Ganoderma zonatum causes butt rot of areca (Dypsis lutescens) and robellini (Phoenix roebelenii) palms in Florida.

Abstract

Ganoderma butt rot is a lethal disease of palms (Arecaceae) prevalent in palm-growing regions in the US that infects at least 58 species of palms (Elliott and Broschat 2001). Early symptoms appear as wilting of older fronds in the lower part of the canopy, and as disease progresses, wilting advances to younger leaves higher in the canopy towards the unopened spear leaf eventually killing the palm. A characteristic sign of the disease is the appearance of fruiting bodies (basidiomata) at the base of the palm trunk close to soil line. Ganoderma butt rot disease was detected on clustering palm species, areca palms, with 9 (82%) clusters showing Ganoderma basidiocarps and dead stumps, and mortality was observed in 5 (45%) clusters. A sterile scalpel was used to transfer the context tissue from Ganoderma basidiomata to full-strength potato dextrose agar selective media supplemented with streptomycin (100 mg/l), lactic acid (2 ml/l) and benomyl (4 mg/l). The pure culture for isolate GAN-33 was grown at 28°C in complete darkness for 10 days. The fungal colony was ivory white in color that grew radially as a dense mycelial mat without any sporulation. To establish the identity of the fungus, DNA was extracted using the Qiagen DNeasy PowerSoil kit (Cat. #12888). Three barcoding genes, nuclear ribosomal DNA internal transcribed spacer (ITS) region, RNA polymerase II subunit 2 (rpb2) and translation elongation factor 1α (tef1α) were amplified using primers ITS1/ITS4 (White et al 1990), bRPB2-6f/bRPB2-b7.1R (Matheny et al 2007) and EF1-983F/EF1-2212R (Matheny et al 2007), respectively. The sequences were deposited in GenBank, accession numbers KX853442, KX853466 and KX853491 for ITS, rpb2 and tef1α, respectively (Elliott et al 2018). Comparison to the NCBI nucleotide sequence database identified isolate GAN-33 as Ganoderma zonatum based on 100, 99 and 99% similarity to ITS, rpb2 and tef1α sequences, respectively. Pathogenicity of G. zonatum isolate GAN-33 was determined on 1-year old seedlings of areca palm (Dypsis lutescens) and pygmy date palm (Phoenix roebelenii). Ganoderma zonatum inoculum was prepared by transferring two-week old cultures to autoclaved wheat berries and allowed to colonize for two weeks. Seedlings were gently removed from the pot and the roots were trimmed before placing them back in the pot ensuring that the roots were in contact with the G. zonatum colonized wheat berries. The inoculated and control seedlings were maintained in a growth chamber at 28°C 60% RH (daytime) and 24°C 50% RH (night time), 12h:8h light:dark period, and watered twice a week. Initial wilting symptoms started appearing approximately one month after inoculation and mortality was observed for a total of four seedlings at three months after inoculation i.e., 2 out of 3 G. zonatum inoculated seedlings died for both areca and robellini palms, whereas the non-inoculated areca and robellini palm control seedlings remained healthy and alive. The pathogen was re-isolated from inoculated roots, and its identity was confirmed by colony morphology and PCR using G. zonatum specific primers (Chakrabarti et al 2022). To the best of our knowledge this is the first report establishing G. zonatum as the pathogen responsible for Ganoderma butt rot of palms.