By Yeshi Wamishe, Extension Rice Pathologist
Rice blast disease can be managed by planting relatively resistant varieties in fields with a history of blast. Current commercial rice varieties grown in Arkansas range from resistant to very susceptible to blast disease (Table 1). Planting blast-susceptible varieties in blast-prone fields can result in near 100 percent yield loss under favorable weather conditions. Therefore, scout and be vigilant to make timely decisions and to take proper action. Note that frequency of scouting and management decisions can vary based on the resistance levels of rice varieties, field history, and field management.
Fields with a history of blast may be categorized as “blast-prone fields” which includes field that: have been over fertilized with nitrogen, are low in fertility particularly potash, contain soil types that are difficult to maintain a deep permanent flood or surrounded by thick tree lines, particularly on the east side, and favor extended morning dew period (entire field or spots within field). Fields lying at river bottoms where the dew period is likely prolonged can also favor rice blast disease. Dew period or leaf wetness for more than 9 hours are favorable for sporulation and further blast disease development. Hence, dew, fog, shade (tree lines), frequent light rains, and overcast skies are included in the list of favorable conditions for blast disease development.
Rice blast caused by the fungus known as Magnaporthe oryzae (synonym of Pyricularia oryzae) has been ranked among the most important rice diseases. The disease prevails in a wide range of environments and the pathogen is wide reaching globally. Blast disease can spread with wind-borne spores and, therefore, is fit to travel miles from region to region or field to field. The fungus is versatile and produces different races. In some fields, more than one race can prevail increasing the complexity of the host-pathogen relationships. The fungus is known to infect rice plants starting at the seedling stage and damage leaves (Figure 1), stems (Figure 2), peduncles (necks) (Figure 3), panicles (Figure 4), seeds, and even roots.
The rice crop remains vulnerable to blast disease regardless of the breeding and screening efforts of breeders and pathologists. Hence, scouting all rice in blast-prone fields is highly recommended. A new race may turn a resistant variety to a susceptible anytime. So far, neither breeding nor fungicides have fully overcome the pathogen’s ability to rapidly adapt or change its genetics for survival. While planting resistant varieties is the best management option, chemical products have been important and effective to a certain degree in rice production systems in Arkansas. To benefit from fungicide applications, fields need to be well managed and applications need to follow recommended rates, frequencies, have adequate canopy coverage and be done at the correct time.
Absence or not being able to detect leaf blast early in the season in a susceptible cultivar does not equate to the absence of neck and/or panicle blast later in the season. Fields where leaf blast is detected early in the season can be planned for fungicide protection since the probability is higher for neck or panicle blast to follow. We need to note that rice blast disease is generally more severe in late planted rice. Weather conditions are also a big factor in making decisions on fungicide applications. However, we need to be diligent in watching weather conditions similar to 2016; several fields did not need fungicide applications since the summer stayed hot and dry until a week or so rain in August that started at the end of July. After the rain, several fields were reported to have blast disease and it was already too late for fungicide applications.
Knowing that different rice cultivars respond differently to leaf blast and neck blast related to developmental stages is key to management. A cultivar can show little or no leaf blast early in the season but can be devastated by neck blast later, as in Roy J. It may be very susceptible for leaf blast early and responds moderately to neck blast similar to Jupiter; or can be susceptible to both leaf blast and neck blast, similar to CL151. Unfortunately, we do not have full information on the responses of Arkansas-grown commercial varieties to blast at various developmental stages.
Tips to manage blast susceptible rice already planted in blast-prone fields
- Avoid using excessive/ beyond the recommended nitrogen fertilizer rates.
- Know your soil types and manage the water in your rice fields to keep deep flood consistently. Recommended flood depth is > 4 inch until drainage for harvest.
- Scout fields starting early in the season for leaf blast on susceptible and moderately susceptible varieties. When scouting, pay attention to leaf blast lesion symptoms. Initial lesion symptoms may not be typical diamond shape (Figure 1) and may not have the ashy centers (Figure 5). When symptoms are not clear enough or are too confusing for correct identification, flag the area and re-visit that spot at least once a week for confirmation (Figure 6).
*Instead of walking all across the field, it is easier to scout for leaf blast in field spots close to tree lines (east side preferred), rice on the levees, and drought-stressed field edges. Note that blast disease can be found in open fields with no trees around if the crop is drought stressed.
4. If leaf blast is seen, raising the flood depth could be enough to suppress it. In cases of leaf burndown, spot fungicide application may be used. However, be prepared to apply preventative fungicides for later protection from neck and panicle blast. Quadris, Stratego, GEM RC, and Quilt Xcel are the listed fungicides for use in Arkansas for blast protection. For better protection higher rates are recommended.
*Recommended timing for the first fungicide application to manage neck blast on susceptible varieties is from late boot stage to 10% head out followed by a second application at 60% to 90% head out but preferably 50% to 75% (not to miss the time window).
**Note that the first application is to protect the main tillers and the second application is mainly to protect the secondary tillers. In a field where there is no uniformity in growth stages, the second application is even more encouraged. Uniform fields in developmental stages benefit more from fungicide applications.
***If a single application is desired, timing is more crucial. A 30 % to 50% head out would work better. However, expect lower protection compared to a secondary application.
****Note that “head out” for both first and second timings means, the necks should still be in boots. Remember that heading is not the same as “headed”. If the necks are fully out of the boots, it is already too late to benefit from fungicide applications. Waiting too long to apply the fungicides is the most common mistake that leads to fungicide failure and yield loss from blast.
- Watch for weather conditions that may favor blast disease. Weather information can serve as an additional tool to plan and make application decisions.
- If you have grown susceptible varieties in a field that should be drained for straighthead or hydrogen sulfide toxicity or water weevils and the field is blast-prone, be careful as to how low you drain and for how long you may keep the ground dry.
- Refer to MP 154 on fungicides and the current label of chemical products for rates. Labels are the rule.