As of the end of May 2013, most rice is either being flooded or is close. Sheath blight is one of the rice diseases we need to watch out starting a few weeks after flooding. Sheath blight occurs in almost every growing season but not necessarily in all rice fields. The rice-soybean rotation system increases the chance for the disease to prevail each year since both crops are affected by the same kind of pathogen resulting in sheath blight of rice and aerial blight of soybean. Sheath blight can cause significant yield loss in susceptible varieties if left unchecked. The disease is more prevalent in fields:
- planted with susceptible semi-dwarf long-grain varieties
- with higher rates of nitrogen fertilizers
- with dense stands and thick canopies (Fig. 1)
- seeded by broadcast
- with infected stubble from the previous year
Warm temperature and high humidity favors sheath blight development. Temperature and humidity begin to increase in June, and under such conditions, it is likely that the sheath blight pathogen will quickly grow and creep up the crop canopy (Fig. 1). The 2012 season was hot and dry, and sheath blight disease of rice in general was slight to moderate. In early-planted rice, only semi-dwarf long-grain cultivars required a fungicide application for sheath blight control. Late-planted rice outgrew the sheath blight that started moving after Tropical Storm Isaac. Although difficult to predict the weather, the likelihood of having this disease in 2013 looks high. The disease starts on the sheath from a compact fungal mass at the water line and then spreads vertically and horizontally (Fig. 2).
To reference former rice pathologists Drs. Rick Cartwright and Fleet Lee, “there is no silver bullet for sheath blight control”. Effective means of managing this disease are growing more tolerant varieties, following recommended cultural practices, and applying fungicides when disease levels go beyond the threshold. When sheath blight affects the upper two leaves before grain fill, it has more impact on yield than any other time in crop development. Sheath blight, in some varieties, weakens the stature of the plants and can cause lodging.
Before you consider a fungicide application, remember not to generate a situation for the development of fungicide-resistant strains of the sheath blight fungus. For this reason, fungicide applications in the absence of sheath blight disease are not recommended. Note that fungicides for management of sheath blight are sprayed to control the disease, unlike those for kernel smut and false smut which are used to suppress and likely to prevent disease from developing in the first place.
Previous research by Dr. Rick Cartwright indicates that Stratego at 16 oz/A will provide14-17 days of sheath blight control, while the 19 oz/A rate about 21-24 days of control. Quadris can provide 10-14 days of control at 6.4 oz/A, 21 days of control at 9 oz/A, and at least 28 days of control at 12.8 oz/A. Research has shown that azoxystrobin (Quadris) is considered somewhat more effective against sheath blight than trifloxystrobin (GEM). However, the difference is considered slight.
According to DD50 enrollment information provided by Dr. Jarrod Hardke, CL XL745, Roy J, Jupiter, CL151, XL753, CL111, CL XL729, and CL152 are the most widely planted cultivars in 2013 in Arkansas. For sheath blight threshold levels for these cultivars see Table 1 and for the recommended fungicides see Table 2.
In order to make timely decisions for management of sheath blight disease in rice, frequent scouting (at least once a week) is needed to effectively monitor disease progress.
While fungicide applications are recommended between panicle differentiation and early heading, additional applications may be required if the disease starts early and the environment encourages disease progress. That is why it is important to scout early and often from early maturity until a few days before heading.
Sheath blight thresholds (when to treat):
Begin scouting for sheath blight about 1/2” internode elongation (jointing) and pay very close attention between 7-14 days after jointing.
For susceptible cultivars (“S” or “VS” in Table 1), treat with a fungicide when 35% of stops are positive for sheath blight.
For moderately susceptible cultivars (“MS”), treat with a fungicide when 50% of stops are positive for sheath blight.
The stop method assumes that scouting is over the bulk of the field and that weather conditions favor vertical development of sheath blight. If weather conditions are not favorable, then fungicide applications may be delayed in some cases where field judgment warrants.
In moderately resistant cultivars (“MR” – none listed here), if the infection is limited to the lower half of the stem and the two upper leaves are clean, fungicide application can be delayed.
For more information on scouting and fungicides, please follow the recommendations given by the University of Arkansas, Division of Agriculture at http://www.aragriculture.org/diseases/rice/sheathblight.htm. Also refer to MP 154-Arkansas Plant Disease Control Products Guide_2013 at http://www.uaex.edu/Other_Areas/publications/PDF/MP154/RiceFung.pdf. Please refer to fungicide labels for more information, restrictions and directions.