Environment and Culture AffectBermudagrass Growth and DeclineThe roles temperature and shade play in ultradwarfbermudagrass health are not totally understood.BY RICHARD H. WHITE

Tifeagle bermudagrass green at the time of treatment initiation in July.

Brmudagrass decline is a devastat-ing root disease of highly man-aged bermudagrass turf, especially

on golf greens in the southern UnitedStates (Elliott, 1991). It is caused by aninteraction of host-predisposing abioticstresses and the soil-borne, ectotrophic,root-infecting fungus Gaeumannomycesgraminis var. graminis(Ggg). Bermuda-grass decline resultsin large areas of turfwith weakened,short, brown-to-black root systemsand an absence offeeder roots androot hairs. Symp-toms include foliarchlorosis, a thin-ning stand, poorresponse tofertilizer andirrigation, andpremature plantdeath. Thepathogen causesroot, rhizome, andstolon rotting.Nutrient andirrigation manage-ment is difficult because of diminishedroot systems. Above-ground symptomsof infection often become evident any-where from spring green-up throughthe spring and summer months, whenheat and moisture stress challenge theweakened root systems of affectedplants. Symptoms also are common

during cloudy, wet periods of latesummer and early fall.

Drs. Joseph Krausz, Philip Colbaugh,Roy Stanford, and Richard White werepart of the turfgrass research team atTexas A&M University that exploredthe effects of cultural, environmental,and plant growth factors on the re-

covery of bermudagrass from damagecaused by Ggg. A strong component ofthe research was to gain a better under-standing of how temperature and lightlevels influence dwarf bermudagrassgrowth and development. The influenceof light and temperature on dwarfbermudagrass growth was of interest

because bermudagrass decline oftenbecomes more severe during persistentcloudy and overcast conditions (Waltz,2003). Texas A&M University researchalso focused on cultural practices to en-hance recovery of turf exhibiting ber-mudagrass decline symptoms. Althoughbermudagrass decline is not inevitable,

when the diseasedoes occur,strategies areneeded to hastenturf recovery.

CULTURALSTRATEGIESTO ENHANCERECOVERYFROM BER-MUDAGRASSDECLINERecommendationsfor hastening turfrecovery from ber-mudagrass declineoften includeraising the mowingheight and applyingacidifying fertilizersand/or foliar feeding(Fermanian et al.,

2003). A four-year-old Tifeagle bermuda-grass putting green with severe symptomsof bermudagrass decline was used toexplore cultural approaches for alleviat-ing the disease symptoms. The greenwas maintained at 0.125 inch, had a soilpH of 9.1, and was previously fertilizedwith a coated urea nitrogen fertilizer.

NOVEMBER-DECEMBER 2004 21

>IPIo~ were rated on a l-to-9 scale. with 9 as the highest quality. A 5 was considered the minimal accepted quality level forputting greens.

2Means within months followed by the same lower-case letter are statistically similar.

IP10~ were rated on a l-to-9 scale. with 9 as the highest quality. A 5 was considered the minimal accepted quality level forputting greens.

2Means within months followed by the same lower-case letter are statistically similar.

Nitrogen August October August October6 I.Sa2 2.Sa 2.8a 4.0c

12 2.0a 2.9a 3.2a S.3b24 2.2a 3.2a 3.Oa 6.4a

symptoms decreased with increasing N,especially for the solid-tine aerificationtreatments. Although 24 lb. N per 1,000sq. ft. is considered excessive, thistreatment, in conjunction with solid-tine aerification, resulted in rapid turfquality recovery and diminished ber-mudagrass decline symptoms. Also, turfquality increased and bermudagrassdecline symptoms decreased to agreater extent for increasing N in con-junction with heavy topdressing thanwhen compared to no and light, fre-quent topdressing (Table 2).

The most advantageous treatmentcombination for recovery from ber-mudagrass decline symptoms andimprovement of turfgrass quality wassolid-tine aerification, heavy topdress-ing, and 24 lb. N per 1,000 sq. ft. Thisimprovement in quality was accom-plished without raising the mowingheight above 0.125 inch. Excessive Ncan cause rapid thatch accumulation,and the combination of the greatestamount of N and no topdressing be-came excessively soft by early October.However, plots that received the greatestamount of N in conjunction withheavy or light topdressing remainedfirm. Surprisingly, the large amountsof N used did not contribute to anexcessive vertical growth rate.

The marked recovery of Tifeaglefrom bermudagrass decline symptomsfor specific treatment combinationsoccurred even though Ggg was stillpresent. Large amounts of N, as used inthis study, should not be applied tobermudagrass for long periods of timedue to the potential to negativelyimpact the environment and because ofthe potential to cause excessive thatchaccumulation and contribute to reducedstress tolerance. Increasing N for shortperiods to enhance recovery followedby lower amounts of N to sustain turfdensity is, however, a more reasonableapproach. Careful rootzone pH man-agement combined with a soundnutrient management plan may reduceseverity of bermudagrass decline(Waltz, 2003).

6.la5.0b

Heavy

3:2c

initiated in early July and ended in earlySeptember. Visual evaluations of turf-grass quality were taken every twoweeks. Microscopy was used to assesspresence of egg.

Aerification is an important manage-ment tool that is used to remove andcontrol thatch, reduce compaction, andimprove root development. In thisstudy, hollow-tine aerification disruptedthe surface more than solid-tine aerifi-cation (Table 1). Aerification of anytype had not been applied to the greenfor several years, resulting in heavythatch, increased disease severity, andpoor rooting. The poor rooting pre-vented the initial hollow-tine aerifica-tion from producing quality cores anduniform coring holes, resulting in greatersurface disruption and a longer healingtime than when solid tines were used.

Turfgrass quality improved andevidence of bermudagrass decline

Table 2 ~'Turf quality of Tifeagle bermudagrass in October as influenced

by none, biweekly dusting, and monthly heavy topdressing'

TopdressingTreatments

None Dusting

3.lb2 3.0b

3.5b 3.6b4.la 4.4a

•Nitrogen6

12

24

Table ITurf quality in August and October for Tifeagle bermudagrass as influenced

by monthly aerification with hollow or solid tines and biweekly fertilizerapplications to provide 6, 12,and 24 pounds of N per 1,000 sq. ft. annually'

HollowTine SolidTine

Previous applications of several fungi-cides in a replicated trial conducted onthe green the previous year were noteffective in controlling the disease.A series of treatments was establishedin early July, including nitrogen (N)regimes, aerification, and topdressingarranged in a split-plot design. Nitrogenregimes included bimonthly applicationsof ammonium sulfate at 0.25,0.5, and1.0 lb. N per 1,000 sq. ft. to supply totalannual N of 6,12, and 241b. per 1,000sq. ft. Ammonium sulfate was used inthis study because of its acidifying effecton soils. Nitrogen regimes were supple-mented with potassium, phosphorus,and micro nutrients based on soil tests.Aerification treatments includedmonthly application of O.5-inch solidtines and 0.5-inch hollow tines withcores removed. Topdressing treatmentsincluded none, 0.125-inch monthly, and0.02-inch bimonthly. Treatments were

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

Tifdwarf bermudagrass growth response to temperature regimes of 95/80°F (left) and 80/66°F day/night(right). Plants were grown in growth chambers with 14 hours artificial light at about one-third of full sunlight.

DWARF BERMUDAGRASSGROWTH ANDDEVELOPMENTIN RESPONSE TOENVIRONMENTThe optimum temperature for growthof bermudagrass is 80 to 95°F (Beard,1973) andfor egg theoptimumgrowth tem-perature inculture is 86°F(Fermanian etal.,2003).Thus, both thebermudagrassand pathogenshould growand developwell at a rangeof 80 to 90°F.Controlledenvironmentchambers wereused toexplore theeffect oftemperatureon bermuda-grass andegg and onsubsequentdiseasedevelopment.

A singlesprig ofTif-dwarf bermudagrass was surfacesterilized and then grown in agreenhouse for several months to createplanting stock for use in theexperiment. Sprigs were obtained fromthe single stock plant and established inindividual containers to receive thefollowing treatment combinations.Treatment combinations includedinoculated and uninoculated plants,nitrogen treatments of 4,8, and 12pounds of N per 1,000 sq. ft., andtemperature regimes of 95/80,80/66,and 66/51°F day/night temperatures.Artificial lighting provided about one-third of full sunlight.

Nitrogen nutrition influenced growthbut was not as influential as the temper-ature regime. The effects of temperatureregimes were robust. Internode lengthwas 0.23,0.51, and 0.83 inch, and leaflength was 0.29,0.45, and 0.49 inch forthe 95/80,80/66,66/51 OF (day/night)

temperature regimes, respectively. Whilethe growth responses exhibited by Tif-dwarf in these growth chamber studieswere consistent with growth under lowlight (shade), temperature was a con-trolling factor in the degree of response.Additional studies were conducted withlight levels of about 10,25, and 50% offull sun within temperature regimesof95/80 and 80/66°F (day/night).Decreasing light caused increases in leafand internode length, but the degree ofincrease was regulated by temperature.The results of this study indicate thattemperature as well as light levels regu-late expression of dwarfuess in Tifdwarf

bermudagrass. Although Tifeagle wasnot a main focus of the growth cham-ber study, Tifeagle bermudagrass wasexposed to the same temperatureregimes to determine if growthresponses were similar in Tifdwarf andTifeagle. Similar responses to tempera-

ture regimeswere observedfor Tifdwarfand T ifeagle.

SUMMARYThe results ofthis studyexplain whyraising themowmgheight is oftenrecommendedas a culturalpractice toreducebermudagrassdeclinesymptoms. Incoastal andother areasaffected bylong periodsof overcast,rainy weather,growth habitof dwarf ber-mudagrassesmay changedramatically

in response to low light and lowertemperatures. The altered growth formof Tifdwarf that may occur duringovercast and rainy weather would notlikely tolerate close mowing heights.Thus, mowing stress would result inincreased sensitivity to pathogenicorganisms such as egg and may resultin more substantial expression of diseasesymptoms. Weather data from TexasA&M University indicate that thehigh and intermediate temperatureregimes and light levels used in thisstudy can occur in southern climatesin late summer and early fall duringperiods of heavy rain and week-long

N a v E M B E R - DEe E M B E R 200 4 23

Effect of monthly solid (front three rows) and hollow-tine (back three rows) aerification, nitrogen at 6, 12,24and 6, 12,24 lb. N per 1,000 sq. ft. (front to back), and heavy, light, and no topdressing (left to right) on theappearance of Tifeagle bermudagrass.

periods of overcast skies. Evidence ofthe altered growth form was observedwithin three to four days of exposure tothe moderate temperature regime andlight levels used in this study .

In addition to the potential implica-tions that the changes in growth habitin response toenvironmentmay have fordisease toler-ance in dwarfbermudagrass,the effect oftemperatureon dwarfbermudagrassgrowth habithas tremen-dous implica-tions for dwarfbermudagrassgolf greenmanagement.The effects oftemperatureon growthhabit of dwarfbermudagrassmay explainwhyexces-sively largeamounts ofNapplied duringsummer didnot causemore robustvertical growth ofTife~gle in the fieldstudy described earlier in this text.Temperature should also be a majorconsideration in the timing and severityof cultivation practices such as coreaerification. Healing of surfacedisruption caused by core aerificationand vertical mowing may occurextremely slowly during July andAugust, periods previously perceived tosupport maximum bermudagrassgrowth. During exposure to the hightemperatures of July and August, forexample, the growth habit of manydwarf bermudagrasses may be extremelycompact and not conducive to recovery

24 GREEN SECTION RECORD

from injury caused by aerification andvertical mowing. Tolerance to pestsand wear also may be less during high-temperature periods. Establishmentrates of dwarf bermudagrasses may bedramatically affected by seasonalchanges in temperature, with slow

establishment occurring at temperaturesgreater than 90°F. This notable dis-covery about the effects of temperatureon dwarf bermudagass growth anddevelopment provides strong rationalefor additional research on numerousaspects of bermudagrass culture, estab-lishment, and pest and abiotic stresstolerance.

RESEARCH SUMMARY POINTS• Tifeagle bermudagrass recovery frombermudagrass decline symptoms wasenhanced by aerification, heavy top-dressing, and aggressive fertilizationwith ammonium sulfate.

• Low light caused increased leaf andinternode length in dwarf bermuda-grasses, but temperature regulatedexpression of the dwarf growth habit.• The alterations in growth form inTifdwarf bermudagrass caused by lowlight and cooler temperatures that often

occur duringovercast rainyperiodsjustifies raisingthe mowingheight toreduce mow-ing stress thatmay con-tribute tobermuda-grass declineseverity.• Hightemperaturescause acompactgrowth habitin dwarfbermuda-grasses andmay slowhealing ofsurfacedamagecaused bycultivation orpests.

REFERENCESBeard,]. B. 1973. Turfgrass: Science and Culture.Prentice Hall, Inc., Englewood Cliffi, N.J.

Elliott, M. L. 1991. Determination of anetiological agent of bermudagrass decline.Phytopathology 81: 1380-1384.

Fermanian, T. W, M. C. Shurtleff, R. Randell, H.T. Wilkinson, and P. L. Nixon. 2003. ControllingTurfgrass Pests. pp. 401-410. Prentice Hall, UpperSaddle River, N.J.

Waltz, Clint. 2003. Fungus Family ProvidesFodder for Several Diseases. Turfgrass Trends.http://www.turfgrasstrends.com/turfgrasstrendsIarticle/ articleDetail.jsp?id= 51998.

RICHARD WHITE, PH.D., is prcifessor<1tuifgrass management and physiology in theSoil and Crop SciencesDepartment at TexasA&M University.