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Severe Dollar Spot Fungus on Bahiagrass in Florida Ann R. Blount, Hank Dankers, and M. Timur Momol, North Florida Research and Education Center, University of Florida, 155 Experiment Road, Quincy 32351; and Thomas A. Kucharek, Department of Plant Pathology, University of Florida, P.O. Box 110680, Gainesville, 32311 Corresponding author: Ann R. Blount. ablount@mail.ifas.ufl.edu Blount, A. R., Dankers, H., Momol, M. T., and Kucharek, T. A. 2002. Severe dollar spot fungus on bahiagrass in Florida. Online. Crop Management doi:10.1094/CM-2002-0927-01-RS. Abstract Cultivar susceptibility to dollar spot fungus (Sclerotinia homoeocarpa F.T. Benn) and environmental stresses during 2001 and early 2002 may have contributed to the general susceptibility of several cultivars of bahiagrass to an outbreak of dollar spot in the Florida Panhandle. Several cultivars of bahiagrass, ‘Pensacola’ and ‘Tifton 9’ (Paspalum notatum Flugge var. saure Parodi, diploid, 2n = 2x), and ‘Argentine’ (P. notatum Flugge, tetraploid, 2n = 4x) make up the majority of the bahiagrass acreage planted in Florida. Severe cases of a disease in Tifton 9 bahiagrass pastures in Jackson and Walton counties, located in northern Florida, resulted in the decline of those pastures. The remaining plants were estimated at less than 5% survival from the original plant stand. Several Pensacola bahiagrass pastures across northern Florida were also heavily damaged by dollar spot, and although stand reduction did not occur, estimates of foliage damage ranged from 15 to 75% leaf tissue death in several counties. In a replicated cultivar trial during 2001 and 2002 at the North Florida Research and Education Center (NFREC), disease severity, under natural field infection, showed Argentine bahiagrass was considerably less susceptible (P < 0.05) to dollar spot in 2001, averaging 15% leaf tissue being damaged compared to Tifton 9 or Pensacola bahiagrass, which had 45% leaf tissue damage (similar results were obtained in 2002). Severe drought conditions followed by heavy rains and warm, humid conditions were most likely contributing factors in the outbreaks that occurred in Florida in 2001. Introduction Bahiagrass is the predominant pasture grass species in Florida (4). Originally from South America, bahiagrass has naturalized well to the similar climate and growing conditions found in the southern Coastal Plain of the U.S. Cultivar development in bahiagrass has mostly been limited because of the apomictic nature of the species, which does not lend itself to genetic manipulation by plant breeders. Pensacola bahiagrass occupies more than 2.5 million hectares in the southern Gulf Coast (3). It is valued as a pasture grass because it is well adapted to the soils and climate of the southern Coastal Plain of the U.S. Pensacola, the narrow-leaf type, dominates the bahiagrass acreage in the southeastern U.S. In Florida, an estimated 60% of the bahiagrass acreage is planted in Pensacola bahiagrass. Selection for improved forage production over nine years of selection for forage types resulted in the release of the cultivar, Tifton 9 (2). Although still growing in popularity because of its forage yield, Tifton 9 only occupies about 10% of Florida bahiagrass pastures. Argentine bahiagrass is a plant introduction that is grown throughout Florida due to its robust growth habit (thick stolons, wide leaves) and ease of establishment from rapidly germinating seed. Argentine comprises about 25% of Florida bahiagrass pastures. The remaining 5% of bahiagrass pastures in Florida are planted in ‘Paraguay 22’, a plant introduction similar in appearance and ploidy to Argentine bahiagrass. (Estimates of bahiagrass acreage were obtained from Carrol Chambliss, Forage Specialist, University of Florida, personal communication.) Sclerotinia homoeocarpa F.T. Benn. causes dollar spot, a leaf and crown disease of many turfgrasses. Bahiagrass in pastures may also become diseased (1). Dollar spot typically develops when weather is mild and accompanied by frequent and prolonged rainy periods (1,5). While outbreaks of dollar spot have been found in Florida, dollar spot has not been an extensive problem of pastures of bahiagrass or bermudagrass. In the 1980s, two occurrences of diseased and dying bahiagrass pastures were attributed to dollar spot in Florida (Gordon Prine, Forage Agronomist, University of Florida, personal communication), but documentation of pasture conditions surrounding these outbreaks was not published and the cultivars of the infected bahiagrass were not known. In spring and summer 2001, following an early spring drought, a severe outbreak of dollar spot in bahiagrass pastures occurred in several counties in the Florida Panhandle. Most of the pastures with reported symptoms were planted with the cultivars Tifton 9 and Pensacola, but several Argentine bahiagrass fields were also mildly affected. Pasture symptoms ranged from mild tip necrosis of the leaves, to significant leaf death, and, in several cases, plant death. Severity was attributed, in part, to plant cultivar susceptibility. Ratings were made in the bahiagrass breeding nursery at the North Florida Research and Education Center at Marianna, FL, to access the relative disease reaction of the popular bahiagrass cultivars: Pensacola, Tifton 9, and Argentine. Laboratory Trials Twenty-four samples of symptomatic plants from twelve pastures were collected in Jackson, Washington, Gadsden, Leon, Walton, Calhoun and Gulf Counties, Florida and were examined at the Plant Disease Diagnostic Clinic at the North Florida Research and Education Center, Quincy, FL. Isolation of the fungus, S. homoeocarpa, was performed by plating small sections of leaf tissue that were sterilized with 5% hypochlorite solution for 2 min., and then aseptically transferred onto potato dextrose agar (PDA) for 1 week. Symptomatic plants from the bahiagrass nursery at the NFREC were also collected and examined at the Plant Disease Diagnostic Clinic at the North Florida Research and Education Center, Quincy, FL. Isolation of the fungus was performed as outlined previously. Isolates of dollar spot were obtained from all 24 pasture samples and from the bahiagrass nursery at NFREC. A characteristic mat of fast growing, fluffy, white mycelium with septa was observed on potato dextrose agar. After two weeks, characteristic black, plate-like stroma, with varying shades of olive-gray, yellow and brown, were produced in culture (Fig. 1).
Field Trials Disease attributed to dollar spot was assessed on naturally infected, two-year old plants of Argentine, Pensacola, and Tifton 9 bahiagrass grown at the North Florida Research and Education Center-Marianna on 20 August 2001 and 2 May 2002. Plants were arranged in a randomized complete block design with 100 replications. Each block consisted of one two-year-old plant of each of the three cultivars. Visual ratings of field plots were made on a 0 to 100 scale (where 0 = no leaf damage to 100 = 100% dead leaf tissue). The same isolation procedure described in the laboratory trials above was used to identify the disease that was rated in the space-plant nursery at the NFREC in 2001 and in 2002. Analysis of variance (ANOVA) was calculated using SAS GLM (SAS Institute, Cary, NC) at P < 0.05. Individual plants were rated in summer, 20 August 2001, and in spring the following year on 2 May 2002 (Table 1). In both disease assessments, the cultivar Argentine exhibited considerably less (15% in 2001 and < 5% in 2002) leaf damage than the cultivars Pensacola and Tifton 9 (45% in 2001 and 15% in 2002). There was no visual difference in host reaction between Pensacola and Tifton 9 (P > 0.05). Infected leaves from bahiagrass were generally chlorotic, with lesions on the leaves, and scorched leaf margins (Fig. 2). Table 1. Bahiagrass cultivar disease severity ratings in 2001 and 2002 at NFREC, Marianna, FL.
† Rating based on 0 to 100%, where 0 = no visible leaf area damaged to 100 = whole plant dead.
Environmental stresses during 2001 and in early 2002 were likely to have contributed to the general susceptibility of bahiagrass. The severe cases of dollar spot that occurred in Jackson and Walton counties in 2001 resulted in the decline of the pastures, which were predominately Pensacola or Tifton 9 bahiagrass, with remaining plants estimated at less than 5% survival from the original plant stand. Severe drought conditions, followed by heavy rains and warm, humid conditions, were most likely contributing factors in the severe outbreaks that occurred. Fungicidal applications on bahiagrass for pastures are not legal for hay or grazing purposes. It is recommended to check soil pH and nitrogen fertility of the pastures. This study did not measure the economic loss due to decreased grazable forage or hay production, but this should be factored into the cost of forage production. While no economic assessment of this loss accompanied the current study, plans to estimate a yield loss value based on cultivar susceptibility will be included in further work planned in 2002-2003. Argentine bahiagrass is more resistant to dollar spot than Tifton 9, although its seasonal forage distribution is shorter and seasonal yields are generally lower. Improvement in diploid bahiagrass for resistance to dollar spot may be achieved using recurrent selection as part of the selection criteria for developing new bahiagrass cultivars. Literature Cited 1. Atilano, R., Freeman, T. E., and Simone, G. W. 1986. Dollar spot of turfgrass. Florida Coop. Ext. Ser. Plant. Path. Fact Sheet PP-36. 2. Burton, G. W. 1989. Registration of ‘Tifton 9’ Pensacola Bahiagrass. Crop Sci. 29:1326. 3. Burton, G. W., Gates, R. N., and Gascho, G. J. 1997. Response of Pensacola Bahiagrass to rates and ratios of nitrogen, phosphorus, and potassium fertilizers. Soil and Crop Sci. Soc. of Florida Proc. 56:31-35. 4. Chambliss, C. G. 1996. Bahiagrass. University of Florida, Agri. Exp. Sta., IFAS, Research Report SS-AGR-36. 5. Freeman, T. E., and Simone, G. W. 1988. Turfgrass diseases and their control. Florida Coop. Ext. Ser. Cir. 221-H. |
