In 2006, the fungus Cercospora janseana, which causes narrow brown leaf spot, did significant damage to the rice crop in south Louisiana. This disease involves linear, reddish- brown spots that usually appear near heading. These spots are slow to develop, taking up to 30 days from infection. Both young and old leaves are susceptible. Seed heads can become infected, causing premature ripening and unfilled grain.
The symptoms of narrow brown leaf spot can be confused with rotten neck and panicle blast lesions. Cercospora symptoms on panicle necks are usually darker brown and develop in the internodal area of the neck. Sheaths and glumes can be infected, causing significant discoloration and necrosis. On sheaths, the disease is referred to as “net blotch” because of the brown sheath cell walls and the tan-to-yellow intracellular areas that form a net-like pattern. Grain infection appears as a diffuse brown discoloration. The disease can also be severe on the second crop.
Rice breeders have found resistance to narrow brown leaf spot, but new races of the pathogen develop rapidly. Low nitrogen appears to favor disease development. Fungicides used to reduce other diseases may reduce narrow brown leaf spot. Propiconazole (Tilt, PropiMax, Bumper, Stratego and Quilt) has the best activity of the labeled fungicides. A limited number of studies suggest the best time to apply fungicides is at boot growth stage (a 4-inch panicle in the boot).
The rate of propiconazole needed to control the disease is approximately equivalent to 6 ounces of Tilt, PropiMax, Bumper, 19 ounces of Stratego or 21 ounces of Quilt. This rate for Tilt, PropiMax and Bumper is very weak against sheath blight and the 21 ounces of Quilt only has 6 ounces of Quadris in it, which would also be weak against sheath blight. These fungicides will need added sheath blight fungicides to control sheath blight. The 1 9 ounces of Stratego has 6 ounces of propiconazole and a full rate of Gem and should work well against sheath blight.
There is no recommended scouting method for Cercospora, except to look at the lower leaves for the narrow brown leaf spot lesions. If the disease is present, there is no treatment threshold for spraying. Fungicides will have to be applied as a preventative. In 2006, it was unusually wet, and the pathogen had extensively over-wintered on rice in crawfish fields, giving it a head start.