With rice fields finally growing and fields at permanent flood, rice farmers need to be on the lookout for diseases. There are many critical areas in rice production and disease management happens to be the one that farmers face at this point in the season.
Whether it is sheath blight, blast, smuts, or cercospora, scouting needs to start now to ensure that the rice fields stay healthy and yields can be maximized. It is also important to know what diseases the rice variety is susceptible to.
Sheath blight is the disease that most commonly affects rice fields in south Louisiana. This disease is caused by Rhizoctonia solani, a pathogen of both rice and soybeans. This disease is favored heavily by thick stands with a developed canopy, warm temperatures, high humidity, and thrives in fields with higher rates of nitrogen. This disease is characterized by large oval spots on the leaf sheaths and can have irregular spots on leaf blades.
Infection can start occurring as early as late-tillering; however, most vulnerable fields become infected once permanent flood has been established.
Lesions are formed just above the water line and the mycelium grows up the leaf sheath. The fungus can spread in the field by growing from tiller to tiller on an infected plant or across the surface of the water to adjacent plants. These lesions have grayish-white or light green centers with a brown or reddish-brown margin.
If not managed, sheath blight can be very detrimental to rice fields. If treating a field, the ideal time to treat with a fungicide is between boot and heading with good effect.
Blast (also called leaf blast, node blast, panicle blast, collar blast and rotten-neck blast – depending on the portion of the plant affected) is another disease that rice farmers have to scout for. Blast has been one of the most important diseases in Louisiana, causing severe yield losses to susceptible varieties under favorable conditions.
Leaf blast typically occurs between seedling and late tillering stages but can also show up later within the growing season. Symptoms start out as small white, gray or blue tinged spots that enlarge quickly under moist conditions. These spots then quickly move into either oval or diamond-shaped spots that have gray or white centers with narrow brown borders.
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During stem node blast, the infection turns black and becomes sunken or shriveled then turns gray as the plant ages. Rotten-neck blast shows up at the base of the panicle starting at the node.
The tissue turns brown and shrivels as well; this causes the stem to snap or lodge. Unfortunately, blast shows up in a variety of ways and shows up at different growth stages. With proper scouting and management of fields, blast can be controlled.
Kernel and false smuts are both late season diseases that affect the grains on the panicle. Kernel smut will appear just before maturity and will be a black mass of smut spores on the rice itself. Disease development is favored by high nitrogen rates.
False smut is similar to kernel smut; instead of being black in color, false smut is characterized by large orange to brown-green fruiting structures on one or more of the grains. It too happens right before maturity. Fungicide timing for both kernel and false smut must be applied at boot growth stage.
Cercospora is another disease that is commonly scouted for across rice fields in south Louisiana. Cercospora shows up as narrow brown leaf spots or lesions on the foliage. Leaf lesions tend to be reddish-brown in color and linear and will show up on both new leaves as well as old leaves. The grain infection appears as a brown discoloration and will affect yield and quality.
Both low and high nitrogen rates tend to increase the vulnerability for rice to become infected. To reduce the chances of cercospora occurring, planting earlier can potentially offer some relief. If a fungicide application is needed, the best time to apply a fungicide is between panicle differentiation and boot.
Please see the figure below for correct fungicide timings.