Four new tropical lowland downy mildew resistant maize populations
--C. De Leon, G. Granados and R. N. Wedderburn

Four genetically broad based downy mildew resistant (DMR) maize populations have been developed by CIMMYT-Asian Regional Maize Program (ARMP) based in Thailand. These are Pops. 100 (Early White DMR), 145 (Early Yellow DMR), 300 (Late White DMR) and 345 (Late Yellow DMR) adapted to tropical lowlands. Until now, the two white and the two yellow populations have undergone five and four cycles of S1-S2 recurrent selection, respectively.

Populations were developed in 1985 by crossing several selected white and yellow maize cultivars in the early (90-95 days to harvest) and late (110-115 days to harvest) maturity groups. Philippine DMR Comp. 1 and Philippine DMR Comp. 2 were used as donors of DMR in the yellow and white populations. The crosses were made in isolated crossing blocks at Suwan Farm, Thailand (14.5^oN, 101^oE, 360 masl). After random mating twice, a S1-S2 recurrent selection program was initiated. During each cycle of selection, approx. 500 S1 families were generated from 500 bulk pollinated ears at Suwan in the dry winter season under no downy mildew conditions and evaluated in DM nurseries at Suwan and the Univ. Southern Mindanao Agric. Res. Center (USMARC) in the Philippines (7^o15'N, 124^o50'E, 300 masl) during the early planting season (April-June). Selected plants in the superior 60% of the S1 progenies were self-pollinated at both Suwan and USMARC. Seeds of approx. 500 S2 ears selected at both places were planted again in disease nurseries at these two locations during the late season (July-Oct.). Screening for DM reaction was done during the early and late planting seasons when disease incidence is high. Information on S2 progeny performance in both nurseries was used to bulk pollinate among 60% of the selected progenies at Suwan. Progenies were selected mostly for desirable agronomic characters and disease resistance.

In the summer of 1990, bulks of Cycles 0, C1, C2 and C3 of the four populations were evaluated at Suwan and USMARC under disease-free conditions for grain yield and days to 50% silking. The response for DMR in the four populations was measured by planting bulks of Cycles 0, 1, 2 and 3 in DM nurseries at two locations.

Data showed that grain yields were higher and plant height lower at Suwan than at USMARC. Throughout the tests CV's were low for yield and days to 50% silking, but high for DM infection. Early populations yielded less and flowered earlier than the late ones (Table 1); in all populations C3's yielded higher than the C0's with a highly significant gain across populations. Highly significant decreases in DM infection were recorded with averages of 63.1% for C0 and 18.8% for C3 of all populations with a highly significant progress from selection for DMR averaging -11% per cycle across populations. During the selection cycles, the S1 progenies showed greater DM infection at USMARC than at Suwan and infection values at both locations were highly correlated.

These new broad based DMR populations can be either directly released by national programs, or used as sources of resistance in breeding programs. Breeder seedstocks are available from CIMMYT-ARMP, P.O. Box 9-188, Bangkok 10900, Thailand.

Table 1. Means of three traits in different cycles of selection in EW-DMR, EY-DMR, LW-DMR, and LY-DMR populations, selected for downy mildew resistance and other agronomic traits.

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