Outbreak of Smut Caused by Tilletia maclaganii on Cultivated Switchgrass in Iowa

Switchgrass (Panicum virgatum L.), a prairie grass native to Iowa, is cultivated for forage and biomass production. During the late 1990s, biomass and seed yields of switchgrass grown in southern Iowa began to decline, and the reduction has been attributed to unidentified diseases. In 1999, many plants in previously low-yielding fields were stunted and flowered prematurely. Glumes had an uncharacteristic purple pigmentation, and seeds had been replaced by fungal spores. A smut fungus identified as Tilletia maclaganii (Berk.) G.P. Clinton (1) was associated consistently with fields that yielded poorly. Teliospores were red-orange when immature and turned dark brown as they matured. Teliospores were globose to slightly irregular, ≈18 to 25 µm in diameter, finely verrucose, with a thick exospore. True sterile cells also were present. T. maclaganii infects switchgrass and has been reported previously in Iowa (2), although it is found only occasionally on the state's native switchgrass. The prevalence and incidence of disease was surveyed in late August 1999. A weighted random sampling procedure was used to select switchgrass production fields from 60 fields involved in the Chariton Valley Biomass Project. Fields were located in Appanoose, Lucas, Monroe, and Wayne counties in southern Iowa. The sampling procedure was designed so the probability of each field being chosen was proportional to its area. This resulted in samples being taken from 17 fields representing ≈50% of the total area of the 60 fields. All sampled fields were planted with the predominant cultivar, Cave-in-Rock. In each field, five 1-m² samples (≈60 to 250 tillers) were taken from arbitrary points. The incidence of smut (percentage of tillers with smut) was calculated for each sample. Smut was found in 15 of 17 fields. We estimated that 50 to 82% of the area in switchgrass production in these counties was infested with T. maclaganii. The mean incidence of smut was estimated at 10.1% of all tillers in the area. Incidence in individual fields ranged from 0 to 70%. Fields with incidence >50% yielded less than half of the expected biomass. Some infested seed-production fields were a total loss in 1999. This disease presents a serious threat to the cultivation of switchgrass for biomass production in southern Iowa. The disease cycle for T. maclaganii is poorly documented, but because switchgrass is a perennial species, it is likely that affected fields will have recurring epidemics. Susceptibility of other cultivars is unknown but needs to be investigated. References: (1) G. W. Fischer. 1953. Manual of the North American Smut Fungi. Ronald Press, NY. (2) J. C. Gilman and W. A. Archer. The fungi of Iowa parasitic on plants. Iowa State College J. Sci. 3:299, 1929.