Richard Latin, professor of plant pathology at Purdue University, specializes in turf diseases. He says that rusts are a component of turfgrass management, but they do not normally produce a deadly infestation. Although uncorrected rust invasions can kill grass, they normally only affect aesthetics.
“Every late July and August, the issue of rust comes up. It’s one of those classic-type diseases,” says Latin. There are several prominent types of rust, the better-known ones being stem rust, leaf rust and crown rust. They are a category of diseases caused by fungi species in the genera Puccinia and Uromyces, with the former more common in the Midwest.
The fungi have complex life cycles and will overwinter in dormant turf. In the spring and summer, the microorganism’s lesions begin to produce spores that disperse and spread the disease to adjacent grass. Latin points out that it can have a lengthy active cycle in a lawn. “I’ve even seen it in early spring; I’ve seen it in mid-November,” he says, although it takes successive stages of the disease to produce a turfgrass pest. In Indiana, that generally first occurs in July and August, and you might notice yellow or orange spores on grass leaves, which stick to shoes and mowing equipment.
Latin says rust can occur in any species of turfgrass, but is most prevalent in perennial ryegrass or Kentucky bluegrass. Varieties have been bred that have varying degrees of resistance to rust, but under the right conditions, even those varieties can suffer a severe outbreak. Turf managers looking for resistant varieties should check out the National Turfgrass Evaluation Program Web site at www.ntep.org, where rust resistance is evaluated. Different types of Kentucky bluegrass, for example can be selected based on their susceptibility to stem rust.
Latin says that damp, shaded parts of a lawn will be the area where rust starts. That is especially true if that area is in shade for much of the day. Rust generally doesn’t like high daytime temperatures, but even then it can take advantage of stressed or slow-growing turfgrass.
The other factor that promotes rust is slow-growing turf, Latin says, because it can take a couple of weeks for the rust spores to cycle inside the plant, and grass that is healthy and growing quickly will be mowed often.
“If the plant is growing adequately, we’re going to be mowing every five to seven days,” he says, and much of the rust will be clipped from the plant and removed from its source of nutrients. It requires a living plant in order to multiply, so regularly chopping off the top leaves will often be enough to curb the disease, and clippings don’t have to be removed from the lawn.
Chemical treatments are available for rust control, but Latin says that these are generally not needed if you can prevent moisture from lingering on the lawn and the grass is kept actively growing. Proper irrigation is necessary for solid growth, but too much water, especially at the wrong time, can promote fungal growth. Avoid watering too much in those shaded areas, and once spores are visible, avoid watering at night if possible. The longer that moisture lingers on the leaves, the more conducive it is for rust.
Good lawn fertility is also important. Latin says that one of the crucial times for rust development is summer when a lawn begins to run low on nitrogen. Then the grass slows its growth, mowing is less frequent and the fungus can overwhelm the lawn. He says that some small fertilizer applications late in the summer, along with adequate irrigation, can kickstart the grass and prevent, or greatly reduce, the possibility of a severe outbreak. The disease can usually be controlled within the span of three or four weeks of mowing.
He cautions that an outbreak under the right conditions may need chemical treatment. “In these cases, the rust can kill the grass. Those are the cases where you have to be proactive about this,” he says. He notes that newly planted grass can be especially susceptible, which he often sees where new lawns in home developments, parks or municipal plantings are seeded on poor soil (it rarely occurs on sodded lawns). The initial fertilizer runs out in the summer, and those lawns can develop sudden and devastating rust infestations.
In those cases, there are two classes of fungicides available. The DMI class of chemistry includes propiconazole (Banner), triadimefon (Bayleton), metaconazole (Tourney), triticonazole (Trinity), myclobutanil (Eagle) and fenarimol (Rubigan). The Qol class of chemistry includes azoxystrobin (Heritage), trifloxystrobin (Compass), pyraclostrobin (Insignia) and fluoxastrobin (Disarm). He says that the two classes can be used interchangeably.
They generally come in wettable granules or liquids, and he recommends applying some fertilizer and water to the lawn before the chemical is applied, which allows the grass to distribute the chemical in the plant tissue and encourages the grass to outgrow the problem once it is treated.
Instead of spraying just the affected areas, Latin recommends treating the entire lawn, or at least the area around the periphery of the infestation if the property is very large. The rust may not be visible in the rest of the lawn, but there will be some level of encroachment since the spores will have been carried all over the area by foot traffic and mowing.
“If you spray everything one time, you won’t have to come back,” he says. However, this may be a year-to-year recurring problem once rust is established, and it will come back in those damp, shady areas where it will overwinter.
Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.