Proceedings of the Fifth Asian Regional Maize Workshop - Search ...

It took almost 10 years to develop the disease resistant double cross hybrid Tainan No. 11.
Tainan No. 11 was the first hybrid ever bred for downy mildew resistance in Taiwan. Its grain
yield is similar to that of Tainan No.5. Tainan selection No.10 was bred for slope-land areas,
it is tolerant to drought and maydis leaf blight, and its grain yield is about 4.5-5.0 t/ha. The
above mentioned corn hybrids are all early-maturing varieties, their profits are not as high as
that of rice, so these were cultivated in a limited area. By 1994, to effectively imp·lement the
government policy of "six-year diversification program for rice production", we need corn hybrids
whose profits can compare with those of rice. However, the climatic conditions in Taiwan are not
ideal for corn cultivation, since the annual average air temperature and planting time varies
between major corn growing areas. The planting time for the spring crop is from mid-January in
the South to early April in the North. For the fall crop, the crop is planted from mid-August in
the North to mid-Qctober in the South. The early growth stage of the spring crop coincides with
the rainy season, when heavy precipitation results in waterlogging which usually retards plant
growth. The latter part of the growing season for the fall crop is very windy, and the prevalent
seasonal wind frequently causes plants to lodge, reducing grain yield by up to 20-40%. Geographically,
Taiwan is located in the tropics and subtropics, where the warm weather and high humidity
all through the year favor the occurrence of several diseases and insect pests, i.e. downy mildew
P. sacchari, Maize dwarf mosaic virus (MDMV-B), stalk rot Pythium aphanidermatum, rust Puccinia
sorghi and the Asian corn borer Ostrinia furnaca7is. In these conditions, the maize breeding
program should aim at developing hybrid corn cultivars with high yielding ability, medium - late
maturity, disease and insect resistance, lodging and waterlogging tolerance, as well as being
suitable for mechanical harvest.
In developing a resistant hybrid corn, breeders are interested in identifying the characters
of resistance and the optimum stage of plant growth for selecting resistant segregates. In this
part of the paper I will discuss techniques to screen for germplasm resistant to downy mildew,
MDMV, stalk rot, the Asian corn borer, and strategies to breed hybrid corn which is suited to
Taiwan's conditions.
1. SCreening for resistance to downy mildew (P. sacchari).
OUtbreaks of downy mildew (P. sacchari) generally occur at a temperature range of 18-22C,
when heavy dew occurs in the early morning. The plants are infected from emergence up to when
they are one month old. After one month, the plants are free from attack by downy mildew. The
disease can be controlled by seed treatment with Ridomil (Chang, 1980), but breeding for a
resistant hybrid is still considered to be one of the most economic and simple means of control.
In developing resistant hybrids, resistant factors should be selected first. The selection
procedures for downy mildew resistant factors are described as follows (Chang and Wu, 1976):
1. Search for sugarcane seedlings infected with downy mildew. Plant these seedlings in a
separate plot to establish the downy mildew nursery. Since sugarcane is also a host
plant of downy mildew, the diseased sugarcane seedlings will provide a source of
inoculum to facilitate the selection for resistant germplasm.
2. Plant the corn seedlings in six rows, each 50 m long and 10 m apart.
3. Plant the corn germplasm to be screened in the spaces between the rows in early April or
mid OCtober.
4. Evaluate the screening materials one month after emergence. If a resistant inbred is
identified, it can be multiplied and its seed collected. If resistant plants are
identified in a segregating population, they can be selfed and the ears will be planted
during the next cropping season until the resistant inbred is identified.
2. SCreening for resistance to maize dwarf mosaic virus (MDMV).
Although MDMV can also infect johnson grass (SOrghum ha7epense (L.) Pers), sorghum (Sorghum
bico7or Rio), and sugarcane (Saccharum officinarum L.), MDMV is spread mainly by aphid vectors
(Chona and Seth, 1960; Sehgal, 1965; Tosic and Ford, 1974).
The corn plants can be infected by MDMV at any growing stage, but the earlier the plant is
infected, the greater is the reduction in grain yield. Tseng (1980) noted that corn plants
infected at the 3rd, 5th, 7th, 9th and 11th leaf stages showed a grain yield reduction of 25, 23,
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