Bob Mugaas, UMN Extension Educator
Rust diseases of our lawn grasses have been on the increase throughout much of this fall period but became especially evident during the drier conditions of October. Rust infestations usually show up as areas of yellow to orange-yellow grass blades, see Picture 1.
Upon closer examination of the grass blade, one will usually see orange colored, tiny tuft-like pustules breaking through the grass leaf surface, see Picture 2. It is these pustules that produce massive numbers of individual spores. These are the same spores that can become air-borne and cover our shoes or lawn mowers in an orange 'powder' as we walk through rust infected areas of the lawn. They can also re-infect other grass plants that in turn can produce more of the same spore producing rust pustules thus carrying on the infection cycle.
What is a rust disease?
Rust diseases have very complex life cycles that include as many as five different stages during a single year. In addition, it is often necessary for various species of rust to spend a portion of their life cycle on one plant species and the other portion on an entirely different plant, often referred to as an alternate host. Such is the case with the specific rust disease known as crown rust (Puccinia coronata) of grass. This disease completes part of its lifecycle on its alternate host, common buckthorn (Rhamnus cathartica) or glossy buckthorn (Frangula alnus, formerly Rhamnus frangula), and the second portion of its lifecycle on some of our lawn grasses, especially perennial ryegrass and tall fescue. Other rust species including Puccinia graminis (Stem Rust) and Puccinia striiformis (Stripe Rust) can also affect Kentucky bluegrass, along with many other grass species.
Rust on turfgrass can overwinter in plant debris but it will need to infect its alternate host before returning to the grass plant. It should also be noted that the disease causing spores can develop in warmer climates to our south and be blown up here during the summer and serve as a source of infection from summer into the middle of fall. This can be an important source of rust infection in this area. In a typical year and under favorable conditions, crown rust will usually start to show up toward the end of June while stem rust can be a bit later. Once begun, the rust infection cycle can continue throughout much of the growing season so long as favorable rust infection conditions persist.
Why rust and why now?
Slow growing lawn grasses are a prime target for rust disease attack. It is usually the combination of warm daytime temperatures, dry weather and heavy amounts of overnight dew production on the grass foliage that creates a favorable environment for rust spores to germinate and infect the foliage. When these common weather conditions are combined with low levels of available nitrogen, an element responsible for active, vigorous growth of our grasses, you have very favorable conditions for a rust outbreak. Shadier areas often experience greater incidence of rust. Note the lighter yellow to orange areas scattered around the lawn underneath the spruce trees in Picture 3.
Rust disease started showing up more frequently around the Twin Cities during late August to early September. However, it wasn't until the very dry conditions lasting nearly the entire month of October that significantly increased the occurrence of rust in our lawns and other turfgrass areas. Frequent enough rainfall combined with an occasional supplemental watering kept our lawn grasses actively growing and utilizing available nitrogen throughout much of the summer period.
Nitrogen can also be lost when it is carried with water down through the soil and beyond the reach of grass roots, a process known as leaching. Thus, due to relatively continuous grass growth during the summer and nitrogen losses due to leaching, it is quite likely that the amount of available soil nitrogen was in short supply by late summer, a time of year when our lawn grasses naturally resume active shoot and root growth. That condition along with the prevailing weather conditions during late September and much of October has contributed to a much higher than usual amount of rust disease on our lawns and other turfgrass areas.
So, what should I do now?
With the rains of the last few days of October, we have improved our previously dry soil conditions. That will be a big help in improving the growing conditions for lawn grasses. While it's late to be putting down nitrogen for this year, it would be a good idea to plan on applying some next spring as our lawns are beginning to show active growth. For the most part, we try to manage rust diseases by changes in our cultural practices. There are fungicides that can be applied in severe cases. However, at this late date in the season, both the rust fungi and the turfgrasses are preparing for winter survival and dormancy. Thus, fungicide applications at this time of year will be of no benefit. Use of protective fungicides can be reevaluated next year should serious rust problems begin to develop.
Where disease levels were quite high and there was some thinning of the lawn, one should be prepared to do some reseeding of those areas as needed. Some overseeding could be done yet this fall in a process known as dormant seeding. Normally this would be done once the ground is cold enough to prohibit germination with the seed remaining in the ground until next spring when it will sprout and grow. One could also wait until early next spring to do some seeding.
For some additional information on rust diseases of lawns, check out the following link to our Gardening Information page, What's Wrong with My Plant?
The author would like to gratefully acknowledge the input and review of Dr. Eric Watkins, Assistant Professor-Turfgrass Science, University of Minnesota Department of Horticultural Science and Michelle Grabowski, Extension Educator - Horticulture & Plant Pathology, University of Minnesota Extension, in the preparation of this article.