By Yeshi Wamishe, Extension Rice Pathologist
As we prepare to start the forthcoming season, planting fields with appropriate varieties is key to managing rice blast disease. Rice blast disease can be well managed by planting relatively resistant varieties in fields with a history of blast. Planting blast-susceptible varieties in blast-prone fields can result in severe blast disease under favorable weather conditions. Fields are more blast-prone if they are over fertilized with nitrogen, contain soil types that are difficult to maintain a deep permanent flood, or are surrounded by thick tree lines, particularly on the east side, that favors an extended morning dew period. Fields low in fertility, particularly potash, or are at river bottoms where dew periods are likely prolonged can lead to the development of blast. Environmental conditions favorable for blast usually are frequent light rains that provide extended leaf wetness, extended overcast days with a light breeze that minimizes leaf dryness, and warmer days with cooler nights to favor dew formation.
Rice blast caused by Magnaporthe oryzae (synonym of Pyricularia oryzae) has been ranked among the most important rice diseases. The fungus is known to cause lesions on leaves, stems, peduncles (necks), panicles, seeds, and even roots. Rice breeders and pathologists spend years developing resistant varieties for blast. Chemical products are also effective to some level in well managed fields if applied with recommended rates, timings, frequencies, and coverage of the canopy. Regardless of these efforts, the rice crop remains vulnerable to blast disease because neither breeding nor chemicals fully overcome the pathogen’s ability to rapidly adapt or change its genetics for survival.
Some races of the pathogen prevail and may be detected every season, while other races occur rarely (Wang, Jia, Wamishe et al., 2017 unpublished data). More than one race may prevail in a field, which increases the complexity of the host-pathogen relationships (Table 1). Some rice varieties grown in Arkansas are known to have useful resistance genes for the common races, but not all resistance genes provide wide spectrum protection to all races (Fig. 1). A variety’s resistance may vary with changes in race types in a particular season, the types and number of resistance genes within the variety, field management, and favorability of the environment/weather for disease development.
In 2016, rice blast in Arkansas was reported around the 3rd week of June. Up to that time, the disease was not a big issue and was slow to develop due to the hot and dry weather. However, when the weeklong rain occurred in late July the situation changed and some blast-prone fields planted with susceptible rice showed various levels of blast disease. The rain even washed down spores likely from flag leaf collars towards the middle of stems causing joint blast (Fig.2a). Unlike neck blast that often blanks panicles (Fig. 2b), joint blast weakened the stems and encouraged lodging.
The asexual spores of the blast fungus can survive on seeds and can move to new locations with seeds. The spores can overwinter in rice residues during mild winters and can also be carried by wind over long distances. The pathogen is very versatile when it comes to survival and widely adapted to different environments and weather conditions. If seed of a susceptible variety is planted in a field with a strong history of blast disease, then the use of azoxystrobin (Dynasty®) fungicide at the labeled rate for seedborne blast should be considered. However, seed treatment does not warrant protection from late season blast.
GENERAL TIPS TO MANAGE RICE BLAST DISEASE
- Weather allowing, plant early to avoid the likelihood of heavy blast pressure later in the season.
- Lower the risk by diseases in general by planting over a range of time during the early planting window. i.e., spread disease risk by spreading out planting dates.
- Plant resistant varieties in blast-prone fields.
- Avoid planting blast susceptible varieties in fields surrounded by heavy tree lines
- Use the recommended nitrogen fertilizer rates.
- Know your soil types and grow rice in fields where a deep flood can be maintained. Maintain a consistent > 4 inch flood especially on susceptible varieties at all times until drainage for harvest.
- Scout fields starting early in the season (June) for leaf blast symptoms on susceptible and moderately susceptible varieties.
- If leaf blast is seen, be prepared to apply preventative fungicides
- Usually fungicides are applied on susceptible varieties at late boot stage to 10% heading followed by a second application at 60-90% heading.
- Remember that heading is not the same as “headed”. During both the 1st and the 2nd heading applications, the necks should still be in the boot.
- Fungicide application after the necks are out of the boot is already too late.
- Waiting too late to apply the fungicides is the most common mistake that leads to fungicide failure to reduce neck blast.
- Refer to the current label of chemical products for rates. Labels are the rule.
Wang, Jia, Wamishe et al., 2017 unpublished data from a project funded in part by Agriculture and Food Research Initiative Competitive Grant number 2013-68004-20378 (Blast Integrated Project, “BIP”) from the USDA National Institute of Food and Agriculture and USDA Agricultural Research Service.