Downy mildew (DM) is one of the most destructive diseases of maize in the humid sub-tropical and
tropical regions of Asia. The most effective and efficient way of managing losses due to DM is to develop
durable host-plant resistance in Asian maize across different DM pathogens. We assessed resistance in
released DM (P. zeae) resistant CIMMYT maize lines (CMLs) against Peronosclerospora sorghi and Perenosclerospora
heteropogoni to develop DM resistant (DMR) maize germplasm with a broad-spectrum of
resistance, and identify and further improve new sources of DMR maize germplasm for use in regional
breeding programs targeting DM-prone environments. Data indicated that out of 19 DM resistant CMLs,
four (CML-433, 465, 466 and 472) were resistant and 15 susceptible to P. sorghi. Screening of new inbred
lines from CIMMYT-Asia (CA) resulted in identification of a set of yellow maize lines with good level of
DM resistance (disease score e 0.0e25%). However, the high quality protein maize (QPM) lines derived
from CIMMYT Population 61 and bi-parental pedigree crosses between elite QPM lines showed very poor
resistance to DM. Progenies of the bi-parental population developed by crossing DMR (P. sorghi) CA lines
showed further enhanced levels of resistance to both P. sorghi and P. heteropogoni. The study resulted in
identification of 10 new inbred lines, in addition to the 4 released CMLs, offering consistent high
resistance to both the DM pathogens. Promising bi-parental populations (13) with combined DM
resistance and other desirable agronomic traits were identified for the extraction of a new generation of
DMR lines with broad-spectrum DM resistance and other commercial traits. Comparison of genotypic
response under different screening methods/DM species clearly indicated that the glasshouse-based
sandwich method of screening against P. sorghi was the best method for identifying promising genotypes
with a high level of DM resistance that were also resistant to DM under field conditions for both
the species. However, the highly controlled conditions used for sandwich screening, which is most
favourable for disease development, resulted in high disease pressure potentially masking the identification
of genotypes with a moderate level of DM resistance.