Daegu, South Korea

Kyungpook National University and the International Corn Foundation

Severe epidemics of downy mildew (Perosclerospora sorghi) on maize in Cambodia, East Timor and Vietnam

--Kim, SK; Yoon, NM; Kim, HJ; Kim, YB; Chhay, N; Kim, SM; Oeun, KS; Bora, P; Glaudino, N; Fontes, L.; Tam, TT; Cho, MC

Downy mildew (Perosclerospora sorghi) is still considered the most damaging disease of maize (Zea mays L.) in South Asia. Breeding works initiated in Thailand four decade ago. Since the early of 1990s, downy mildew (DM) has been a minor problem in Thailand, Indonesia, Philippines and Taiwan. Breeding for resistance materials and the uses of chemicals such as Ridomil have played a catalytic role to reduce the spread of the disease in the region. However, the same disease produces epidemics in Cambodia, East Timor and Vietnam, recently. The study of DM was carried out in East Timor and Ben Tre Province in Vietnam for three years, respectively and at the ICF/Cambodia Banteay Dek Agricultural Research Station, for two years (2005-2006). This paper reports the results of DM infections at the station and farmers’ fields.

In East Timor, severe infections of DM were observed in farmers’ fields and the Lapos station of the CIMMYT/Australia maize trials in Feb., 2003. A Thailand open-pollinated variety (OPV), Suwan 1 was selected. Seeds of DM resistance (R) were introduced from Thailand and multiplied with urgency. Additional DMR materials of IITA, Nigeria and CIMMYT, Mexico were introduced and tested. Among the IITA DMR materials, TZDMR-ESR-Y appears to be the best. Using Suwan 5, Kalinga (Indonesia), TZDMR-ESR, the project has developed several DMR variety crosses. Both OPV and variety crosses are being tested at research stations and farmers’ fields. Only DMR materials shall be recommended to East Timor.

In Ben Tre Province of Vietnam (the first province of Mekong Delta), DMR materials from IITA and Thailand have tested for three years (October, 2002) with the assumption that DM would be the key biotic constraint of maize cultivation in the country. However, the first observation of DM occurred in March, 2006. DM appears to be widely presence in the country. Introduced waxy hybrids from Thailand and locally bred field corn hybrids were found to be highly susceptible. Five DMR OPVs were selected. The best known DMR OPV, Suwan 1 showed an unknown black ear rot. Maize programs in Vietnam must focus on DMR breeding to block further spread of DM nation wide.

In Cambodia, DM looks like No.1 production constraint of maize cultivation nation wide. Severe epidemics of DM infection were observed from several farmers’ fields in Phnom Penh area in August, 2005. The program has focused on to breed only DMR materials for Cambodia. Ten different plantings have been made to screen DMR and segregating materials using the ICF/Cambodia Banteay Dek Agricultural Research Station. The station was established by the Government of Hungary 15 years ago. Among the 50 materials tested, four DMR OPVs showed acceptable level of tolerance. They are Suwan 5 (coded as KC35), Suwan 1 (coded as KC6) from Thailand and TZDMR-ESR-Y (coded as KC25) and TZDMR-LSR-Y (coded as KC4) from IITA. A CIMMYT DMR conversion, EV28-DMR and several other DMR materials were segregating for resistance. DMR genes are being incorporating into farmers preferred local waxy materials.

Breeding DMR cultivars are considered the most sustainable, economic and environmentally friendly way of tackling of DM problem in the three countries. However, a serious misunderstanding on the host expression of DMR is that tolerance plants with the early stage of infection on the lower leaves are considered as susceptible cultivar because of the DM symptoms. This has been the most common general misunderstanding on horizontal resistance or tolerance with quantitatively inherited gene. Similar misunderstanding of host tolerance was reported on Puccinia rusts (sorghi and polysora), maize streak virus (MSV), and parasitic weed, Striga species (hermonthica, asiatica and aspera). The senior author has developed the “co-survival tolerance principle” as the most durable, sustainable, and pro-environmental host plant resistance breeding approach in crops and other living organisms. To avoid this misunderstanding on DMR plants, breeding must be done under the high and uniform infection of the disease either under natural or artificial conditions. Individual plants with onto-genetic change of tolerance upon plant growth must be selected. Otherwise, infection escaped susceptible plants may be selected. A policy adoption of co-survival tolerance DMR cultivar release to farmers might be the best way to cut down DM spread in the region. Tolerant plants give 5% to the disease for their evolutional survive in nature. Otherwise, a mutant race might be occurred and over-come the high resistance (100% control). QTL and genomic approach seems to be a similar principle. GMO products also would be developed under the same co-survival tolerance principle. In nature all living organisms must be co-survived and co-evolved. This principle has been highly misunderstood and lacked in host plant resistance breeding.