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Discouraging DiseasesWith Cultural

Nitrogen application is a key element in the management of turfgrassdiseases. For example, both Microdochium patch (pink snow mold) andTyphula blight (gray snow mold) can be exacerbated by nitrogenapplication. Apply nitrogen cautiously if these diseases are a problem onyour turf.

By Dr. J.M. Vargas Jr.

Cultural management is, of course,only one aspect of a comprehensiveprogram for managing turfgrass

disease. The other major components ofa disease-management program areresistant cultivars and chemical man-agement. This article deals with theeffects of soil fertility, soil pH, wateringand mowing on turfgrass disease.Soil Fertility

Adding nitrogen to soil makes someturfgrass diseases worse but reducesthe severity of others. Some diseasesin each category are listed in Table A.

Once you know the effect of nitro-gen on disease development, timing thenitrogen applications becomes the nextimportant consideration. Nitrogen appliedin the spring is not going to solve astem-rust problem in the fall. Timingnitrogen applications to make a partic-ular disease less severe is not as simpleas it may seem. In the course of onegrowing season, a single turfgrass speciesmay be subject to a variety of diseases,all of which must be taken into account.It would be ideal if each turfgrass specieswere susceptible to only one disease,for then it would be easy to plan nitro-gen applications. Unfortunately, asthings are, a program of nitrogen fer-tilization that alleviates one diseasemay worsen another.

20 sporfsTURF

You must balance disease managementwith the nutrient needs of the turfgrassplant. Grass plants need nitrogen, soyou cannot simply eliminate all nitrogenapplications in the interests of diseasemanagement. You should apply nitrogenat the time of year when it will assistdisease management the most.

After nitrogen, the two most impor-tant elements for growing plants arephosphorus and potassium. The role ofthese elements in controlling diseases hasnot been determined. If most turf grasseswere subject to only one serious dis-ease, it might be worthwhile to maintainrelative concentrations of nitrogen, phos-phorus and potassium designed to mit-igate that disease. But, unfortunately,most turfgrass species have many diseasesduring a single growing season, andwhile it is fairly easy to regulate the nitro-gen level, it is difficult to alter theamounts of phosphorus and potassiumavailable to the plant. Phosphorus and

Managementpotassium tend to be insoluble and thusare not readily leached. It is easy toraise the level of these nutrients in thesoil but hard to bring it back downquickly. It would be very difficult toimplement any program that requiredraising and lowering the levels of phos-phorus and potassium throughout the sea-son. The simplest solution is to maintainadequate levels of phosphorus and potas-sium and vary the amount of nitrogen.

Sulfur is known to be a fungicide. Itmay act to reduce disease directly as afungicide or indirectly as a plant nutri-ent. R.L. Goss and C.J. Gould demon-strated control of Microdochium patchwith sulfur, and P.H. Dernoeden showedtake-all patch was reduced by sulfur.While all this may be well and good,other fungicides and cultural practicescan be used to manage these diseaseswithout the potential problems that sul-fur poses. Sulfur has been shown to bethe primary cause of the black layer.Managing Microdochium patch andtake-all patch with sulfur is a little likecuring the common cold with chemother-apy: It might do the job, but God help thepatient. These two diseases can bemanaged using less drastic means, butit is very difficult to manage a blacklayer once it occurs.

Iron is a minor element that is appliedwhen turf becomes chlorotic because ofan iron deficiency. This deficiency mayoccur when low levels of iron are presentin the soil or when adequate levels of ironare present in the soil but unavailabledue to the high soil pH. When soluble iro~

Table A: Effects of Nitrogen on Selected Turfgrass DiseasesSeverity increased by nitrogen Severity decreased by nitrogen

Pythium blight Dollar spotBrown patch RustGray leaf spot Red threadStripe smut Pink patchMicrodochium patch AnthracnoseTyphula blight Necrotic ring spot

Summer patchMelting-outLeaf spot

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is applied to foliage, it is a matter of onlyhalf an hour or so before the turf looksdarker and greener. Iron sulfate is gen-erally used, although other forms of sol-uble iron are available. If you desire adarker green turf, adding iron is a muchsafer way to get it than applying nitro-gen. Adding excessive amounts of nitro-gen will only encourage developmentof certain diseases, and you may beleft with brown turf or bare groundinstead of the lovely dark-green grassyou envisioned.

Iron sulfate may be used to maskthe symptoms of yellow tuft disease.While this won't control the problem, itwill make turf infected with yellow tuftlook better.Soil pH

Most of the literature tells you that thesoil pH should be maintained at theoptimum level for turfgrass growth(between 6 and 7). This is really part offolklore and is based on research done onwheat and corn. Having a pH between6 and 7 is important so the wheat and complants can have the phosphorus in the

soil available at heading time because itis difficult to run a fertilizer down the fieldwhen the corn is 6 feet high. On theother hand, you can run a fertilizerspreader over your turf every day ifneed be. It is much safer to add the fer-tilizer as needed than to try to lower thepH with sulfur and wind up with ablack layer. In most instances it is alsoimpossible to lower the pH of soils above7.5 with sulfur because of the highbuffering capacity of the soil.

Although some diseases do respond tochanges in soil pH, it is not practical tocombat turfgrass diseases by adjustingpH. For example, it is impossible tochange the soil pH from 5 to 7 to fight onedisease and then change it back againwhen a different disease comes along.Since the effect of pH on turf grass dis-eases is usually related to the levels ofnitrogen, phosphorus and potassium,it is much simpler to live with the pH youhave, adjust the levels of phosphorus andpotassium in the soil, and add nitro-gen as needed.

IrrigationProper irrigation can help minimize

turf grass diseases. The best time to irri-gate is in the afternoon, lightly anddaily. It not only supplies the plant withwater but helps cool it off, so it can bet-ter make it through the stress of midday.The worst time to irrigate turf is earlyto late evening. This wets the turf grassplant and debris (mat and thatch) andallows foliar pathogens to germinate, growand infect all night, since normally verylittle drying takes place before sunrise.Watering early in the evening also coolsoff the plants and promotes the forma-tion of guttation water, which is rich innutrients and encourages even moredisease development.

If irrigating during the day is imprac-tical, the second-best time to irrigate isjust before sunrise. The water dilutes thenutrient-rich guttation water and,by breaking up the droplets, allowsquicker drying after the sun rises.

continued on page 22

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consequently contribute to the devel-opment of thatch.

Many people say that thatch causesdisease, but there is little evidence thatit actually does so. It is suggested thatthatch harbors the pathogens that causeturf diseases. However, many diseasesare airborne, like the rusts and powderymildew, and the rest do just as well ina soil environment as in a thatchenvironment. The one problem withturfs that are maintained in thatch is thatthey are more susceptible to droughtthan turfs that are maintained in soils.Consequently, diseases like necroticring spot, summer patch and stripesmut, which develop symptoms under con-ditions of drought stress, will be moresevere. However, this is quite differentfrom the theory that thatch is the placethat harbors all these nasty pathogens,as proposed by many turfgrass experts.The suggestion that if we somehow elim-inate the thatch all our disease problemswould go away just isn't true. Believe me,that's not going to happen.

Removal of the clippings for a crownor root-rot disease, like necrotic ringspot and summer patch, has little effecton inoculum levels. There are enough air-

borne spores of the fungi that causemelting-out, rusts, powdery mildew andsmuts to negate what little could bedone by removing clippings. The perfectexample again is golf course greens,where clippings are always removed,yet there are as many disease problemson golf course greens as there areanywhere else.

Should you dethatch your lawn? Theanswer to that question depends onhow much weight you have gained dur-ing the winter. If you gained a lot ofweight, the exercise of running thepower rake might be good for you. On theother hand, if you are too far out ofshape, it might kill you! But it will donothing for your thatch problem. You willremove the winter-killed grass, whichwould break down anyway, but you willnot remove that thatch or correct athatch problem. Where severe thatchexists, the lawn should be cored(aerated) and the soil in the coresreturned to help facilitate thatch break-down. It may be necessary to repeatthis operation several times where severethatch problems exist.

Rust, red thread, melting-out andleaf spot can be managed by mowing.

Recommendations for managing thesediseases call for increasing the nitrogenlevel. However, if you add nitrogen anddo not mow, you will actually make thedisease worse, not better. This has beendemonstrated many times in thelaboratory. In the field, where mowingoccurs at least once a week, infectedfoliage is mowed off before the above-mentioned fungus has a chance tocomplete the disease cycle. Since thesepathogens have a 10- to 14-day cycle frominfection to sporulation, mowing once aweek keeps it from becoming aserious problem. •

Dr. J.M. Vargas Jr. is a professor ofbotany and plant pathology at MichiganState University. This article is reprint-ed with permission from Advances inTurfgrass Science: Management ofTurfgrass Diseases, Second Edition byJ.M. Vargas Jr. Copyright: LewisPublishers, an imprint ofCRC Press, BocaRaton, FL. For information on how toobtain a copy of the book, contact CRCPress, Inc., 2000 Corporate Blvd., NW,Boca Raton, FL 33431.

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