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© 2005 Plant Management Network.
Accepted for publication 9 March 2005. Published 28 March 2005.

Tolerance of Cool-Season Turfgrasses to Rapid Blight Disease

Paul D. Peterson, S. Bruce Martin, and James J. Camberato, Entomology, Soils, and Plant Sciences Department, Clemson University, Pee Dee Research and Education Center, Florence, SC 29506

Corresponding author: Paul D. Peterson. ppeters@clemson.edu

Peterson, P. D., Martin, S. B., and Camberato, J. J. 2005. Tolerance of cool-season turfgrasses to rapid blight disease. Online. Applied Turfgrass Science doi:10.1094/ATS-2005-0328-01-RS.

Abstract

Forty-nine different cool-season grass cultivars representing 24 species were tested for their tolerance to the causal agent of rapid blight, Labyrinthula spp. Seeds of each grass were sown in 7.6-cm pots and grown in the greenhouse. Plants were watered daily with deionized water for three weeks, then with artificial sea water at a concentration of 3.5 deciSiemens per meter. Experimental design was a randomized split-plot with four replications where treatment (inoculated or non-inoculated) was the main-plot factor and cultivars were the sub-plot factor. The entire experiment was replicated twice. Bulk inoculum of five Labyrinthula isolates was prepared by growing isolates on modified serum seawater agar for four days, adding colonized agar plugs to serum broth and vortexing. Cell counts of the bulk inoculum at 1.4 × 10⁵ per ml were used for each inoculation. Plants in each pot were inoculated with 1 ml of inoculum placed on leaf tissue in the form of droplets from a syringe after wounding by trimming with scissors. Plants were rated for percent of diseased foliage. A high degree of susceptibility was observed in cultivars of rough bluegrass (Poa trivialis L.), colonial (Agrostis capillaris L.) and velvet bentgrasses (Agrostis canina L.), and perennial ryegrass (Lolium perenne L.) while certain cultivars of alkaligrass (Puccinellia distans (Jacq.) Parl.), slender creeping red fescue (Festuca rubra spp. littoralis (G. F. W. Meyer) Auquier), and creeping bentgrass (Agrostis stolonifera L.) appear to have moderate levels of tolerance.

Rapid Blight Emerges as a Problem on Turfgrass

Fig. 1. States with current confirmed cases of rapid blight.

Since 1995, when it was first diagnosed in California on annual bluegrass (Poa annua L.) putting greens (14), rapid blight disease has been a frequent problem for the golf course industry. More than 100 golf courses in 11 states over the southern United States (Fig. 1) have confronted extensive and costly damage to annual and rough bluegrass (Poa trivialis L.), perennial ryegrass (Lolium perenne L.), and sometimes to creeping bentgrass (Agrostis stolonifera L.) from this disease (L. J. Stowell, S. B. Martin, and P. D. Peterson, unpublished).

Recently Olsen et al. (17) showed rapid blight to be caused by a species of Labyrinthula on the basis of morphological characteristics (11). Utilizing ssRNA gene sequences, Craven et al. (3) confirmed the genetic identity and phylogenetic relationships among 11 rapid blight pathogens isolated from four species of diseased turfgrass collected across the U.S., verifying the previous morphological characterization.

Labyrinthula spp. produce spindle-shaped vegetative cells that move within extensive branching and anastomosing ectoplasmic networks (18) (Fig. 2). These slime nets are why this group of organisms is referred to as marine slime molds or net slime molds. Their ability to survive in plants that live in saline environments is one of their defining characteristics. Traditionally, Labyrinthula spp. are known mostly to inhabit marine environments and some species have been shown to cause diseases on aquatic grasses. Labyrinthula zosterae D. Porter & Muehlst. is a pathogen associated with eelgrass (Zostera marina L.), an important marine plant that serves as a nursery bed for larval shrimp, oysters, and scallops. Catastrophic declines in eelgrass in North America and Europe during the 1930s and 1940s were probably caused by L. zosterae (15,16). Prior to the discovery of rapid blight disease, Labyrinthula spp. were not known to parasitize terrestrial grasses.

Fig. 2. The rapid blight organism growing in culture. Vegetative cells of Labyrinthula sp. at 100× magnification.

Rapid blight symptoms on golf course turfgrass appear as irregular shaped chlorotic or necrotic reddish-brown patches ranging from 0.15 to 1.8 m in diameter (Figs. 3, 4). In overseeded grasses on close-cut greens, patches may have a dark border of water-soaked turf (Fig. 4). Upon close examination, the chlorotic leaves appear mottled and water-soaked. Microscopic examination reveals hyaline spindle-shaped vegetative cells in leaf epidermal and mesophyll cells. Choroplasts degrade in infected leaf tissues as disease progresses.

Fig. 3. Symptoms of annual bluegrass putting greens infected with rapid blight disease.		Fig. 4. Rough bluegrass overseeding infected with rapid blight disease.

Rapid blight can affect juvenile as well as mature turf. In the southern United States, winter-dormant bermudagrass is overseeded often with a temporary stand of cool-season grasses. Overseeded grasses in the seedling stage at first mowing are particularly vulnerable to infection. Extensive damage can result and stands may be eliminated within a week if disease occurs at establishment. However, rapid blight also can be severe on putting greens of mature annual bluegrass and creeping bentgrass and established stands of overseeded perennial ryegrass.

Rapid blight outbreaks appear to be associated with dry periods in the fall and spring. Dry weather may increase levels of soluble salts in the soil and plants, consequently making the host environment more conducive to the pathogen and increasing the susceptibility of cool-season turfgrasses to infection and symptom expression. The major source of the soluble salts appears to be irrigation water, which typically has salinity levels ranging from 1.0 to 3.5 deciSiemens per meter (dS/m).

The use of turfgrasses with tolerance to rapid blight would provide a useful tool for management of the disease. The objective of this study was to evaluate 49 different cool-season turfgrass cultivars among 24 species for their relative tolerance to rapid blight disease.

Screening for Tolerance

Forty-nine different grass cultivars from 24 species were selected for this study (Table 1). Seeds of each cultivar were planted on sterilized sand/peat (85:15) in 7.6-cm² plastic pots. Seeding rates were 85.5 g/m² for the weeping alkaligrasses (Puccinellia distans (Jacq.) Parl.), bluegrasses, and tufted hairgrass (Deschampsia caespitosa (L.) Beauv.); 38.8 g/m² for the bentgrasses; 118.6 g/m² for the crested dogstail (Cynosurus cristatus L.); and 207.5 g/m² for all other grass types. The soil mixture was amended with ground triple super phosphate at 0.1 g/kg soil and dolomitic limestone at 0.25 g/kg soil. Soluble fertilizer (24-12-12) was applied in two separate applications, 7 and 14 days after establishment at the rate of 1.5 mg/kg soil. Pots were placed in trays on the greenhouse bench at 24 to 27°C and the plants were watered daily with deionized water for approximately three weeks. Three days before inoculation, the watering regime was changed to artificial seawater at a concentration of 3.5 dS/m (NaCl 1.765 g/liter; MgSO₄ 0.21 g/liter; MgCl₂ 0.164 g/liter; CaCl₂ 0.071 g/liter; NaHCO₃ 0.015 g/liter; KCl 0.014 g/liter) (4) and pots were irrigated daily until saturation.

Table 1. Grass type, genus, species, cultivar, and source of seed evaluated for tolerance to rapid blight disease.

 † SRO = Seed Research of Oregon, PS = Pennington Seed, TS = Turf-Seed Inc., JS = Jacklin Seeds, TM = Turf Merchants Inc., PW =Pickseed West Inc., VSS = Valley Seed Service, JRS = J.R. Simplot Company, DLF = DLF International Seeds, CU = Clemson University, BA = Barenbrug, BM = Blue Moon Farms, AVS = Arkansas Valley Seed Solutions

 ‡ LSD(0.05) for comparing disease means is 14.

Bulk inoculum of five Labyrinthula isolates was prepared by growing isolates on petri plates containing modified serum seawater agar (SSA) (18) for approximately four days. Equal numbers of agar plugs with each isolate were transferred into modified serum seawater broth (SSB) and mixed for two minutes. Inoculum was quantified to 140,000 cells per ml using a hemacytometer and Tween (0.25% v/v) was added to the solution.

All plants were wounded prior to inoculation by trimming with scissors. Inoculum was applied arbitrarily to leaf tips at the rate of 1 ml per pot using a 3-cc needle syringe which resulted in the formation of droplets on leaf tissue. A solution containing SSB and Tween (0.25% v/v) was applied at the same rate to each cultivar to serve as an non-inoculated control. Trays with all plants (inoculated and non-inoculated kept separate) were placed in a sealed container for 48 hours post-inoculation, after which time the lids were removed and the plants continued to be watered daily with saline water as described.

Plants were checked daily for disease symptoms. Plants were rated for the percentage of the total leaf area diseased when the level of disease on the known susceptible intermediate ryegrass check cv. Transeze reached > 50%. Final disease values were attained at 21 days post-inoculation.

The experimental design was a randomized split-plot with four replications where treatment (inoculated or non-inoculated) was the main-plot factor and cultivars were the sub-plot factor. The entire test was repeated an additional time resulting in eight replications in total. Run effects were not significant; therefore, data are presented averaged over runs. Data were transformed using a square root transformation and analyzed in SAS (SAS Institute, Cary, NC) using the PROC GLM procedure.

Tolerance Levels Among Different Species and Cultivars

Symptoms of rapid blight were first noted approximately 8 days after inoculation in ‘Transeze’ ryegrass and the time to 50% foliar disease was approximately 14 days after inoculation. ‘Transeze’ ryegrass reached 90% foliar disease 21 days post-inoculation. No symptoms were observed on the non-inoculated plants; therefore, the main plot factor of treatment (inoculated or non-inoculated) was highly significant (Table 2). The sub-plot factor of cultivar was also highly significant, with different cultivars exhibiting a wide range in their levels of susceptibility to rapid blight (Table 2).

Table 2. Analysis of variance of rapid blight disease response on 49 cool-season turfgrass cultivars evaluated in a greenhouse study.

Mean percent disease ranged from 0.6% in the slender creeping red fescue (Festuca rubra spp. littoralis (G. F. W. Meyer) Auquier) cultivar Dawson to 95% in the crested wheatgrass (Agropyron cristatum (L.) Gaertn.) cultivar Ephriam (Table 1). Grasses were designated as highly tolerant to rapid blight if, on a per pot basis, less than 10% of the grass was diseased, tolerant if 11 to 30% of the grass was diseased, moderately tolerant if 31 to 65% of the grass was diseased, and susceptible if greater than 65% of the grass was diseased. The most tolerant grasses to rapid blight were the fescues (0.6 to 28.1% disease), creeping bentgrasses (5.0 to 21.9%), and alkaligrasses (12.5 to 30.6%). With the exception of the creeping bentgrasses, the other bentgrass species [colonial (Agrostis capillaris L.), Idaho (Agrostis idahoensis Nash), redtop (Agrostis gigantea Roth), and velvet (Agrostis canina L.)] were all highly susceptible to rapid blight (46.2 to 90.6%). The bluegrass species and cultivars varied widely in their tolerance. Canada (Poa compressa L.), Kentucky (Poa pratensis L.), and annual bluegrasses (Fig. 5) were moderately tolerant and tolerant to rapid blight (26.9 to 56.2) with the exception of the Kentucky bluegrass cultivar North Star, which was highly tolerant (7.5%). The rough bluegrasses were susceptible (73.7 to 80.0%). Annual ryegrass (Lolium multiflorum Lam.) (39.4%) and some cultivars of perennial ryegrass were moderately tolerant (‘Hawkeye’ and ‘Peregrine’), but other perennial ryegrass cultivars (45.0 to 78.1%) and the intermediate ryegrass, ‘Transeze’ (90.0%), were highly susceptible. Of the less commonly utilized grasses, tufted hairgrass (37.5%) and slender wheatgrass (Elymus trachycaulus (Link) Gould ex Shinners) (39.4%) were moderately susceptible to rapid blight. Crested dogstail (93.1%) and crested wheatgrass (95.0%) were highly susceptible.

Fig. 5. The devastating effects of rapid blight on annual bluegrass (noninoculated, left; inoculated right), 16 days post-inoculation.

Since epidemics of rapid blight have been associated with high salinity irrigation water and soils, we hypothesized that grasses with high salt tolerance would also be highly tolerant to the disease. For many grass types this perception was validated in our experiment. Alkaligrass (1,6,7,9,12), creeping bentgrass (12,13), and the fescues (10,12) are considered moderately tolerant to tolerant of salinity and were highly tolerant to tolerant of rapid blight in our experiments. Several of the grasses considered very sensitive to salinity, such as colonial (5,12,13) and velvet (13) bentgrasses as well as the annual and rough bluegrasses (2), were also highly susceptible to rapid blight.

However, there were some notable discrepancies in the correlation of salt tolerance and rapid blight susceptibility. Several perennial ryegrasses showed high susceptibility to rapid blight although as a group they are rated as moderately tolerant to tolerant of salinity (2). ‘Brightstar SLT’ in particular was selected for enhanced salinity tolerance (19), but was one of the most susceptible ryegrasses to rapid blight.

In our study the most variation in rapid blight tolerance within a species was exhibited by the Kentucky bluegrasses, ranging from 7.5% to 40% disease (Table 1). Substantial variation in salinity tolerance has also been shown to exist among cultivars of Kentucky bluegrass (5,8,20,21). Unfortunately none of the cultivars we examined for rapid blight tolerance were evaluated for salinity tolerance in the earlier studies.

There was little variation in rapid blight tolerance within the 8 cultivars of creeping bentgrass we evaluated even though considerable variation in salinity tolerance exists in creeping bentgrass (13,22). Among the 5 cultivars common to our study and that of Marcum (13), there were only weak correlations between our assessment of rapid blight tolerance and his assessment of salinity tolerance by determination of green leaf area, relative leaf dry weight, root length, and root dry weight (r = -0.22, -0.38, 0.06, and 0.22, respectively) . The most rapid blight tolerant cultivar, Providence, had low salt tolerance and one of the most salt tolerant cultivars, Seaside II, had intermediate rapid blight tolerance.

The most tolerant grass tested in our experiments was ‘Dawson’ slender creeping red fescue (Fig. 6). This finding confirmed a field experiment conducted in 2001 on a South Carolina golf course with severe rapid blight, where ‘Dawson’ was the only grass unaffected by the disease. Three cultivars of rough bluegrass, a colonial bentgrass, a velvet bentgrass, and a chewing's fescue (Festuca rubra L. spp. commutata Gaudin) were killed by the disease (S. B. Martin, unpublished). ‘Dawson’ has excellent salt tolerance (10), even rated more tolerant than ‘Fults’ alkaligrass in one study (21).

Fig. 6. ‘Dawson’ slender creeping red fescue, the most tolerant grass to rapid blight tested in this greenhouse experiment (noninoculated, left; inoculated, right), 16 days post-inoculation.

Our experiments have shown several cool-season turfgrasses to be tolerant of rapid blight under conditions of moderately high salinity stress. Some of these grasses may be suitable for overseeding where rapid blight is a chronic problem. In South Carolina, some golf courses are using seed blends of rough bluegrass and alkaligrass with acceptable results. Potential exists for the use of certain creeping bentgrass cultivars or slender creeping red fescues for overseeding as well, although rates of germination and establishment may be complicating factors to consider. In short, blending fast-establishing susceptible grasses (rough bluegrasses or moderately susceptible grasses like the perennial ryegrasses) with tolerant grasses (alkaligrasses, creeping bentgrasses, and slender creeping red fescues) is a promising strategy to reduce the risk of devastating epidemics of rapid blight. Field experiments and further screening of species and cultivars for tolerance to rapid blight and for overseeding suitability will be important now and in the future as the quality of the irrigation water supply degrades.

Acknowledgments

Technical Contribution No. 5067 of the Clemson University Experiment Station. This material is based upon work supported by the CSREES/USDA, under project number SC-1700256.

The authors would like to thank Debbie Cottingham, Sheila Godwin, Cristal Robbins, and Amy Turner for technical assistance. We also thank Dr. Dawn Fraser for her assistance with statistical methods. Finally, we would like to acknowledge the United States Golf Association for partial financial support of this project.

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