By Yeshi Wamishe, Extension Plant Pathologist, and Jarrod Hardke, Extension Rice Agronomist
If your rice crop has reached or is past green ring (internode elongation) it’s time to start scouting for sheath blight. Conditions that favor sheath blight are susceptible cultivars, high N rates, dense stands with thick canopies, and field history of sheath blight or aerial blight.
Sheath blight (Fig. 1) 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 cultivars if left unchecked.
Sheath blight disease is favored by warm temperature and high humidity. There is no question about the increase in temperature and humidity in the last two weeks, especially in the dense micro-environments in rice fields with dense canopy.
The disease usually starts on the sheath at the water line and then spreads vertically and later horizontally from plant to plant contact. 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 cultivars weakens plants and can cause lodging.
Managing sheath blight is accomplished by growing more resistant cultivars and following recommended cultural practices. Fungicides should be used when disease levels go beyond the recommended threshold. Excessive applications result in unnecessary expenses and potentially result in the development of fungicide resistance. It is important to note that fungicides for management of sheath blight are sprayed to control the disease (unlike smut or blast where they’re used for prevention).
While one fungicide application is recommended between panicle differentiation and early heading for sheath blight in Arkansas, additional applications may be required if the disease starts early and the environment continues to encourage disease progress. Therefore, consider scouting starting from green ring and continue until after heading. For susceptible cultivars (S or VS) 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. See examples in Table 1. The positive stop counts assume that scouting is over the bulk of the field not including the edges of the field and that weather conditions favor vertical development of the disease. If weather conditions are not favorable, then fungicide applications may be delayed in some cases where field judgment warrants.
Before you consider a fungicide application, remember not to create conditions 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.
Previous research indicated 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 (R. D. Cartwright). The maximum rates (Table 2) are preferred for better management of the disease.
For more information on scouting and fungicides, please follow the recommendations given by the University of Arkansas, Division of Agriculture at: http://www.uaex.edu/publications/pdf/mp192/mp192.pdf Chapter 11 and Pages 124-127. Also refer to MP 154-Arkansas Plant Disease Control Products Guide_2015 at: http://www.uaex.edu/publications/pdf/mp154year2016/pg52thru53rice.pdf. Please refer to fungicide labels for more information, restrictions and directions.