Rust of Turfgrasses
Click for larger image Classic rust spots on leaf blade from a Kentucky bluegrass lawn (Poa pratensis)

There are nearly 5,000 different species of rust of which about 10 may be important pathogens on turfgrass. They attack only live grass plants, and two or more rusts may attack the same grass plant at the same time. Different races of the pathogens occur and they differ in their ability to attack cultivars (varieties) within a grass genus or species. This is one of the reasons that some grass cultivars lose their rust resistance after being grown for a period of years.

Rusts in the genus Pucciniaoccur on all commonly grown warm- and cool-season turfgrasses, and a few are responsible for significant stand losses. Among warm-season turfgrasses, zoysiagrass lawns are often the target of severe rust damage, while bermudagrass, St. Augustine grass, and centipede grass lawns are far less likely to suffer serious injury. Perennial ryegrass is the most susceptible of commonly grown cool-season turfgrasses, but the disease is rare on tall fescue and bentgrass. With few exceptions, the host range of rust fungi is limited to a few turfgrasses.

Symptoms and Diagnosis

Diseased plants initially develop light yellow flecks on the leaves. As the spots enlarge, the surfaces of the leaves rupture, exposing masses of powdery, brick red spores. Continuous heavy infection causes many grass blades to turn yellow, wither, and die. A severely rusted lawn may winterkill. From a distance, rust-infected turf appears dull yellow or light brown. Individual plants may die and the turf becomes noticeably thin. The disease tends to be more severe in partially shaded areas, such as under trees or along fences.

Life Cycle

The cycle of development for the rust fungus is very complex because of the many species involved and the numerous alternate hosts, mostly woody shrubs and herbaceous ornamentals. Alternate hosts are not believed to play an important role in the disease development of the fungi that attack turf grasses.

The rust fungi may overwinter in infected plants or be reintroduced into lawns each summer by windblown spores. Each spore must come in contact with a water droplet on the leaf or stem surface to germinate and infect the plant. Infection of leaf blades is favored by moderate temperatures (68 degrees to 85 degrees F) and extended wet periods. This cycle is repeated about every two weeks under conditions favorable for rust development.

Integrated Pest Management Strategies

1. Fertilizer. Use a balanced fertilizer at recommended levels, according to a soil test. Low nitrogen levels and nutrient imbalance can encourage rust.

2. Mowing. Mow regularly to remove infected leaf blades before the spores are produced. Avoid mowing lower that the recommended height, as this can lead to stunted root systems and decrease the ability of the turf to withstand drought.

3. Watering. Water early in the day to allow the leaves to dry before night. Avoid water stress by irrigating during dry periods.

4. Cultivars. Select resistant cultivars. Before seeding, consider recommended cultivars that are resistant to rusts.

Organic Strategies

Strategies 2, 3 and 4 are strictly organic approaches. Using an appropriate organic fertilizer would be a viable organic approach to Strategy 1.

More images:

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Rust spot on leaf blade from a zoysiagrass lawn (Zoysia)
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Rust spots on leaf blade from a Kentucky bluegrass lawn (Poa pratensis)
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Rust spots on leaf blade from a fescue lawn (Festuca)
 
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