Close up of leaves of Manhattan ryegrass being colonized by Corticium red thread pathogen.

Recent Insights on the Nature andControl oi Corticium Red Threadby DR. HOUSTON B. COUCH, Department of Plant Pathology,Virginia Polytechnic Institute and State University, Blacksburg, Virginia

CORTICIUM RED THREAD hasthe distinction of being the firstreported foliar disease of turf-

grass. The fungus that incites the dise~sewas first observed on ryegrass in Australia,in 1854. The next account of an outbreakof red thread came from England, in1873, where it was reported to be causingsevere damage to ryegrass. The diseasewas first identified in the United Statesin 1931. At the present time, Corticiumred thread is known to occur on bent-grass, bermudagrass, Kentucky blue-grass, creeping red fescue, and perennialryegrass in the cooler humid areas ofNorth America, Europe, and Australia.

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Since the first description of thedisease, all the symptoms associatedwith outbreaks of red thread have beenthought to be caused by a single speciesof fungi. In recent years, however,research has shown that there are atleast two fungus species that can causesimilar symptom patterns on the grassthey are infecting, and that they canoccur simultaneously on the same standof grass. Although both of these orga-nisms produce pink mycelia and are inthe same major taxonomic group offungi, they are clearly distinguishablefrom each other. Also, while the symp-tom patterns of the diseases they inflict

on turfgrass have features in common,they also have certain key features thatdistinguish them from each other.

One of the components of what wasformerly grouped under the blanket titleof red thread is now known to be causedby the fungus Limonomyces roseipellis.Based on the primary features of itssymptom pattern, this disease has beennamed Limonomyces pink patch. Onlyperennial ryegrass and creeping redfescue are known to be susceptible topink patch. The disease is confined tothe above-ground portions of the plant.Symptoms are usually seen first alongthe margins of the leaves where they

Overall view of Corticium red thread on Pennfine ryegrass.

appear as small, irregularly shapedblotches of pink color bordered bylight green to yellow bands of discoloredleaf tissue. Eventually, the entire widthof the leaf takes on a distinctive pinkishcast. When this occurs, a light brownto tan tip dieback of the leaves thendevelops.

On stands of grass that are mowedfrequently and grow under optimumnitrogen fertilization, affected areasseldom reach more than 20 inches indiameter. Also, the severity of thedisease within these individual locationsis minimal. Consequently, under thesemanagement conditions, Limonomycespink patch is generally regarded to beof minor importance. However, on turfthat is mowed infrequently and thatgrows under low nitrogen fertilization,damage from pink patch may be severe.In these instances, the affected areasassume a distinctive pinkish tinge, afterwhich all ofthe above-ground parts maybecome completely blighted.

From an economic viewpoint, the redthread component of this total syndromeis by far the most important. Corticiumred thread affects a wider range of grassesthan does Limonomyces pink patch, andit has a much greater potential for thedestruction of large areas of turf.

A recent research report on the natureof the fungus that incites Corticium red

thread has recommended that its namebe changed from Corticium fuciformeto Laetisaria fuciformis. The rationalefor this change appears to be a valid one;therefore, the proposed new name for thefungus will probably receive generalacceptance. However, for the sake ofcontinuity in written and oral reports onthe nature and control of the disease,its present standard name, Corticiumred thread, should not be changed.

THE PRIMARY diagnostic featurefor Corticium red thread is the

presence of fine, thread-like, coral pinkstructures 1/ 16 to 1/4 inch in length atthe terminals of the leaves. These struc-tures are never present in cases ofLimonomyces pink patch. However,many of the other signs and symptomsof the two diseases overlap. Therefore,in field diagnosis, one can usually statewith a reasonable degree of certaintythat only Limonomyces pink patch ispresent. However, in the instances ofpositive diagnosis of the presence ofCorticium red thread, one cannot becertain through field examination onlythat Limonomyces pink patch is not alsopresent. This can only be determined bylaboratory-based procedures. Whetheror not only one or both of these diseasesis present is more than just an academicquestion, for it could be the basis for

deciding whether or not a spray programshould be initiated and what fungicidesshould be included in it.

Although the creeping bentgrasseshave been known for years to be suscep-tible to Corticium red thread, the proba-bility of the occurrence, of the disease onputting green managed grass has notbeen given much consideration. One ofthe reasons for this is that the bent-grasses are one of the more resistantspecies to this disease. Also, outbreaksof red thread are most common whenbentgrass greens are at their highestnitrogen levels and thus less prone toinjury by the disease. In addition, inyears past, the organic and inorganicmercury-based fungicides were a main-stay in greens management. These com-pounds are very effective in the controlof Corticium red thread; consequently,the probability of the appearance ofthe disease was preempted as a sideeffect of standard fungicide applicationpractices.

With the cessation of the use ofmercury fungicides for control ofdiseases that occur during the growingseason and the general trend towardlower nitrogen fertilization, the incidenceof red thread on bentgrass greens hasincreased. In these instances, the diseasehas not been of any major consequence.However, it is capable of doing somedamage to both the grass and the super-intendent's confidence in his ability tocontrol what he considers to be dollarspot.

Under close mowing conditions, thesymptom pattern for red thread is some-what different from that of taller cutgrass. The affected areas range from twoto six inches in diameter and are irregularin outline. The affected leaves are tan,and a cursory examination of the areacan result in a misdiagnosis of the prob-lem as being a case of fungicide-resistantatypical Sclerotinia dollar spot. In theinstances of occurrence of Corticium redthread, however, close examination ofthe leaves will usually reveal the presenceof a light reddish tinge to the sheaths.Also, although the frequency levels willbe low because of the close mowing, thecharacteristic red threads typical of thisdisease can be found in these areas.

The optimum weather conditions forthe development of Corticium red threadare air temperatures in the 68° -75°range, coupled with prolonged periodsof rainfall. With creeping red fescue,soil moisture stress effects on the suscep-

NOVEMBER/DECEMBER 1983 9

(Top) The positive diagnostic feature of red thread is the presence of finethread-like, coralpink structures at the terminal portion of the leaves. (Above)Corticium red thread on Penncross bentgrass underputting green management.Note that the characteristicpink colorationproduced by the red threadpathogenis present even wider close mowing conditions.

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tibility of plants to the disease appear tobe related to the variety in question.Rainier red fescue, for example, is moresusceptibleto red thread under conditionsof low soil moisture content, while thesusceptibility of the variety Pennlawndoes not appear to be affected by soilmoisture levels. Plants grown undercalcium deficiency are more susceptibleto red thread than those grown atadequate calcium levels. Of the variousmanagement practices, nitrogen fertili-zation has the most pronounced effectson development of this disease. Theincidence of Corticium red thread isusually much lower when the plants aregrowingunder a program of high nitrogenfertilization.

OF THE CULTIVATED turfgrassspecies that are susceptible to Cor-

ticium red thread, perennial ryegrass,creeping red fescue, and Kentucky blue-grass are most vulnerable. Within thesespecies,there is a wide range in the degreeof susceptibility among certain varieties;therefore, comparisons among speciesneed to take this into consideration.However, as a general rule, the perennialryegrasses rank at the top of the suscep-tibility list, with red fescues second.Kentucky bluegrasses, as a group, holddown third place. The increasing use ofthe fine-leaved perennial ryegrasses inrecent years, then, has introduced a newdimension to the potential for outbreaksof red thread in areas where the diseasehas been known to occur but has notbeen considered to be of major impor-tance. The eastern seaboard of the UnitedStates is a good example of this phe-nomenon. The past history of Corticiumred thread in this region has been markedby occasional outbreaks of the disease,but not of sufficient magnitude to causeany general concern. However, with thesteady increase in the use of perennialryegrasses has come a higher frequencyof reports of localized but severe out-breaks of red thread.

During 1982-83, in certain locationsof Virginia, the disease was active instands of ryegrass throughout the winterand continued to be in high incidenceuntil late June. The year, then, was aparticularly good one for comparativeevaluation of the susceptibility ofperennial ryegrass varieties to Corticiumred thread. In the ryegrass variety trailsat the Virginia Tech Turfgrass ResearchCenter at Blacksburg, red thread wasboth severe and uniformly distributedover the plots for an extended period of

time. Comparative ratings ofthese plotswere made for relative disease develop-ment, using an incidence-severity scoringsystem. The results were subjected tostatistical procedures that identified thefollowing comparative disease resistancegroups:

we found that certain of the ergosterolinhibitors show good promise for thecontrol of this disease. Chevron'sexperimental sterol inhibitor XE-779gave complete control of the disease.DuPont's DPXH6753 also gave a highlevel of disease control. The ergosterolinhibitors that are currently labeled forturf use [Elanco's RUBIGAN (fenarimol)and Mobay's BAYLETON(triadimefon)]also gave good disease control. Bayletonispresently labeled for red thread control.

Other compounds that gave very goodcontrol of red thread were Mallinckrodt'sCADMINATE (cadmium succinate) andVORLAN (vinclozolin). Mobay'sDYRENE (anilizine) and Ciba-Geigy'sBANNER (propiconazole) were also veryeffective in control of the disease.

The future possibility of having afairly wide selection of materials tochoose from for red thread control looksvery good. In the meantime, however, inview of the possibility of varying perfor-mance patterns in different localitiesand local regulations concerning fungi-cide usage on turfgrass, one should seekthe advice of an established advisoryservice before initiating a spray programfor the control of Corticium red thread.

INTHE PAST,cadmium- and mercury-based fungicides were the principal

compounds used for control of Cor-ticium red thread. Although certain ofthe newer, organic fungicides have beenshown in field tests to control the disease,from time to time reports of their per-formance does not correspond with theexpected control level. The reasons forthis could be such factors as differinglevelsof resistance to ,themodes of actionby local strains of the red thread patho-gen, or the possibility that the user isunknowingly dealing with both Cor-ticium red thread and Limonomycespink patch and the material in questionis not active against one of the twopathogens. In any event, there is a clearand present need for an expandedprogram of field screening of candidatefungicides to search out those that arehighly effective in control of red thread.

In the 1983 Virginia Tech field testsfor red thread control, we includedseveral fungicides that are presentlycommercially available for use on turf-grass, as well as recently developedcompounds that are still in the earlyscreening stage of development. Withinthe group of recentlydevelopedmaterials,

Red ThreadResistanceGroup.IMost

Resistant

PerennialRyegrassVarietyLinnCitation

II DerbyEatonEpicYorktown

III GameEnsortaPeloDiplomat

IV PennfineOmega

Most ManhattanSusceptible Caprice*The numerical values used to establish eachof these ran kings were subjected to analysisof variance, and the differences betweengroups are statistically significant at the 5percent level of probability.

Effect of nitrogen fertilization on the incidence of red thread on Kentucky bluegrass. Left, lownitrogen; right, high nitrogen.