Downy mildew (DM) caused by the fungus Peronosclerospora sorghi is one of the most serious diseases reducing maize (Zea mays L.) production in East Timor. We have conducted three trials of DM resistant (R) and non-DMR source populations in three locations; Loes, Dili and Lospalos. Materials were developed by using DMR-ESR-Y and MO5-5-RUVT-EE (TZEE-Y) (IITA) and Suwan 5 (Thailand). Three sets of random mating, incomplete diallel crosses and parent populations were planted with three replications in one meter row with RCBD. Data assessment of DM infection was rated based on 1 – 9 rating score (1 = highly tolerant and 9 = highly susceptible) from 2 and 4 weeks before flowering. In Comoro, Dili, 32 genotypes exhibited very low DM infection with coefficient of variance and mean 17.2% and 3.9, respectively. BISI-2 (Indonesian hybrid) and TH2 (Timor Hybrid 2 crossed from two DMR OPVs, Indonesian Arjuna and Suwan 5) showed outstanding commercial value (CV). Twelve cultivars of MO5 were varied in CV. At Loes Research Station, genotypes showed high variation in DM infection. Average heterotic effect of 36 F1 crosses for DM tolerance was 29.5%. Mean parent value and F1 indicated negative correlation for DM infection. The result showed that to breed high resistance is difficult. However, breeding for host tolerance with polygenic system with a threshold nature is possible.

• Claudino Ninas Nabais(International Agricultural Research Institute (IARI), Kyungpook National University) Corresponding author
• Soon-Kwon Kim(International Agricultural Research Institute (IARI), Kyungpook National University)
• Lourenco Fontes Borges(Ministry of Agriculture and Fisheries)
• Albertino Jeronimo(Ministry of Agriculture and Fisheries)