Rapid Blight Disease ofCool-Season GrassesResearch reveals that certain cultivars toleraterapid blight and may be suitable for overseeding.BY PAUL PETERSON, BRUCE MARTIN, AND JIM CAMBERATO

Although rapid blight can result in extensive damage at establishment, it also can be severe on puttinggreens of mature annual bluegrass and creeping bentgrass.

Since 1995, the golf course industryhas confronted a new diseaseenemy. Rapid blight disease has

caused extensive and costly damage toannual and rough bluegrass, perennialryegrass, and occasionally to creepingbentgrass in the southeastern and west-ern United States. First diagnosed nineyears ago in California on annual blue-grass putting greens; the destructivedisease has been identified on morethan 100 golf courses in 11 states.

Recently, rapid blight disease hasbeen shown to be caused by a relativelyobscure microorganism known asLabyrinthula, also referred to as a netslime mold~ Labyrinthula is a single-cellorganism that occurs in net-like aggre-gations when growing in culture. Thisorganism's ability to survive in plantsthat live in saline environments is oneof its defining characteristics. Tradition-ally, Labyrinthula spp. is known mostly toinhabit coastal environments, and somespecies have been shown to causediseases in aquatic grasses. For instance,Labyrinthula zosterae is a pathogenassociated with eelgrass, an importantmarine plant that serves as a nurserybed for larval shrimp, oysters, and scal-lops. Catastrophic declines in eelgrass inNorth America and Europe during the1930s and 1940s were probably causedby L. zosterae.3

,4,6 Even though theoccurrence of rapid blight on variouscool-season turf grasses has been associ-ated with increased soil and/or irriga-tion water salinity (Martin, Peterson,and Camberato, unpublished), the dis-

20 GREEN SECTION RECORD

covery of Labyrinthula affecting grassesgrown on land was unexpected.

When it emerges on golf courses,rapid blight symptoms appear asirregularly shaped patches of yellow(cWorotic) or brown (necrotic) turfranging from 6 inches to 6 feet indiameter. Sometimes these patches havea darkened edge where the turf iswater-soaked. Rapid blight can affectjuvenile as well as mature turf. Over-seeded grasses in the seedling stage atfirst mowing are particularly vulnerableto infection. Extensive damage canresult and stands may be eliminated

within a week if disease occurs atestablishment. However, rapid blightalso can be severe on putting greens ofmature annual bluegrass and creepingbentgrass.

Rapid blight outbreaks appear tobe associated with dry periods in thefall and spring. Dry weather increasessoluble salts in the soil and plants,increasing the susceptibility of cool-season turf grasses to infection andsymptom expression. The major sourceof the soluble salts appears to be irriga-tion water, which typically has salinitylevels ranging from 1.0 to 3.5 dS/m.

Over the past year research wasinitiated to investigate the biology,epidemiology, and management of thisemerging threat to the turf industry.Identifying specific cultivars withtolerance to the rapid blight pathogenwould provide useful, timely informa-tion for turf grass professionals. The ob-jective of this study was to evaluate 49different cool-season turf grass speciesfor their relative tolerance to rapidblight disease.

SCREENING FORSUSCEPTI BIL1TYForty-nine different grass cultivarsrepresenting a range of cool-seasongrass species were selected for this study.Seed of ~ach cultivar was planted onsterilized sand/peat (85:15) in three-inch square plastic pots. The soil mix-ture was amended with ground triplesuper phosphate at 0.1 g/kg soil anddolomitic limestone at 0.25 g/kg soil.Soluble fertilizer (24-12-12) was appliedin two separate applications (just afterestablishment and prior to inoculation)at the rate of 1.5 mg/kg soil. Pots wereplaced in trays on the greenhouse benchand watered daily with deionized waterfor approximately three weeks to estab-lish the grasses. Three days beforeinoculation, the watering regime waschanged to artificial seawater at a con-centration of 3.5 dS/m, and pots wereirrigated each day to field capacity.

Bulk inoculum of five Labyrinthulaisolates was prepared by growing equalnumbers of petri plates of each isolatein serum seawater agar (SSA) forapproximately four days. Equal numbersof agar plugs containing each isolatewere transferred into serum seawaterbroth (SSB) and quantified to 140,000cells per ml.

All plants were wounded prior toinoculation by trimming with scissors.Inoculum was applied to the treatedplants at the rate of 1 ml per pot. Acontrol solution containing SSB andTween (0.25% v/v) was applied atthe same rate as the inoculum to anequivalent set of cultivars (non-inocu-

Table ISpecies, cultivar, and source of 49 different

cool-season turfgrasses selected for this study

Species 'Cultivar Source

Weeping Alkaligrass Fults Seed Research of OregonSalty Pennington Seed

Colonial Bentgrass Alister Turf-Seed, Inc.Glory Turf-Seed, Inc.SR 7100 Seed Research of Oregon

Creeping Bentgrass Brighton Seed Research of OregonPennA-I Turf-Seed, Inc.Penncross Turf-Seed, Inc.Providence Seed Research of OregonSeaside II Turf-Seed, Inc.SR 1119 Seed Research of OregonSandhill Seed Research of OregonSRX IGPO Seed Research of Oregon

Idaho Bentgrass GolfStar Jacklin Seeds

Redtop Barracuda Turf Merchants; Inc.

Velvet Bentgrass Greenwich Pickseed West Inc.SR 7200 Seed Research of OregonVesper Pickseed West Inc.

Annual Bluegrass Annual Bluegrass ValleySeed Service

Canada Bluegrass Reubens J. R.Simplot Company

Kentucky Bluegrass Arcadia Seed Research of OregonNorth Star Turf-Seed, Inc.Kingfisher Seed Research of OregonSR 2284 Seed Research of OregonSR 2100 Seed Research of Qregon

Rough Bluegrass Laser Seed Research of OregonSabre II Seed Research of OregonWinterstar Turf-Seed, Inc.

Crested Oogstail Shade Star Turf-Seed, Inc.

Blue Hard Fescue Bighorn Turf-Seed, Inc.

Hard Fescue SR 3100 Seed Research of Oregon

Chewings Fescue SR 5100 Seed Research of Oregon

Sheep Fescue Quatro OLF International Seeds

Slender Creeping Red Fescue Dawson Seed Research of OregonSeabreeze Turf-Seed, Inc.SRX 55 Seed Research of Oregon

Strong Creeping Red Fescue SR 5210 Seed Research of Oregon

TallFescue Tomcat Clemson University

Tufted Hairgrass Barcampsia Barenbrug

Annual Ryegrass Acella DLF International Seeds

Intermediate Ryegrass Lolium x hybridum Transeze Seed Research of Oregon

Perennial Ryegrass Experimental Blue Moon FarmsBrightstar SLT Turf-Seed, Inc.Hawkeye Seed Research of OregonPeregrine Seed Research of OregonPenguin Seed Research of OregonSR4420 Seed Research of Oregon

Crested Wheatgrass Ephriam Arkansas ValleySeed Solutions

Slender Wheatgrass San Luis Arkansas ValleySeed Solutions

SEPTEMBER-OCTOBER 2004 21

In research trials,Transeze intermediate ryegrass showed a relatively high level of susceptibilityto rapid blight disease (right = inoculated).

Rapid blight symptoms appear as irregularly shaped patches of yellow or brown turf ranging fromsix inches to six feet in diameter. At times these patches have a darkened edge where the turf iswater-soaked.

Mean percent disease ranged from0.63% in the slender creeping red fescue"Dawson" to 95% in the crested wheat-grass "Ephriam." The grass species mosttolerant to rapid blight were the fescues,creeping bentgrasses, and alkaligrasses.With the exception of the creepingbentgrasses, the other bentgrass species(colonial, Idaho, redtop, and velvet)were all highly susceptible to rapidblight.

Similar to bentgrasses, bluegrassesshowed a wide variation in levels oftolerance among species. Canada andKentucky bluegrasses were moderatelytolerant to rapid blight (with the excep-tion of the Kentucky bluegrass "NorthStar," which was highly tolerant),whereas the annual and rough bluegrassspecies were highly susceptible. Annualryegrass and some cultivars of perennialrye grass were moderately susceptible,but other perennial ryegrass cultivars andthe intermediate ryegrass "Transeze"were highly susceptible. Of the lesscommonly utilized grasses, hairgrass andslender wheatgrass were moderatelysusceptible to rapid blight. Crested dog-tail and crested wheatgrass were highlysusceptible.

Since epidemics of rapid blight havebeen associated with high-salinity irri-gation water and soils, it was hypothe-sized that grasses with high salt toleranceare also highly tolerant. Based on theseexperiments, for many grass types thisproved to be true. Alkaligrass, creepingbentgrass (many .cultivars), and the fes-cues are considered moderately tolerantto tolerant of salinity and highly tolerantof rapid blight.1 Several of the grassesconsidered very sensitive to salinity,such as colonial and velvet bentgrass, aswell as annual and rough bluegrass,were also highly susceptible to rapidblight. However, there were somenotable exceptions in the correlation ofsalt tolerance and rapid blight suscepti-bility. Crested wheatgrass, rated tolerantto salinity, was the grass most susceptibleto rapid blight. Several perennial rye-grasses also showed high susceptibilityto rapid blight, although they rate

HOW DIFFERENTSPECIESAND CULTIVARSSTACKED UPAll of the grasses tested were susceptibleto rapid blight, but there was a largevariation in the level of susceptibility.

perennial ryegrass check ("Transeze")reached more than 50% infectionsymptoms.

lated controls). All plants (inoculatedand non-inoculated) were covered witha clear plastic lid for 48 hours afterinoculation, after which time the lidswere removed and the plants continuedto be watered daily with saline water.Plants were checked daily for diseasesymptoms. Plants were rated for percentof diseased foliage when the level ofdisease on the known susceptible

22 G R E ENS E C T ION R E COR 0

Shade StarEphriam

disease of rough bluegrass and perennial ryegrass.Plant Disease 87:1267.6. Porter, D. 1987. Labyrinthulomycetes.p. 110-111. In M. S. Fuller and A. Jaworski (eds.)Zoosporic fungi in teaching and research.Southeastern Publishing Corp., Athens, Ga.

PAUL D. PETERSON, PH.D., postdoctoralfellow}' BRUCE MARTIN, PH.D., professor;and JAMES CAMBERATO, PH.D., professor;Tuifgrass Program} Entomology, Soils} andPlant Sciences Department} ClemsonUniversity, Pee Dee Research and EducationCenter, Florence}S. C.

Greenwich

FultsBarcampsia

San Luis

_ NorthStarII)

.~UQ)a..

V>

o 10 20 30 40 50 60 70 80

Percent Disease• Ryegrass • Other • Fescue • Bluegrass • Bentgrass

2. Martin, S. B., L.J. Stowell, W D. Gelernter,and S. C.Alderman. 2002. Rapid blight: A newdisease of cool-season turf grasses. Phytopathology(Abstr.) 92:S52.

3. Muehlstein, L. K., D. Porter, and F. T. Short.1988. Labyrinthula sp., a marine slime moldproducing the symptoms of wasting disease ineelgrass, Zostera marina. Marine Biol. 99:465-472.

4. Muehlstein, L. K., D. Porter, and F. T. Short.1991. Labyrinthula zosterae sp. Nov., the causativeagent of wasting disease of eelgrass, Zosteramarina. Mycologia 83:180-191.

5. Olsen, M. W, D. M. Bigelow, R. L. Gilbertson,L.J. Stowell, and W D. Gelernter. 2003. Firstreport of a Labyrinthula sp. causing rapid blight

Editor's Note: A more detailed editionof this article and many others in theUSGA's Turfgrass and EnvironmentalResearch Online at:hUt):/ / us,gatero.msu. edu.

LITERATURE CITED1. Carrow, R. N., and R. R. Duncan. 1998. Salt-affected turf grass sites assessment and manage-ment. Ann Arbor Press, Chelsea, Mich.

moderately tolerant to tolerant tosalinity. 1

The most tolerant grass tested inthese experiments was "Dawson" slendercreeping red fescue. This finding con-firmed a field experiment conducted in2001 on a South Carolina golf coursewith severe rapid blight, where "Dawson"was the only grass not affected by thedisease. Three cultivars of rough blue-grass, a colonial bentgrass, a velvet bent-grass, and a chewings fescue were killedby the disease (Martin, unpublished).

These experiments have shownseveral cool-season turf grasses to betolerant of rapid blight under conditionsof moderately high salinity stress. Someof these grasses may be suitable foroverseeding where rapid blight is achronic problem. In South Carolinasome golf courses are using seed blendsof rough bluegrass and alkaligrass withacceptable results. Potential exists forthe use of certain creeping bentgrasscultivars or slender creeping red fescuesfor overseeding as well, although ratesof germination and establishment maybe complicating factors to consider.In short, blending rapid-establishingsusceptible grasses (rough bluegrassor moderately susceptible grasses likeperennial ryegrass) with tolerant grasses(alkaligrass, creeping bentgrass, andslender creeping red fescue) is a promis-ing strategy to reduce the risk of devas-tating epidemics of rapid blight. Fieldexperiments and further screening ofspecies and cultivars for tolerance torapid blight and for overseeding suit-ability will be important now and inthe future, especially under reduced-quality irrigation water.

S E P T E M B E R - 0 C T 0 B E R 2 0 0 4 23