Petiole (Rachis) Blight of Palm - Manatee County Extension Office ...
Petiole (Rachis) Blight of Palm - Manatee County Extension Office ... Petiole (Rachis) Blight of Palm - Manatee County Extension Office ...
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
<strong>Petiole</strong> (<strong>Rachis</strong>) <strong>Blight</strong> <strong>of</strong> <strong>Palm</strong> 3Figure 3. Close-up <strong>of</strong> discolored petiole streak shown inFigure 2. Credits: M.L. ElliottFigure 5. Sporulating structures <strong>of</strong> 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 <strong>of</strong> 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 <strong>of</strong> the diseasedpetiole segment into a moist chamber (e.g., sealedplastic container with wet paper toweling) will <strong>of</strong>teninduce their development (Figure 6).Figure 6. Sporulating structures <strong>of</strong> fungus causing<strong>Petiole</strong>/<strong>Rachis</strong> <strong>Blight</strong> on Latania. This piece was incubatedfor two weeks before the structures appeared. Credits:M.L. ElliottWhile some <strong>of</strong> the pathogens that cause petioleblight can be cultured on artificial media, manycannot. The Florida <strong>Extension</strong> Plant Disease Clinic(FEPDC) network is available for pathogenidentification. Contact your local county <strong>Extension</strong>
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