Fungicide Timing Critical for Maximum Return

Donald Groth  |  4/11/2006 3:12:40 AM

Donald E. Groth and Rick Cartwright

Rice diseases pose a major threat to rice production. The two major diseases, sheath blight and blast, cause significant yield and quality reductions that cost farmers millions of dollars each year. Grain smuts have also become significant problems in rice production, causing significant quality reductions. Planting rice varieties with disease resistance is the best control method, but resistant rice often is not available or the resistance breaks down over time after a variety is released.

Most long-grain varieties are susceptible to sheath blight, and several major varieties are also susceptible to blast. How a crop is managed in the field can reduce disease development, but reducing inputs can limit yield, too. As a result, rice farmers often rely on fungicides to control diseases. Fungicide timing is critical for maximum return.

Deciding to use a fungicide and when to apply it are the two most critical decisions a producer must make. The correct decision will make money, and the wrong one will lose money. Each disease has its own cycle, and control practices are effective only at certain stages when the pathogen is susceptible to the chemical control and before irrevocable damage occurs in the crop. Typical sheath blight fungicide timing is at the boot growth stage; however, several fungicides also control blast when applied at heading. Consequently, a general trend has been to apply rice fungicides later in the season to control both blast and sheath blight with a single application. Several fungicides most effective against smuts cannot be applied after heading, making them ineffective against blast.

Timing and rate trials
Fungicide timing and rate trials have been conducted at the LSU AgCenter Rice Research Station, the University of Arkansas Rice Research Station and in grower fields in both states for a number of years. Typically, fungicides are applied to small plots using pressurized sprayers, although several aerial trials have been conducted. Fungicides were applied at either 7 days after panicle differentiation (panicle 0.125 inches in length), 2-inch boot (2-inch panicle in the sheath), 50 percent heading (at least half of the heads emerging from the sheath), or 5, 10 or 15 days after heading. Varieties selected were susceptible either to sheath blight, blast or smut and were managed to encourage disease (inoculated, fertilized with high nitrogen rates, planted late and/or located where disease pressure is high).

Timing is an important consideration in fungicide control in sheath blight, blast and smut. The booting stage appears to be the best time for smut control (Figure 1). Earlier applications may require high rates to be effective, and applications after heading can be ineffective as well as illegal because restrictions on the labels of some fungicides prevent applications after the heads emerge.

Sheath blight control in these tests during drier years has been best at the 7 days after panicle differentiation. But often higher fungicide rates are necessary for season-long control (Figure 1). Applications at the boot growth stage have been the best timing for sheath blight control. Applications at heading have been effective; however, sheath blight can spread up the plant readily and cause more damage by this growth stage. Blast control has been best when fungicides were applied at heading (Figure 3). Applications after heading lost effectiveness on both sheath blight and blast (Figures 2 and 3).

Time for the worst
Fungicide timing must be based on the most damaging disease present in a field. This is determined by knowing the varietal susceptibility, field disease history, what is occurring in the area and, most importantly, by scouting for disease in the field multiple times during the growing season.
  • If sheath blight and smuts are both significant in a field, a boot application would be best. Earlier applications, with higher rates, would be advisable only if sheath blight had started early in the season and was causing significant damage before the boot growth stage.
  • If blast and sheath blight are both present, applying a fungicide with both sheath blight and blast activity at heading would be best because blast can be more damaging than sheath blight and applications at heading were effective on sheath blight.
  • If blast and kernel smut are both significant in fields, a fungicide application at heading would be advisable because blast is more destructive, and applications at heading are somewhat effective for kernel smut.
  • If kernel smut has been a major problem in a field, applying the smut fungicide at boot stage and the blast fungicide at heading may be advisable but much more costly because two fungicide applications costing $15 to $30 each would have to be used.
Split applications for sheath blight and blast at boot and heading are also more effective, but economic constraints of rice production limit this practice. Most important, fungicides must be applied at or before 50 percent to 70 percent heading to maximize disease control and yields.

Donald E. Groth, Professor, Rice Research Station, Crowley, La., and Rick Cartwright, Extension Plant Pathologist, University of Arkansas Cooperative Extension Service, Little Rock, Ark.

(This article was published in the winter 2006 issue of Louisiana Agriculture.)
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