Petiole (Rachis) Blight of Palm - Manatee County Extension Office ...

PP-221Petiole (Rachis) Blight of Palm 1Monica L. Elliott 2Summary• Petiole (rachis) blight is caused by numerousfungal pathogens, but the symptoms thesepathogens cause are similar for all of them.• Palm host range is unknown for this disease.• The disease often results in discolored (usuallybrown or reddish-brown) elongated lesions orstreaks along the petiole of the oldest (lowest)leaves.• The pathogens infect only the petiole, not theleaf tissue. However, destruction of vasculartissue (xylem and phloem) deep into the petioleresults in a one-sided or uneven death in the leafblade.• Fungal structures can sometimes be observedon the infected petiole surface.• Since very little is known about this disease,only general disease management strategiesregarding sanitation, water management, andfungicide use can be provided.IntroductionTo discuss petiole (rachis) blight, a briefintroduction to palm leaf anatomy is required. Thereare three types of palm leaves. All possess a base(leaf sheath), a petiole and a blade. The leaf base isthe portion that attaches to the stem (trunk). The leafpetiole is the stalk or stem of the leaf connecting theleaf base with the leaf blade.The leaf blade is either: 1) fan-like (palmate)with individual leaflets (segments) merged togetherand the petiole terminated where the blade begins; 2)an elongated version of fan-like (costapalmate) withindividual leaflets (segments) merged together butwith the petiole extending into the leaf blade; or 3)feather-like (pinnate) with individual leaflets clearlydelineated and attached to the petiole, with the petioleextending the entire length of the leaf blade. Theextension of the petiole into the leaf blade is calledthe “costa” on a costapalmate palm, and the“rachis” on a pinnate palm.Unfortunately, the term petiole and rachis areoften used interchangeably. Since diseases of thepetiole were first observed on Phoenix palms,pinnate-leaf palms that have a rachis, the common1. This document is Fact Sheet PP-221, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food andAgricultural Sciences, University of Florida. Original publication date January 2006. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.2. M. L. Elliott, professor, Plant Pathology Department, Fort Lauderdale Research and Education Center--Ft. Lauderdale, FL; Florida Cooperative ExtensionService, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL.The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty theproducts named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition.The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information andother services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. LarryArrington, Dean

Petiole (Rachis) Blight of Palm 2name of these diseases became “rachis blight.”However, as we now know, palms that do not have arachis also are susceptible to petiole diseases. In thisfact sheet the disease will be referred to as "petioleblight."In general, the “typical” or “classic” petioleblight is a disease in which the pathogen only infectsthe petiole. The disease symptoms may look like thepathogen has also infected the leaf blade, but this is asecondary affect from petiole infection. Anexception to the “typical” petiole blight is a diseasecaused by Pestalotiopsis. This pathogen causes botha leaf spot and blight as well as a petiole or rachisblight, usually at the same time. It is discussed in aseparate fact sheet at http://edis.ifas.ufl.edu/pp141.Pathogens and HostsThere are a number of different pathogensassociated with petiole blight. These includeCocoicola and Serenomyces, which are only found inassociation with palms. Macrophoma, Phoma,Phomopsis, Diplodia, and Pestalotiopsis may alsocause this disease, but these fungi also cause diseasesof numerous other plant hosts. The host range of allof these fungi within the palm family is unknown.SymptomsFigure 1. Overview of Washingtonia robusta group: Thecenter palm is severely affected by Petiole Blight causedby Cocoicola. Credits: A. Wilsondestroyed vascular tissue are killed (Figure 4). Thisresults in a one-sided or uneven death of the leafblade, the symptom most often observed first. ForPhoenix canariensis (Canary Island date palm), thesesymptoms are quite similar to those caused byFusarium wilt. See the fact sheet on this disease athttp://edis.ifas.ufl.edu/pp139.Mature palms (palms with trunks) are morelikely to be affected by petiole blight than juvenilepalms. On mature palms, the oldest (lowest) livingleaves will be affected first. The disease will move upthrough the canopy to younger leaves. The diseasemay severely debilitate the palm, but the diseaseseldom kills the palm (Figure 1).No matter which pathogen is causing the disease,the symptoms of petiole blight are essentially thesame. Discolored (usually brown or reddish-brown)elongated lesions or streaks often occur along thepetiole and rachis (Figures 2 and 3). The fungusinvades deep into the petiole and is destroying alltissue, including the vascular tissue (xylem andphloem). This vascular tissue moves water andcarbohydrates between the leaf and the stem (trunk).Vascular tissue destroyed in the petiole or rachisresults in localized death in the leaf blade becauseonly the leaf segments or leaflets connected to theFigure 2. Reddish-brown streak along petiole ofWashingtonia robusta due to Petiole Blight. Credits: A.WilsonAgain, while it appears the fungus is alsoinfecting the leaf blade, it does not. The leaf bladesymptoms are a result of the fungal damage in the

Petiole (Rachis) Blight of Palm 3Figure 3. Close-up of discolored petiole streak shown inFigure 2. Credits: M.L. ElliottFigure 5. Sporulating structures of Cocoicola on aWashingtonia robusta petiole. Credits: M.L. ElliottDiagnosisFigure 4. Reddish-brown lesion on petiole disruptsvascular tissue which kills leaflets within the leaf blade.Credits: M.L. Elliottpetiole and rachis. Thus, for diagnostic purposes, thepetiole or rachis is the tissue that must be examinedand sampled, and not the leaf blade. A cross-sectionthrough the petiole or rachis lesion or streak willreveal the discolored internal tissue resulting fromthe fungal invasion. It is critical to use a saw to cutthrough the petiole or rachis. A tool such as a pruningshear may bruise internal, healthy plant tissue in thecutting process. This bruise can then be mistaken forfungal damage.Sometimes, the surface of the diseased petiole orrachis will have erupted with fungal structures whilethe leaf is still attached to the palm trunk (Figure 5).These structures either are or can be induced toproduce spores used to identify the pathogen.As stated above, fungal structures may bepresent on the petiole or rachis at the time thesymptoms are observed. These structures may also besporulating while the leaf is still attached to the trunk,which makes pathogen identification relativelysimple. If not producing spores, it is usually possibleto induce spore production. If these structures are notimmediately evident, placement of the diseasedpetiole segment into a moist chamber (e.g., sealedplastic container with wet paper toweling) will ofteninduce their development (Figure 6).Figure 6. Sporulating structures of fungus causingPetiole/Rachis Blight on Latania. This piece was incubatedfor two weeks before the structures appeared. Credits:M.L. ElliottWhile some of the pathogens that cause petioleblight can be cultured on artificial media, manycannot. The Florida Extension Plant Disease Clinic(FEPDC) network is available for pathogenidentification. Contact your local county Extension

Petiole (Rachis) Blight of Palm 4office or FEPDC for information on samplesubmission and cost of laboratory diagnosis. Ingeneral, the entire leaf should be placed in acardboard box or tube and padded with dry paper.The leaf can be cut into pieces with a handsaw foreasier packaging. Do not place the pieces in a plasticbag.Disease ManagementVery little is known about petiole blight,including the specific environmental conditions thatencourage disease development. It is presumed thathigh humidity is required for fungal infection anddisease development, as that is a requirement formany plant diseases that affect above-ground tissue.It is also presumed that spores are the primary meansof fungal movement from palm to palm. Based onthese assumptions, sanitation and water managementare the basis for petiole blight management.Removal and destruction of severely infectedleaf fronds would be suggested as a means ofreducing inoculum (available spores to infect otherleaves and palms). This would be especially criticalin a nursery situation where palms are planted closertogether and are more numerous. However, if thepalms are in the landscape and under nutrient stress,pruning should be minimal as removal of too manyleaves may be detrimental to the palm rather thanbeneficial.In a nursery, increasing distance between plants,increasing air movement, and irrigating in the earlymorning hours to reduce leaf wetness at night arecritical water management components. Whenpossible, overhead irrigation should be eliminated.fungicide sprays to protect the petiole and rachis maybe useful. For palms where the canopy is no longeraccessible, broad-spectrum systemic fungicide soildrenches may be useful, although information as tomovement of systemic fungicides in palms isextremely limited.It is critical to understand that fungicides do notcure the damage already present on the palm. Planttissue does not “heal” itself. Once the damageoccurs, it will remain for the duration of the life ofthat particular leaf. Fungicides are used to preventfurther spread of the disease by protecting a leafpetiole and rachis that has not yet been infected by thefungal pathogen.Selected ReferencesBarr, M. E., H. D. Ohr, and M. K. Murphy.1989. The genus Serenomyces on palms.Mycologia 81:47-51.Hyde, K. D., and P. F. Cannon. 1999. Fungicausing tar spots on palms. Mycologial Papers,No. 175. CABI Publishing, Wallingford, U.K.Simone, G. W. 2004. Rachis blight. Pages33-34 in: Compendium of Diseases andDisorders of Ornamental Palms. M. L. Elliott, T.K. Broschat, J. Y. Uchida, and G. W. Simone,eds. American Phytopathological Society, St.Paul, MN.Simone, G. W. 1998. Prevention andmanagement of palm diseases in Floridaslandscapes. PP-Mimeo 98-4. FloridaCooperative Extension Service, Gainesville, FL.Since no fungicides have been evaluated forefficacy in managing petiole blight, fungiciderecommendations are educated guesses at best.Fungicides would only be recommended in thosesituations where a palm is being seriously affected, orthe potential for an epidemic is high, such as in anursery. Never initiate a fungicide program withoutfirst initiating a cultural control program as describedabove.For palms where the canopy is still accessible forspraying, broad-spectrum contact or systemic foliar