KORELASI ANTARA AGRESIVITAS INOKULUM SPORANGIA DENGAN TOKSISITAS FILTRAT Phytophthora capsici ASAL TANAMAN LADA (Piper nigrum L.)

Abstract

ABSTRAK Penggunaan varietas lada tahan penyakit paling praktis dan efektif untuk menekan serangan Phytophthora capsici, penyebab penyakit busuk pangkal batang (BPB) lada, tetapi varietas lada tersebut belum tersedia. Seleksi dini ketahanan lada dapat dilakukan di laboratorium menggunakan inokulum sporangia atau filtrat biakan P. capsici. Tujuan penelitian ini ialah membandingkan agresivitas inokulum sporangia dengan toksisitas filtrat biakan (FB) P. capsici. Penelitian dilakukan dari bulan Juni sampai September 2009 di laboratorium Biokimia BB Biogen dan Hama dan Penyakit Balittro. Penelitian menggunakan 50 isolat P. capsici dari berbagai daerah pertanaman lada yang diisolasi tahun 1982 sampai 2009. Daun-daun lada yang diambil dari varietas Natar-1 (agak tahan terhadap P. capsici) dan Petaling-1 (rentan terhadap P. capsici) diinokulasi dengan potongan agar mengandung sporangia atau 20 µl FB P. capsici. Percobaan dilakukan secara faktorial (2 varietas × 50 isolat) dengan rancangan acak kelompok dan tiga ulangan. Derajat agresivitas isolat dan toksisitas FB diukur berdasarkan luas bercak nekrotik. Hasil penelitian menunjukkan bahwa agresivitas inokulum sporangia dan toksisitas FB bervariasi antar isolat P. capsici. Bercak nekrotik yang disebabkan oleh inokulum sporangia lebih luas (0,0–2.535,2 mm 2 ) dibandingkan dengan FB (0,7– 233,0 mm 2 ). Derajat agresivitas isolat dan toksisitas FB sangat dipengaruhi oleh asal isolat (P<0,0001), tetapi tidak dipengaruhi oleh varietas dan interaksi isolat×varietas (P>0,05). Derajat ketahanan pada varietas tahan Natar-1 diduga tidak berbasis genetik dan tidak berbeda nyata dari varietas rentan Petaling-1 sehingga kedua metode seleksi ketahanan tidak dapat dibedakan keefektifannya. Luas bercak nekrotik yang diinduksi oleh FB tidak berkorelasi nyata dengan yang ditimbulkan oleh inokulum sporangia (R 2 =0,002; P>0,05), sehingga secara umum FB P. capsici tidak dapat digunakan sebagai standar pengujian ketahanan lada. Oleh karena itu masih perlu dikembangkan metode inokulasi yang konsisten untuk seleksi dini ketahanan lada. Kata kunci: lada, penyakit busuk pangkal batang, Phytophthora capsici, agresivitas, filtrat biakan, seleksi dini ABSTRACT Resistant varieties are the most practical and effective means to control Phytophthora capsici, the pathogen of foot rot disease of black pepper. However, no resistant cultivars are available. Early selection of black pepper resistance can be performed in laboratory using P. capsici inocula or culture filtrate. The objective of this study was to compare P. capsici isolate aggressiveness with culture filtrate (CF) toxicity. The study was conducted from June until September 2009 at the Biochemistry Laboratory of the Indonesian Center for Agriculutral Biotechnology and Genetic Resources Research and Development and the Plant Pest and Disease Laboratory of the Indonesian Research Institute of Spice and Medicinal Crops. The study used 50 P. capsici isolates collected from various black pepper plantations during 1982 until 2009. Detached leaves of two black pepper cultivars, i.e. moderately resistant cv. Natar-1 and susceptible cv. Petaling-1, were inoculated with agar blocks containing sporangia or 20 µl CF of P. capsici. The experiments were designed as factorial experiments (2 cultivars × 50 isolates) under a randomized completely block design. Isolate aggressiveness and CF toxicity were measured based on the necrotic area of the inoculated leaves. The results from the two inoculation methods showed varying levels of aggressiveness and CF toxicity among isolates. Necrotic lesions incited by sporangial inoculum were more extensive (0.0-2,535.2 mm 2 ) than those induced by CF (0.7-233.0 mm 2 ). Degree of isolate aggressiveness and CF toxicity were significantly affected by origins of isolate (P<0.0001), but not by cultivar and isolate×cultivar interaction (P>0.05). Resistance degree in the moderately resistant cv. Natar-1 was presumably not genetically based and was not different to that in the susceptible cv. Petaling-1, and hence both selecting agents were unable to discriminate resistance level between the two cultivars. Necrotic sizes induced by CF did not well correlate with those incited by sporangial inocula (R 2 =0.002; P>0.05), indicating that CF is generally not suitable to be used as early selection agent of resistant plants. Therefore, further study is justified to find more reliable inoculation method for early detection of resistant black pepper. Key words: black pepper, foot rot disease, Phytophthora capsici, aggressiveness, culture filtrate, early selection in laboratory