91 PENDAHULUAN Bulai (downy mildew) pada tanaman jagung (Zea mays L.) merupakan salah satu penyakit yang sangat penting, dapat menyebabkan kehilangan hasil di daerah penghasil jagung, seperti Negara Asia, Afrika dan Amerika (CIMMYT, 2004). Penyakit ini dapat menyebabkan kehilangan hasil 40-60% dan dapat mencapai 100% untuk jagung manis di Filipina. Di India kehilangan hasil dapat mencapai 70% (Magil et al., 2006). Di Indonesia kerugian karena penyakit bulai sangat bervariasi Hubungan Antara Keragaman Gulma dengan Penyakit Bulai pada Jagung (Zea mays L.) Stadium Pertumbuhan Vegetatif I MADE SUDARMA 1) , I KETUT SUADA 2) , KETUT AYU YULIADHI 3) , DAN NI MADE PUSPAWATI 1) 1) Laboratorium Ilmu Penyakit Tumbuhan, 2) Laboratorium Bioteknologi Pertanian, dan 3) Laboratorium Gulma, Jurusan/Program Studi Agroekoteknologi Fakultas Pertanian Universitas Udayana Jl. PB. Sudirman Denpasar Bali, E-mail sudarma_made@ymail.com ABSTRACT Relationship between Weeds Diversity and Disease Incidence on Downy Mildew of Maize (Zea mays L.) in Vegetative Stage Weeds play an important role in the spread of downy mildew disease, competition, and lower crop yields of corn. Research using the survey method in 3 locations of Sanur village, Denpasar City. Each location is taken 5 samples, and each sample area was 1m 2 , which was arranged diagonally. For weeds identification was using reference i.e. Weeds of rice in Indonesia, and identification of downy mildew disease using reference i.e. Maize Disease: A Guide for Field Indentification and microscopically, while the weeds diversity was determined based on Diversity Index of Shannon-Wiener. Relationship between number of weed species and weed populations, and disease incidence were determined by regression and correlation analysis. Weed diversity on ecosystem of maize in vegetative stage is very low with the diversity index of 0.779, this means weed ecosystem unstable, and dominated by a particular weed species was Paspalum commersonii Lamk. (Fam. Poaceae/Graminaceae) of 23.3%, with the dominance index of 0.709. Relationship between the number of weed species and disease incidence of the downy mildew disease was highly significant (P<0,01), while the relationship between weed population and disease incidence of the downy mildew disease is not significant. Plant height and leaf number of maize was influenced by the type of weed, but weed populations only affect plant height of maize in vegetative stage (V3-V11). Keywords: weeds diversity, disease incidence, vegetative stage AGROTROP, 2(1): 91-99 (2012) ISSN: 2088-155X Fakultas Pertanian Universitas Udayana Denpasar Bali - Indonesia C pada tempat tertentu. Areal tanaman dapat menderita kerugian 90%, sehingga beresiko kegagalan yang cukup tinggi. Di Sulawesi Selatan penyakit dapat menyebabkan kehilangan hasil mencapai 60-90%, pada November 2000 sampai 2001 (Pakki et al., 2005). Penyakit bulai pada tanaman jagung dapat disebabkan oleh 10 spesies jamur dari tiga genus. Genus pertama yaitu Peronosclerospora, ada tujuh spesies yang telah dilaporkan menyebabkan penyakit bulai pada jagung yaitu P. maydis, P.