**1. Introduction**

Oil palm (*Elaeis guineensis*Jacq.) is an important economic crop in many tropical areas. In particular, Indonesia, Malaysia, and Thailand are the leading palm oil producing countries of this region. The oil palm plant typically has a productive life of 20 or more years, and oil can be harvested several times each year. Consequently, it holds an advantage over all other oil-producing crops [1]. However,

the plant is often damaged by fungal infections, and these can cause a decrease of crop yields and result in the death of oil palm trees.

Fungal pathogens mainly infect the stems and leaves of oil palm trees during all stages of growth, from seedlings to the mature stage, and consequently can affect both the quality and quantity of palm oil. Basal stem rot (BSR), or *Ganoderma* rot disease, is the most severe disease of oil palm trees in Southeast Asian countries, especially Malaysia and Indonesia [2]. In addition to these countries, BSR has also destroyed oil palm plantations in Africa, Colombia [3], Papua New Guinea [4], and Thailand [5]. The causative fungus *Ganoderma boninense* is a basidiomycetous fungus and belongs to the order *Polyporales* and the family of *Ganodermataceae*. Fruiting bodies of *Ganoderma* typically form on the exterior of the oil palm trunk and then release and spread the spores to the soil. The usual method of controlling BSR in oil palm plantations is the use of chemical fungicides. Many fungicides, such as azoxystrobin, benomyl, carbendazim, carboxin, cycloheximide, cyproconazole, drazoxolone, hexaconazole, methfuroxam, nystatin, penconazole, thiram, triadimefon, triadimenol, tridemorph, and quintozene, could inhibit the growth of *Ganoderma* [6–10]. However, the fungicides cannot actually cure infected palm trees; they can only delay the spreading of the disease [9]. Furthermore, the applications of these chemical treatments have some worrying effects on human health and ecosystems. Examples of this would be toxicity to organisms and the suppression of beneficial microbes [9,11–13]. Nowadays, raising concerns about the high cost of chemicals, and the environmental problems they are associated with, have encouraged researchers to seek alternative strategies for BSR suppression.

The use of biological control agents represents a major alternative approach in the management of oil palm diseases. Fungal species, such as *Trichoderma harzianum, Trichoderma viride*, and *Gliocladium viride*, have been studied for their anti*-Ganoderma* activity, and their effectiveness against *Ganoderma* in a glasshouse and in a field trial [2,14–16]. Certain *Trichoderma* species are known as mycoparasites and have been utilized to control fungal pathogens. One of the biocontrol mechanisms of *Trichoderma* spp. is the release of glucanase and chitinase enzymes that are involved in the cell-wall degradation of *G. boninense*, and these can be elicitors in inducing a plant defense response [17,18]. Several strains of bacteria, especially *Pseudomonas aeruginosa, Pseudomonas syringae,* and *Burkholderia cepacia,* have also been studied for their potential to be applied as biological control agents [19–22]. Their potential abilities to inhibit the spread of *G. boninense* and to reduce the incidence of the disease have been documented [19–22]. Although the control mechanisms of these bacteria have not yet been clarified, they may control *Ganoderma* by producing antifungal secondary metabolites. In addition, several actinomycetes were screened for their antagonistic activity against *G. boninense.* Actinomycetes, especially the genus *Streptomyces*, are well known for their ability to produce a wide variety of bioactive metabolites [23–26]. Many *Streptomyces* species, such as *Streptomyces hygroscopicus*, *Streptomyces ahygroscopicus*, *Streptomyces abikoensis*, and *Streptomyces angustmyceticus*, were found to be promising biocontrol agents for BSR disease [27,28]. *Streptomyces violaceorubidus* released not only secondary metabolites towards *G. boninense* but also released cell-wall degrading enzymes involved in the control of this pathogen [29,30].

Actinomycetes associated with the oil palm rhizosphere may have an important role in protecting plants from *Ganoderma* infection by releasing antibiotics and enzymes. Thus, we isolated actinomycetes from the rhizosphere of healthy oil palm plants and screened the antifungal activities of their culture broth against *G. boninense*. One actinomycete strain, CMU-AB204T, showed significant antimicrobial activities against, not only *G. boninense* but also phytopathogenic fungi and several bacteria. We had previously identified this strain and proposed that it could serve as a novel species, namely *Streptomyces palmae* CMU-AB204T [31]. This actinomycete was selected to investigate antimicrobial secondary metabolites. This report describes the results of the isolation, structural elucidation, and antimicrobial activities of six new compounds, AB204-A–F (**1**–**6**), and three known compounds, anguinomycin A (**7**), leptomycin A (**8**), and actinopyrone A (**9**), that were produced by *S. palmae* CMU-AB204T.
