Managing Rust in Cool Season Pastures

Monitoring for Rust in Cool Season Pastures

Preventing and Controlling Rust in Cool Season Pastures

Hand holding two grass blades with yellow streaks, showing disease. — **Figure 1.** *Rust on perennial ryegrass leaves in August.*

Pasture grasses tend to be fairly disease-free, with one exception, rust. Rust is frequently observed by farmers and does impact pasture yield and quality. It shows up prominently with its dusty orange spore pustules that appear in warm, humid summer conditions. Unfortunately, some of the most popular pasture grasses are the most susceptible to rust: orchardgrass, meadow fescue, ryegrass, and a few others.

In the last couple of decades, scientists have added significantly to the understanding of the genetics of rusts and their life cycle. Rusts that infect pasture grasses are often in a complex of more than one species, with seven species in the genus Puccinia able to infect pasture grasses. Individual species are only distinguishable by the primary host they infect, and more than 45 genera of grasses are affected by this disease. Most of the recent research has been focused on higher-value crops like small grains and turf grasses. To be honest, there is little new research and no new tools to combat rust in pastures. Using Integrated Pest Management principles (IPM) can guide management of this disease, but first, what are the impacts of rust on pastures?

Impacts of Rust on Pasture Production

Palatability

Palatability is one of the most frequently seen impacts of rust infection. There has been no research quantifying the impact of refusal on forage intake and animal health, but most farmers who have susceptible pastures know that refusal of rust-infected forage may be one of the most challenging issues with the disease.

Pasture Yield

There is limited, older research on rust impacts on pasture yield, but it is well documented that all plants respond to disease infection by shifting resources to combat the disease, leaving less energy for growth and photosynthesis. Premature leaf death further reduces yield potential. Research conducted on pure stands of perennial ryegrass in New Zealand (published in 1987) compared yields on plots with rust controlled with fungicide sprays, versus yields on non-sprayed plots. The average yield for a single cutting of the rust-controlled plots was 1,759 lbs. dry matter per acre compared to 1,233 lbs./acre for the non-sprayed plots, a 30% reduction. Other research reported yield reductions of 9-23% in rust-infected grass stands.

Nutritional Quality

Rust infection affects the nutritional value of forage in several ways. The disease increases the proportion of dead and dying leaves in the infected sward. It also impairs the plant’s ability to store nutrients, primarily carbohydrates and digestible fiber. Another New Zealand study on ryegrass indicated that rust can reduce dry matter digestibility by 1.4 to 4.2 percentage points, and water-soluble carbohydrates by 3.6 to 8.5 percentage points. These are not huge decreases, but combined with the other impacts, they can be significant.

The nutritional value of infected leaves is further compromised by foreign chemical compounds produced by the rust fungus and by chemicals produced by the plant to combat the disease. A study comparing digestion of rust-infected and healthy orchardgrass leaves in rumen fluid showed that the rust organism produces compounds that can affect the rumen microbes’ ability to digest the forage by interfering with production and/or functioning of enzymes. That means that regardless of the quality of the forage going into the rumen, the rust pathogen impairs the ability of the animal to use those nutrients.

Managing Rust in Cool-Season Pastures

What can be done to control rust in cool season pastures? Elimination of the disease is not a realistic goal, but managing its impact is very doable. Managing rust and other diseases of pastures simply requires understanding the biology of the pathogen and using the concepts of integrated pest management to reduce disease severity on pasture-grass growth and intake.

Integrated pest management involves three strategies: monitoring, prevention, and control. Monitoring to detect the disease at the earliest possible stage allows for the largest range of options for managing the disease cycle and reducing damage. This starts long before you see evidence of the disease and requires tracking weather and environmental conditions that are conducive to an outbreak before it happens. Prevention involves cultural control or managing the environment and conditions of the growing plant to reduce the potential for disease infection. Control measures include chemical control with fungicides (though rarely recommended) or mechanical control such as mowing or grazing.

Monitoring

Conditions for rust development include temperature, humidity and wind. Daytime temperatures of 68 to 77 degrees F and nighttime temperatures of 59 to 68 degrees F are ideal. Moisture is required for rust to infect the plant and any conditions that favor formation of dew on grass leaves will enhance rust infection. Light daytime winds help move spores around and spread the infection.

Close-up of a rugged tire with deep treads and orange dust in the wheel well. — **Figure 2.** *Orange dust clinging to equipment tires or shoes after passing through a pasture is often an early sign of rust infection.*

Prevention and Control

Chemical Control

Although farmers who grow annual crops may reach for a fungicide as a first line of defense against rust, this is not the best option for pasture rust control. Fungicides can be effective in monoculture crop fields but haven’t worked well in pasture settings. Most fungicides function mainly as a barrier to infection, meaning that they need to be applied prior to infection and may need to be reapplied after rainfall. In other words, if you’re seeing the orange pustules on the plants, it is likely too late to treat. So, rust in pastures can be controlled by fungicides, but treatment is often ineffective and seldom pays off economically in a pasture setting.

Cultural Control

There are a few reliable management strategies that should be used on every operation to minimize the occurrence of rust and other grass diseases. They include:

Diverse pasture mixes. Because rust susceptibility varies among species, a diverse pasture with multiple grass and legume species present will reduce the ability of the rust organism to multiply and spread. Just like a monoculture of corn or soybeans, a single species pasture planting creates conditions that magnify pest and disease problems. Because the cool season pasture workhorse species (orchardgrass, ryegrass, and meadow fescue) are all susceptible to rust, it can help to include at least three or more grass species in your pasture, including a couple that are less susceptible like smooth brome, timothy, or reed canarygrass (Note: reed canarygrass may be considered “invasive” and prohibited for planting in some states).
Plant rust resistant varieties. Susceptibility to rust infection varies among varieties within a species, and breeding for resistance is one of the primary tools in combating the disease. This is an area where improvements have been made in the last quarter century. Most seed companies are breeding for disease resistance in the major pasture grass species: orchardgrass and ryegrass in particular, but others as well.
When selecting pasture seed, review yield and quality data that compare resistant and non-resistant varieties. Some resistant varieties may yield less than susceptible ones and some may have lower quality. Often resistance is imparted through enhanced physical barriers to disease entry. This may mean more lignin and other undigestible fiber in the leaf tissue.
Maintain appropriate soil fertility levels. Adequate availability of plant nutrients will result in a healthier stand and increased ability of the plants to resist rust infection. Some research suggests that increased nitrogen availability, in particular, can reduce rust problems. Ideal growth temperatures for cool season grasses are 60 to 75 degrees in daytime and mid-50s at night. As temperatures increase, they exceed optimum for cool season grasses and reach optimum for rust. Grass growth slows and rust can gain a foothold in the stand. However, this summer slump can be mitigated when nitrogen and moisture are adequate. There is some research that suggests that added nitrogen at this time may increase growth and lessen the effects of the rust, but the data is sparse. For fields with high levels of rust infection, an application of 30 to 50 lb actual N per acre might help the plants resist rust, if rainfall and soil moisture are adequate to allow nitrogen uptake and increase grass growth during the summer slump.

Field with rows of cut grass under partly cloudy sky with sun rays and distant tree line. — **Figure 3.** *Cutting pastures to harvest as hay may be the single most effective method of handling rust.*

Physical Control

A timely mechanical harvest may be the single most effective rust control measure. A midsummer hay harvest removes the plant material and rust spores from the field and increases air flow and drying. Grazing the stand will be less effective as the herd will remove less infected plant material and potentially move inoculum to other pastures as they graze. Harvested hay can be fed safely as there are no mycotoxins associated with rust. However, if the rust infection is excessive, supplementation may be needed to balance losses in palatability, quality, and digestibility. Timing hay harvest with the first signs of rust will minimize these challenges.
Manage grazing for greater air flow and lower canopy humidity. For paddocks susceptible to rust, keeping the sward shorter to allow greater air movement and drier condition seems to reduce the severity of infection. Bring the animals into the paddock at a relatively low height (8 to 10 inches), but make sure you leave adequate residual (4 to 6 inches) for rapid regrowth.

Grassy field with grazing cows under partly cloudy sky, with trees along the horizon and one central tree. — **Figure 4.** *Rust-infected grasses can be grazed before they have reached the recommended height or rest period, but care must be taken to maintain adequate residual.*

There are times when grazing pastures with high rust severity is the best or maybe the only option. When this is the case, graze them early, when the rust infection is minimal. Give the herd small paddocks and move them frequently to minimize refusal. Consider supplemental feeding to compensate for the reduced forage quality.
Graze shorter in fall. Rust pathogens can overwinter on grass leaves, so grazing susceptible pastures shorter going into the winter may help reduce inoculum. This advice is counter to what is recommended for overall winter survival but may be appropriate for specific paddocks that routinely have rust infestations. If winter damage does occur, this may be an opportunity to interseed rust resistant species or varieties.
Remove alternate hosts. Many pathogens and insect pests have alternate hosts: another plant species that the disease lives on for part of the year or part of its life cycle. Both hosts are needed for the disease to spread from year to year. Crown rust (one of the types of rust that can be found on pasture grasses) is no exception. The most prominent alternate host for crown rust is a familiar one – buckthorn. Buckthorn is an invasive shrub that outcompetes native species in woodlands and provides winter habitat for soybean aphids. In addition, it serves as a crucial host for early summer spore production of crown rust. While spores can spread long distances from various sources, without buckthorn, it is very difficult for rust to spread into pastures. So, there’s more than one reason to control buckthorn in woodlots and fencerows!

Close-up of green leaves with yellow-orange spots, indicating rust infection. — **Figure 5.** *Managing invasive buckthorn in nearby fencerows and woodlots can help to delay the spread of rust and minimize the extent of infection in pastures. Photo by SDSU Extension.*

Summary

Rusts are a fact of life in many temperate, cool season pastures, and there is no silver bullet for controlling them. It does have an impact on palatability, quality, and yield of pasture grasses, but impacts can be mitigated through a combination of cultural and mechanical practices. Keeping diverse pastures with rust resistant varieties, maintaining soil fertility levels, controlling alternate hosts, and timely mechanical harvesting are all tools graziers can use to effectively manage rust.

Updated: Sept. 5, 2025