Management Options - Rice Blast

Varietal resistance

The use of resistant varieties is the most practical and economical approach to control blast. However, its use has not been completely successful due to the presence of lineages (that may consist of different physiologic races) overcoming host resistance. Effective resistance can be achieved by combining into the same cultivar different race-specific genes and genes conferring quantitative resistance. Another method is by deploying resistance genes in mixed plant populations. Recent studies indicated that use of cultivar mixture is an effective tool in blast management. IRRI scientists introduced the practice of interplanting glutinous rice varieties with blast-resistant hybrid varieties in Yunnan province, China. Blast caused great yield loss on traditional glutinous rice varieties and farmers were spraying fungicides for up to seven times. Interplanting has prevented the fungus from continuous build-up of inoculum that had previously occurred in the monoculture fields of the glutinous varieties.

Cultural Practices

Studies suggest that the application of fertilizer, specifically nitrogen, based on the actual requirements of the crop may control blast epidemics while achieving high yield. An oversupply of nitrogen will result in luxuriant crop growth that would favor the disease while a deficiency will result in low yield.

Considering that silica increases the resistance of varieties to blast, soils known to be low in plant-available silicon should be amended with calcium silicate slag.

Water management practices that lessen the likelihood of stress aid in the control of blast in rainfed areas. Flooding the field, when possible, can reduce the severity of blast.

In tropical upland rice, crops sown early after the onset of the rainy season are more likely to escape blast infection than late-sown crops. Early sowing allows escape from the build-up of inoculum originating from neighboring farms.

Biological Control

Avirulent isolates of P. grisea and the non-rice pathogen Bipolaris sorokiniana were found to reduce blast when sprayed on plants. Since the fungus has a wide distribution, it is believed that avirulent isolates, which can be used as biological control agents, can be easily found in various rice-growing areas. Recently, a study showed that a specific pheromone produced by Saccharomyces cerevisiae can minimize infection by inhibiting production of appressorium, a specialized cell in the fungus that allows the pathogen to penetrate host plant cells.

Chemical Control

Studies at IRRI several years ago showed that a number of chemicals such as benomyl, edifenphos and mancozeb were found effective against leaf and neck blasts. Among the current blasticides are probenazole, tricyclazole, pyroquilon and fthalide that allow blast control with fewer applications and provide systemic control. Since 1995, pyroquilon and tricyclazole fungicides have been observed effective as seed treatment that may result in long term blast control. If seed treatments are effective, fungicides may be economical in developing countries. However, seed treatment may be of little use where eradication of seedborne inoculum would have minimal effect on the amount of inoculum. Seedborne inoculum can be effectively eliminated with benomyl slurry at 0.3% by seed weight.

Probenazole, the leading blasticide in Japan, is claimed to control rice blast via activation of natural defense mechanisms in susceptible rice plants. It has a weak direct antifungal activity but treated plants are able to mount a rapid and effective resistance response to infection by the pathogen. Other chemicals reported to induce resistance to blast and elicit similar biochemical responses in rice include N-cyanomethyl-2-chloroisonicotinamide and dichlorocyclopropanes.