EFFECTS OF THE FUNGUS SETOSPHAERIA ROSTRATA ALONE AND IN COMBINATION WITH HERBICIDES ON RED SPRANGLETOP (LEPTOCHLOA CHINENSIS (L.) NEES) CONTROL IN DIRECT SEEDED RICE

摘要

Leptochloa chinensis is ranked as the second most important weed after Echinochloa crus-galli in direct seeded rice in Vietnam. In the Mekong delta where direct seeding practice is common, this weed is becoming abundant and causes concern to farmers. One of the reasons is that many rice herbicides in this country have low efficacy on this weed. Other factors affecting the high infestation of red sprangletop are the shortage of water during the onset of rainy season and secondly, remaining pockets of dry areas due to poorly levelled rice fields. An attempt was made to find out an alternative method for checking this emerging problem. The collaboration between CLRRI and the Australian Center for International Agricultural Research (ACIAR) from 1995 until now has resulted in finding out a fungus namely Setosphaeria rostrata which showed the high efficacy on this weed and safe to rice. Results from field experiments during the year of 1999-2000 showed that spraying of the spore concentration at 7 days after seedling transplanting (DAST) of weed had resulted in high dead plant percentage as compared to those at 14, 21 and 28 DAST. The percentages of infected leaves of Leptochloa chinensis sprayed at 7 DAST are 99.4; 100; 100 and 100% observed at 3, 7, 14 and 21 days after application (DAA). The application of spore concentration of Setosphaeria rostrata with 5 x 10~(11) spores/ha brought down weed density significantly as compared to check. The percentages of reduction were: 45.5%; 51.1% and 57.5% observed at 7, 14, and 55 DAA. Weed dry weight reduced 69% at 56 DAST. Under treatments of 10~(12) spores/ha, weed population and dry matter accumulation of Leptochloa chinensis tend to be reduced as compared to those in the treatments of 5 x 10~(11) spores/ha but the difference is not significant. The combination of fungi Setosphaeria rostrata and Cochliobolus lunatus; for controlling Echinochloa crus-galli, or with bispyribac-sodium herbicide had good efficacy on both grass species of Echinochloa crus-galli and Leptochloa chinensis respectively.