One type of smut or another can bedevil turfgrass throughout the growing season in many northern areas of the country. Smuts can shoot through many grass types, weaken the plants and allow other diseases or pests to kill the host. The result can be severely damaged areas of grass that look bad and are difficult to replace because of the persistence of the disease.

The good news is that it rarely comes to that. Stripe smut, caused by the fungus Ustilago striiformis, and flag smut, caused by Urocystis agropyri, are the two primary culprits, according to Henry “Hank” Wilkinson, a professor emeritus in crop sciences at the University of Illinois at Champaign-Urbana.

“There are also head smuts, which affect the head florescence of the grasses,” Wilkinson says, but they are usually manifestations of the same species that have spread to seed heads. Flag and stripe smuts are similar in action but attack different parts of the grass plant, though in general they affect leaves and sheaths. It is extremely difficult to sort out which fungus species is responsible for damage, lab analysis by a plant pathologist familiar with these fungi is required, but it’s usually enough to simply recognize that it is a smut and understand how such a fungus works.

“They often cause a lesion to develop on the plant tissue, and it will be yellow,” he says. Yellowish streaks may appear between the veins of the leaves, and a lawn may appear clumpy or patchy. In the worst cases, this can lead to patchy death and browning of a lawn. Drought conditions and heat stress can accelerate the decline. As the grass plant dies from one cause or another, smut spores form in the dead and dying tissues. These are the survival form of the fungus, and they are distinctively black. These unsightly spores are spread by wind and water and may lie dormant in the winter for up to three years.

Wilkinson notes that leaf smuts are most prevalent in bluegrasses, but can also occur in some cool-season grass species, including fescues, bentgrasses and perennial ryegrasses. One or more of the smut species can also occur in weed grasses, such as timothy and wheatgrass, and can thrive in those longer than in turfgrasses that are mowed and maintained regularly. Those can include certain native grasses. Thus, once leaf smuts gain inroads in an area they may persist and move among several species in and around the lawn.

“You can have resistant cultivars within a species,” Wilkinson says, and that is the first step in avoiding smuts. By selecting grass species or cultivars resistant to smuts when planting, or by mixing species and cultivars, smut can be diminished to the point where it is not harmful to the lawn or facility. In addition, overseeding with resistant cultivars can help renovate an infested lawn. There are a number of resistant cultivars, and some of these can be found by looking at a University of Illinois report at http://ipm.illinois.edu/diseases/series400/rpd409/index.html.

Leaf smuts can almost always be managed through cultural controls, Wilkinson says. The first aspect of management that he recommends examining is the use of fertilizer in the summer months. By minimizing fertility to what is absolutely required, particularly nitrogen, any smuts that crop up can usually be kept under control. Over-fertilizing in northern climates in general is an invitation to leaf smuts.

Another important element in managing smuts is irrigation. It is recommended that in the summer managers avoid frequent, short irrigations that keep leaves wet during much of the day.

As with all grass diseases, an important element in keeping smuts to a minimum is growing healthy, disease-resistant turf. That would include keeping thatch to a minimum; smut fungi don’t live in the thatch, but thick thatch is a sign that the grass may be growing too fast due to over-fertilization. Wilkinson notes that removal of smut-infested grass cuttings is not necessary, since once the fungi are there, removing cuttings won’t control them.

Leaf smut is caused by a fungus and is most prevalent in bluegrasses, but can also occur in some cool-season grass varieties.
Photos courtesy of Roch Gaussoin, UNL.

He also says that the black spores may be unsightly, but they are not harmful to humans. If you want to reduce the amount of spores, apply a light irrigation, which should wash some of them off the leaves.

Several fungicides are available to help control leaf smuts. Gabe Towers, plant pathologist with Target Specialty Products in Tempe, Ariz., says he doesn’t see leaf smuts in an amount requiring treatment there, however, he is from New Jersey originally and is familiar with the chemicals used at facilities where high-quality turfgrass must be maintained and there is low tolerance to smutty grass.

Fenarimol (Rubigan), myclobutanil (Eagle) and propiconizole (Banner Maxx) are available in liquid formulations. Triadimefon (Bayleton) is available in both liquid and water-soluble packet, Towers says. Other brand names may be available in some or all of those chemistries. He notes the disease is treatable through cultural controls, so one chemical application should be sufficient to manage the fungi if other measures, such as fertilizer management and proper irrigation scheduling, are taken at the same time.

Towers also points out one other important item of trivia regarding smuts. The ’70s rock band Jethro Tull is named after a real-life 18th century British agronomist. Apparently, Tull, in addition to being the inventor of the seed drill, was the first person to examine and describe a smut pathogen.

Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.