Certain rice pathogens that survive in soil are waiting for the right conditions to start their activities. Sheath blight is one of the major diseases of rice in Arkansas that is caused by a soil pathogen, and flooding is one of those right conditions for the growth of the pathogenic fungus. The fungus has several hosts including soybean and corn, among others. It causes aerial blight in soybean during prolonged periods of high humidity and high temperatures. Similar conditions favor sheath blight disease in rice. In rice/soybean (or other host crop) rotation system, the pathogen builds up and survives in the soil from both crops of the previous seasons. The fungus mostly survives as “sclerotia” (tiny masses of fungal structure called “mycelia”) (Picture 1) or infected host residues.
When rice is flooded, the sclerotia or the infected plant residue float to the surface. They can also move around by flood water. The floating fungal structures coming in contact with the growing rice infect the sheath at or just above the waterline and spread throughout the plant tissue. The infection progresses upward to the canopy and sideways to neighboring plants (Picture 2). Infection can start early from tillering and continue throughout the season. Therefore, sheath blight is the most common and most important disease of rice we need to scout and manage. Scouting for sheath blight is recommended from green ring to pre-heading. Symptoms of sheath blight may be confused with other diseases such as aggregate sheath spot, bordered sheath spot, and others that have not required fungicide application to date. Therefore, correct diagnosis of the disease before fungicide application is important.
Management: The extent of damage would be dependent on the susceptibility level of the cultivar and favorable conditions including weather. Sheath blight is more prevalent in fields with infected stubble from the previous year, planted with susceptible semi-dwarf long-grain varieties, with higher rates of nitrogen fertilizers, with dense stands, with thick canopies and seeded by broadcast. If all is done right, fungicide application would give the desired level of sheath blight control. It has been shown that fungicides work better in well-managed fields.
Automatic application of fungicides is highly discouraged for the purpose of mainly slowing down fungicide resistance development. Remember, fungicides for sheath blight are for control of the disease not for protection as in kernel and false smut. Therefore, scouting for the disease is required to make timely decision on the timing of application. For susceptible cultivars (“S” or “VS”), treat with a fungicide at or above 35% positive stops. For moderately susceptible cultivars (“MS”), treat with a fungicide at 50% positive stops. If the infection is limited on the lower half of crop height at pre-heading and the upper two leaves are clean, fungicide application can be delayed for moderately resistant cultivars. To read more on disease threshold levels, scouting and disease symptoms go to http://www.arkansas-crops.com/wp-content/uploads/2014/02/MP192.pdf.
Fungicide applications for sheath blight are recommended between panicle differentiation and early heading. However, additional application may be required if the disease starts early and is actively moving. Past research by Dr. Rick Cartwright indicated sheath blight control from 16 oz/A rate Stratego for 14-17 days and from 19 oz/A rate for 21-24 days. Quadris provided 10-14 days disease control at 6.4 oz/A, 21 days at 9 oz/A, and at least 28 days at 12.8 oz/A. It was also shown azoxystrobin (Quadris) to be slightly more effective against sheath blight than trifloxystrobin (GEM). The recommended fungicides currently used are shown on Table 1. To read more on sheath blight management go to http://www.arkansas-crops.com/2013/06/10/scout-for-sheath-blight-disease-of-rice-from-green-ring-until-pre-heading/
|Quadris||8.5 – 12.5||azoxystrobin||–|
|Quilt||14 – 34.5||azoxystrobin||propiconazole|
|Quilt Xcel||14 – 27||azoxystrobin||propiconazole|
|Stratego||16 – 19||trifloxystrobin||propiconazole|
|GEM RC||3.8 – 4.7||trifloxystrobin||–|
Table 1. Recommended fungicides for sheath blight disease of rice with minimum and maximum rates and compositions.
*propiconazole is not effective against sheath blight at these rates but is included for suppression of smut diseases if applied at the proper timing.
In case you have similar situations, below are answers to some questions we (Cartwright and Wamishe) received in 2013 on rice sheath blight disease management.
Q. 1. I want to spray 2-4-D for duck salad and I see active sheath blight at green ring. The variety is semi-dwarf and I do not want to take a risk. Could I tank-mix the strobi fungicide with 2-4-D and spray?
A. 1. It has successfully been done before, but always check the new labels for restrictions before you tank-mix.
Q. 2. I have a large field but not-uniformly planted due to mishaps with the planter. Some spots in the field have thicker canopy than others. I see sheath blight moving actively in the thicker spots. Should I spray for sheath blight?
A. 2. In such a field it may be difficult to know the threshold level. Therefore, it is a judgment call. Historically, controlling sheath blight is most advantageous in fields with high yield potential. So this field may not be a good candidate for treatment.
Q. 3. If sheath blight appears at green ring, moving fast and reached threshold level, how many times do I need to spray for sheath blight in a season to minimize yield damage?
A. 3. If sheath blight is this aggressive at green ring, which is before canopy closure on a well-managed field, then it is very likely that the field has been excessively fertilized with N at the preflood stage, or the field is suffering from severe potassium deficiency. These factors need to be addressed over the long term. In general, spraying more than once for sheath blight is not economically justifiable, but in this case, an application of Quadris would be warranted, then scout at booting to see if the disease is active again. If so, and yield potential seems good, then a logical judgment could be made to address late sheath blight and smut if the field has this history. Our concern would be why sheath blight is active so abnormally early in this field though.
Q. 4. If I plan for just a one-time application for sheath blight control on a susceptible variety, what is the optimum developmental stage I should spray the fungicide?
A. 4. In Arkansas, it is usually not economical to treat for sheath blight more than once, if the field is well managed and the fungicide rate and timing is correct. Naturally, there may be exceptions, but these would be rare in our experience. Typically, a fungicide for sheath blight would be applied from 5-20 days after mid-season (1/2 inch IE), developmentally 2 inch IE to mid-booting.
To read on fungicide related answers to questions and other rice diseases in 2013 go to http://www.arkansas-crops.com/2013/07/19/answers-to-recent-questions-regarding-rice-disease-management/