwinter and give rise to basidiospores. Airborne basidiospores can infect alternate hosts, causing new infections. For instance, stem rust’s alternate host is common barberry, with basidiospores leading to aeciospores that complete the life cycle. Buckthorn serves as the alternate host for P. coronata.

 

Credit: Smiley, R. W., Dernoeden, P. H., & Clarke, B. B. (2005). Compendium of Turfgrass Diseases (3rd ed.). American Phytopathological Society.

 

Figure 14.8.1:  The diagram details the cyclical life stages of rust fungi, starting with the overwintering of mycelia and urediniospores in infected turf foliage. In climates with year-round growing seasons, these serve as initial inoculum for new infections. Wind plays a critical role in long-distance dispersal of urediniospores, particularly from warmer regions to colder ones where overwintering is not possible. The cycle showcases the repeating stage facilitated by urediniospores, capable of rapid and exponential spread under optimal conditions. Additionally, the role of teliospores and alternate hosts, like common barberry, in the lifecycle is explored, introducing the complex dynamics of spore types like basidiospores, pycniospores, and aeciospores in the propagation and cross-infection between different host plants.

 

Epidemiology

The optimal temperatures for growth and sporulation of rust fungi vary depending on the species. Stem rust development is generally favored by temperatures of 20 to 30°C (68 to 86°F), while crown and stripe rust development prefer temperatures of 10 to 20°C (50 to 68°F). Leaf moisture is necessary for rust spores to germinate and infect turfgrass leaves, although rust symptoms are typically most severe under dry conditions.

Rust diseases tend to be more severe in grasses that are experiencing slow growth under stressful conditions. Common stress factors include the following:

  • drought
  • nutrient deficiency
  • low mowing height
  • shade
  • soil compaction

In temperate climates, on cool-season turfgrasses, rust severity is often highest in late summer and early fall when stressors are more prevalent.

 

Symptoms

Severe rust infections can result in thin and weak areas of turf, with irregular patches of red, brown, or yellow turf. Heavily infected plants may wither and die due to excessive moisture loss and blocked sunlight.

 

Credit: John Kaminski/Penn State University

 

Figure 14.8.2: (a) Rust symptoms can vary, but generally take on an appearance of weak or thinned-out turfgrass. (b&c) The disease is more prominent in areas receiving low fertility. (d) In some cases, the symptoms appear as a browning or discoloration of individual leaves. 

 

Signs

Early signs of rust diseases in turfgrass include light yellow flecks on leaves or culms. These yellowed areas gradually enlarge and cover a significant portion of the leaf surface. Inside the infected spots, the fungus produces pustules called uredinia, which are fruiting bodies of the pathogen. These pustules contain spores called urediniospores, which can be yellow, orange, brownish yellow, chestnut brown, or brick red in color and appear powdery. Urediniospores are easily dispersed by wind and can travel long distances before landing on susceptible plants. Teliospores, black overwintering spores, are also produced by rust fungi and can be used for identification purposes under a microscope.

 

Credit: John Kaminski/Penn State University

 

Figure 14.8.3: (a-c) A close inspection of individual leaves often reveals the dusty orange rust pustules. Many homeowners will inquire about the safety of their children and pets who often get covered in the rust spores. (d) Rust found in turfgrass is not pathogenic to people or pets. 

 

Management

Maintaining optimal turfgrass growth conditions and utilizing resistant cultivars are key factors in managing rust disease. Rust development can be minimized through the following cultural practices:

  • proper irrigation
  • fertilization
  • aerification
  • traffic management
  • sunlight exposure
  • mowing height maintenance

Different turfgrass species and cultivars exhibit varying levels of rust resistance, with tall fescue and fine fescues generally being less susceptible compared to perennial ryegrass and Kentucky bluegrass. Incorporating a mixture of resistant cultivars or cultivar blends can contribute to reducing overall rust severity. While fungicides are not typically necessary for rust control as rusts seldom cause turfgrass mortality, severe outbreaks may require the use of demethylation inhibitor fungicides for effective rust management.

 

Cultural

To minimize rust development, it is important to maintain optimal turfgrass growth conditions. This includes proper irrigation and fertilization to avoid water and nutrient-related stresses that can impede grass growth. Other cultural practices that can help reduce rust include aerification to alleviate soil compaction; traffic management to avoid excessive wear; ensuring adequate sunlight for turf growth and leaf drying; and maintaining a proper mowing height.

There are significant differences in rust resistance among turfgrass species and cultivars. Tall fescue and fine fescues tend to be less susceptible to rust compared to perennial ryegrass and Kentucky bluegrass. Within Kentucky bluegrass, certain cultivars such as ‘Midnight’ and those genetically related to it are more susceptible to rust.

Incorporating a mixture of resistant cultivars or cultivar blends in the turfgrass stand can help reduce overall rust severity.

 

Chemical

Fungicides are generally not recommended for rust control since rusts typically do not kill the turfgrass. However, in severe outbreaks, fungicides may be warranted. Fungicides from the QoI and DMI , and chlorothalonil + acibenzolar (Daconil Action) are commonly used and tend to be effective for rust control.