PERANAN KESEHATAN TANAH DAN PENEKANAN INOKULUM PATOGEN DALAM PENGELOLAAN PENYAKIT BUSUK PANGKAL BATANG KELAPA SAWIT

Abstract

Penyakit Busuk Pangkal Batang (BPB) yang disebabkan oleh cendawan Ganoderma boninense merupakan salah satu penyakit paling merusak pada pertanaman kelapa sawit. Kehilangan hasil akibat penyakit ini dilaporkan dapat mencapai 50–70%, sehingga menimbulkan kerugian ekonomi yang signifikan. Tingginya insidensi penyakit BPB diduga berkaitan erat dengan terganggunya keseimbangan segitiga penyakit tanaman, terutama akibat perubahan kesehatan tanah yang dipengaruhi oleh perubahan sifat fisika, kimia, dan biologi tanah. Perubahan tersebut meliputi penurunan kandungan dan keseimbangan unsur hara tanah. Selain itu, akumulasi biomassa sumber inokulum penyakit seiring bertambahnya generasi tanaman dan tekstur tanah serta penurunan fungsi komunitas biologi tanah turut memperparah kehilangan hasil akibat infeksi penyakit BPB. Ketersediaan biokontrol dan kompetitor substrat tumbuh (food base) Ganoderma, juga diduga memengaruhi perkembangan penyakit BPB di alam. Penelitian ini bertujuan untuk (1) mengkaji intensitas penyakit BPB pada berbagai generasi tanaman kelapa sawit di Sumatera Utara, (2) menganalisis peranan faktor kesehatan tanah meliputi sifat fisika, kimia, dan biologi tanah terhadap insidensi dan severitas penyakit, (3) mengevaluasi potensi agen hayati berupa bakteri penghasil senyawa organik volatil (SOV) dan senyawa organik terdifusi (SOT), serta (4) memperoleh calon makrofungi saprofitik yang mampu berperan sebagai kompetitor substrat tumbuh (food base) patogen penyakit BPB. Penelitian ini dilaksanakan melalui empat subkegiatan utama yang saling terintegrasi. Subkegiatan pertama berupa survei lapangan penyakit BPB yang dilakukan di tujuh kebun perusahaan yang tersebar di tiga kabupaten di Sumatera Utara, yaitu Simalungun, Asahan, dan Serdang Bedagai. Survei insidensi dan severitas penyakit dilaksanakan di 36 blok di 7 perkebunan kelapa sawit. Subkegiatan kedua yakni analisis faktor kesehatan tanah yang meliputi aspek fisika, kimia, dan biologi tanah. Data diperoleh secara langsung (data primer; dilakukan pengambilan sampel dan dianalisis di laboratorium) dan secara tidak langsung (menggunakan data sekunder). Aspek fisika mencakup tekstur tanah, yakni komposisi pasir dan debu. Aspek kimia mencakup unsur hara tanah, termasuk pH, kandungan C-organik, N, P, K, Ca, Mg, Mn, Si, dan B, serta data unsur hara daun, termasuk N, P, K, Ca, dan Mg. Aspek biologi meliputi total bakteri, total cendawan, populasi bakteri pelarut P dan K, bakteri pereduksi Mn, dan cendawan lignolitik. Pemeriksaan pengaruh faktor kesehatan tanah tersebut dilakukan terhadap insidensi dan keparahan penyakit BPB yang dianalisis dengan regresi linier berganda. Subkegiatan ketiga difokuskan pada pengujian potensi bakteri Serratia surfactantfaciens sebagai biokontrol melalui produksi SOV dan SOT. Isolat Serratia surfactantfaciens (SK018, S085, S108, dan SK909) yang digunakan merupakan isolat koleksi Departemen Proteksi Tanaman. Pengujian dilakukan terhadap 17 isolat Ganoderma dari koleksi kelompok peneliti proteksi tanaman Pusat Penelitian Kelapa Sawit (PPKS) dari berbagai kebun kelapa sawit di Jawa, Sumatera dan Kalimantan. Pengujian yang dilakukan meliputi uji penghambatan pertumbuhan koloni patogen oleh SOV dan SOT bakteri Serratia surfactantfaciens secara in vitro, dan uji pengaruh SOV dan SOT bakteri terhadap perkembangan penyakit secara in planta. Pengujian in vitro dilakukan menggunakan rancangan acak lengkap faktorial dengan tiga ulangan, sedangkan pengujian in planta di rumah kaca menggunakan rancangan acak kelompok lengkap. Selanjutnya, subkegiatan keempat meliputi koleksi makrofungi yang dilakukan di beberapa kebun di Jawa, Sumatera, dan Papua. Proses seleksi dalam penelitian ini meliputi uji ligninolitik, uji reduksi biomasa, dan produksi enzim ligninolitik serta uji antagonis makrofungi terhadap Ganoderma. Hasil survei menunjukkan bahwa seluruh kebun yang diamati mengalami serangan BPB dengan tingkat serangan yang bervariasi, mulai dari kategori netral hingga berat, di mana kategori serangan berat cenderung mendominasi. Kabupaten Simalungun menunjukkan insidensi dan severitas tertinggi, masing-masing sebesar 41,53% dan 30,94%. Pada generasi ke-3, berdasarkan pengamatan insidensi penyakit, pada blok-blok tanaman kelapa sawit yang diamati memiliki kategori serangan sedang dan berat. Pada pengamatan blok kelapa sawit generasi ke-3 ini, tidak ditemukan blok tanaman yang memiliki kategori serangan ringan dan sehat. Analisis sifat fisika tanah menunjukkan bahwa kandungan pasir berkorelasi dengan peningkatan insidensi dan severitas penyakit BPB, yang mengindikasikan peran tekstur tanah (pasir) dalam mendukung perkembangan patogen. Analisis sifat kimia tanah menunjukkan bahwa kandungan C-organik serta unsur hara makro seperti N, P, K, Ca, dan Mg berkorelasi negatif dengan perkembangan penyakit. Penurunan kandungan hara, terutama kalium dan silikat, serta peningkatan kandungan mangan di tanah maupun penurunan kandungan kalium dan magnesium di jaringan daun, berhubungan erat dengan meningkatnya perkembangan penyakit BPB. Dari aspek biologi tanah, hasil analisis menunjukkan bahwa kerapatan mikroba tanah fungsional berkorelasi negatif dengan insidensi penyakit. Semakin tinggi populasi mikroba tanah, termasuk bakteri pelarut fosfat penyedia kalium, mikroba pendegradasi lignin, dan terutama bakteri pereduksi mangan, insidensi BPB cenderung semakin rendah. Hasil pengujian in vitro S. surfactantfaciens terhadap 17 isolat Ganoderma, menunjukkan bahwa dua isolat bakteri terbaik, yaitu S108 dan SK909, memiliki kemampuan penghambatan tertinggi terhadap pertumbuhan G. boninense. Pada uji SOV, kedua isolat menunjukkan tingkat penghambatan rata-rata kumulatif masing-masing sebesar 85,15% dan 82,54%, sementara pada uji SOT masing-masing mencapai 90,80% dan 87,20%. Pengujian in planta menunjukkan konsistensi kedua isolat dalam menekan insidensi dan severitas penyakit BPB, sehingga berpotensi besar untuk dikembangkan sebagai agen pengendali hayati. Pada subkegiatan seleksi makrofungi untuk kompetitor Ganoderma, sebanyak 64 isolat makrofungi berhasil dikoleksi dari berbagai bagian tanaman dan lingkungan kebun kelapa sawit. Selanjutnya, terpilih 14 isolat dengan kemampuan tumbuh yang baik dan aktivitas ligninolitik tinggi. Kemudian diseleksi lebih lanjut hingga diperoleh lima isolat makrofungi saprofit obligat. Di akhir pengujian, tiga spesies makrofungi terbaik berhasil terseleksi, yakni Agaricus sp., Macrolepiota sp., dan Polyporus sp.

Basal stem rot (BSR), caused by the fungus Ganoderma boninense, is one of the most devastating diseases in oil palm plantations. Yield losses due to this disease can reportedly reach 50–70%, resulting in significant economic losses. The high incidence of BSR is thought to be closely related to the disruption of the plant disease triangle, primarily due to changes in soil health influenced by changes in physical, chemical, and biological properties. These changes include a decrease in soil nutrient content and balance. Furthermore, the accumulation of disease inoculant biomass with increasing plant generations and soil texture, as well as a decline in the function of soil biological communities, exacerbates the loss of yield due to infection of BSR. The availability of biocontrol agents and competitors for the Ganoderma food base are also thought to influence the development of BSR in nature. This research aims to (1) examine the intensity of BSR disease in various generations of oil palm plants in North Sumatra, (2) analyze the role of soil health factors, including physical, chemical, and biological properties, on the incidence and severity of the disease, (3) evaluate the potential of biological agents in the form of volatile organic compound (VOC) and diffusible organic compound (DOC) producing bacteria, and (4) identify potential saprophytic macrofungi capable of competing with the food base of the BSR pathogen. This research was conducted through four main, integrated sub-activities. The first sub-activity was a field survey of BSR disease, conducted in seven company plantations across three regencies in North Sumatra: Simalungun, Asahan, and Serdang Bedagai. The survey of disease incidence and severity was conducted in 36 blocks across seven oil palm plantations. The second sub-activity analyzed soil health factors, covering physical, chemical, and biological aspects of the soil. Data were obtained directly (primary data; sampling was conducted and analyzed in the laboratory) and indirectly (using secondary data). Physical aspects include soil texture, namely the composition of sand and silt. Chemical aspects include soil nutrients, including pH, C-organic content, N, P, K, Ca, Mg, Mn, Si, and B, as well as leaf nutrient data, including N, P, K, Ca, and Mg. Biological aspects include total bacteria, total fungi, populations of P and K-solubilizing bacteria, Mn-reducing bacteria, and lignolytic fungi. The examination of the influence of these soil health factors was carried out on the occurrence and severity of BPB disease which was analyzed using multiple linear regression. The third sub-activity focused on testing the potential of Serratia surfactantfaciens bacteria as a biocontrol through the production of VOC and DOC. The Serratia surfactantfaciens isolates (SK018, S085, S108, and SK909) were collection of the Department of Plant Protection. Testing was conducted on 17 Ganoderma isolates from the collection of the plant protection research group of the Oil Palm Research Institute (IOPRI) from various oil palm plantations in Java, Sumatra, and Kalimantan. The tests conducted included in vitro inhibition of pathogen colony growth by VOC and DOC of Serratia surfactantfaciens bacteria, and in planta testing of the effect of VOC and DOC of bacteria on disease development. In vitro testing was conducted using a completely randomized factorial design with three replications, while in planta testing in the greenhouse used a completely randomized block design. Furthermore, the fourth sub-activity included macrofungal collection conducted in several fields in Java, Sumatra, and Papua. The selection process in this study included ligninolytic tests, biomass reduction tests, and ligninolytic enzyme production tests, as well as macrofungal antagonist tests against Ganoderma. The survey results showed that all observed plantations experienced BSR infestation with varying degrees of infestation, ranging from neutral to severe, with severe infestation tending to dominate. Simalungun Regency showed the highest incidence and severity, at 41.53% and 30.94%, respectively. In the third generation, based on disease incidence observations, the oil palm blocks observed were categorized as having moderate and severe infestations. In this third generation of oil palm blocks, no blocks were found with mild or healthy infestations. Analysis of soil physical properties showed that sand content correlated with increased incidence and severity of BSR disease, indicating the role of soil texture (sand) in supporting pathogen development. Analysis of soil chemical properties showed that organic carbon content and macronutrients such as N, P, K, Ca, and Mg were negatively correlated with disease development. Decreased nutrient content, particularly potassium and silicate, as well as increased manganese content in the soil and decreased potassium and magnesium content in leaf tissue, were closely associated with increased BSR disease development. From a soil biology perspective, the analysis results indicated that functional soil microbial density negatively correlated with disease incidence. The higher the soil microbial population, including potassium-suppressing phosphate-solubilizing bacteria, lignin-degrading bacteria, and especially manganese-reducing bacteria, the lower the incidence of BSR. Results of in vitro testing of S. surfactantfaciens against 17 Ganoderma isolates showed that the two best bacterial isolates, S108 and SK909, had the highest inhibitory capacity against G. boninense growth. In the VOC test, both isolates showed average cumulative inhibition levels of 85.15% and 82.54%, respectively, while in the DOC test, they reached 90.80% and 87.20%, respectively. In planta testing demonstrated the consistency of both isolates in suppressing the incidence and severity of BSR, thus demonstrating their potential for development as biological control agents. In the sub-activity of macrofungal selection for Ganoderma competitors, 64 macrofungal isolates were collected from various plant parts and oil palm plantation environments. Furthermore, 14 isolates with good growth ability and high ligninolytic activity were selected. Then, further selection was carried out to obtain five isolates of obligate saprophytic macrofungi. At the end of the test, the three best macrofungal species were successfully selected: Agaricus sp., Macrolepiota sp., and Polyporus sp.