Pathogenicity of Fusarium oxysporum causes wilt disease in shallots at peat soils in West Kalimantan. Fusarium wilt is a disease that can attack all phases of shallots growth. This disease develops easily in peatlands due to the low pH, high humidity and high carbon availability. The aim of this study was to examine the level of pathogenicity of Fusarium oxysporum isolated from shallots from mineral soils in Bengkayang Regency, West Kalimantan. Experiment research method used a completely randomized design consisting of 5 treatments with concentrations of F. oxysporum spores (105 spores ml-1, 107 spores ml-1, 109 spores ml-1, 1011 spores ml-1, and control (whitout F. oxysporum inoculation). Each treatment was repeated 6 times. Inoculation of the F. oxysporum was carried out by spraying evenly on the growing media at a dose of 100 ml of conidia suspension per 1 kg of growing media before planting. The variables observed were the incubation period of the disease (days), the incidence of disease (%), disease severity (%), the rate of infection and the value of area under the disease progress curve (AUDPC). The results showed that pathogenicity of concentration of 1011 spores ml-1 caused the fastest incubation period (12.57 days after planting) and the highest disease incidence (90%), significantly different from 105 spores ml-1 and 107 spores ml-1, but not significant compared to with a concentration of 109 spores ml-1. The concentration of 1011 spores ml-1 also caused the highest disease severity, 76.88%, the highest infection rate were 6.54% per day during the observation period. The value of area under the disease progress curve of the 1011 spore ml-1 concentration was the highest (216.6% per day) compared to other concentration levels. The concentration of 109 spores ml-1 was sufficient to accelerate the incubation period of fusarium wilt disease in shallots if planted in peat soil, whereas in the control treatment there wasno Fusarium wilt disease attack. Key words: disease incidance, disease progress curve, incubation period, infection rate, peat soilABSTRAKLayu fusarium merupakan penyakit dapat menyerang seluruh fase pertumbuhan tanaman bawang merah. Penyakit ini berkembang dengan mudah di lahan gambut karena pH rendah, kelembaban tinggi dan tingginya ketersediaan karbon. Penelitian bertujuan menguji tingkat patogenitas Fusarium oxysporum yang diisolasi dari bawang merah yang berasal dari tanah mineral di Kabupaten Bengkayang Kalimantan Barat. Percobaan menggunakan rancangan acak lengkap terdiri dari 5 taraf perlakuan konsentrasi spora F. oxysporum (105 spora ml-1, 107 spora ml-1, 109 spora ml-1, 1011 spora ml-1, dan kontrol (tanpa inokulasi F. oxysporum). Setiap perlakuan di ulang 6 kali. Inokulasi jamur F. oxysporum dilakukan dengan cara disemprotkan secara merata pada media tanam dengan dosis 100 ml suspensi konidia per 1kg media tanam sebelum penanaman. Variabel yang diamati yaitu periode inkubasi penyakit (hari), insiden serangan penyakit (%), keparahan penyakit (%), laju inveksi dan nilai kurva perkembangan penyakit layu fusarium (AUDPC). Hasil penelitian menunjukkan bahwa konsentrasi 1011 spora ml-1 menyebabkan masa inkubasi tercepat (12,57 hari setelah tanam) serta insiden penyakit tertinggi (90%), berbeda secara nyata dengan 105 spora ml-1dan 107 spora ml-1, tetapi tidak signifikan dibandingkan dengan konsentrasi 109 spora ml-1. Konsentrasi 1011 spora ml-1 juga menyebabkankan tingkat keparahan penyakit tertinggi, 76,88%, tingkat infeksi terbesar (6,54% per hari) sepanjang periode pengamatan. Nilai area di bawah kurva kemajuan penyakit dari konsentrasi 1011 spora ml-1adalah yang tertinggi (216,6% per hari) dibandingkan dengan tingkat konsentrasi lainnya. Namun, konsentrasi 109 spora ml-1 cukup untuk mempercepat masa inkubasi penyakit layu fusarium pada bawang merah, sedangkan pada perlakuan kontrol (tanpa inokulasi jamur F. oxysporum) tidak terjadi serangan penyakit layu Fusarium. Kata Kunci: insiden penyakit, kurva perkembangan penyakit, periode inkubasi, laju infeksi, tanah gambut
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