Donald E. Groth

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 rice farmers millions of dollars each year. Disease resistance is the best control method, but often it is not available or breaks down after varietal release. Most long-grain varieties are susceptible to sheath blight, and several major varieties are susceptible to blast. Cultural control can reduce disease development, but practices to reduce disease by reducing inputs can limit yield, too. As a result, rice farmers often rely on fungicides to control diseases. Several new fungicides are available, and timing is critical for maximum return. 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.

Studies were conducted to determine the best rice growth stage and fungicide rate for the control of sheath blight and blast at the LSU AgCenter’s Rice Research Station at Crowley, La., from 1997 to 2001. Standard commercial cultural management practices were used to maintain the plots. Application growth stages studied each year were 1) panicle differentiation, when the panicle can be first identified (2 mm in size); 2) at booting, when the head is 2 to 4 inches in length; and at heading, when 50 percent to 70 percent of the heads are starting to emerge from the boot (just after boot split) but before heads are fully exerted.

The fungicide Quadris was used at 6, 9 or 12 fluid ounces per acre. In 2000 and 2001, additional fungicide treatments were applied five, 10 and 15 days after heading. Sheath blight plots were inoculated with enough Rhizoctonia solani, grown on a rice grain-hull medium, to produce an infection level above the treatment threshold of 5 percent to 10 percent of tillers infected. Blast inoculum came from natural sources. Disease development was evaluated one to two weeks before harvest by determining infection levels of sheath blight (percent tillers infected) and blast (percent heads infected).Sheath blight severity was determined using a 0-9 scale where 0 indicated no disease and 9 indicated sheath blight on the upper canopy and plants collapsing. Plots were harvested with a small plot combine, and yields adjusted to 12 percent moisture were determined. A subsample of grain was collected and milling percentages of head and broken rice measured. Studies were conducted on susceptible varieties from 1997 to 2001 in separate plots for sheath blight and blast.

Sheath blight

In multiple-year studies, yield increases were as high as 2000 pounds per acre with timely fungicide application. Disease control, yield increases and head rice yields were significantly different from the unsprayed checks for all of the Quadris applications. The yield of 6 and 9 fluid ounces per acre Quadris rates were lower than the 12 fluid ounces per acre rate at panicle differentiation, and disease control was weak for the 6-ounce rate at this timing. At booting the 9-ounce rate was similar to the 12-ounce rate but the 6-ounce rate was weak again. At heading there was no significant difference between the three rates. Head and total milling were significantly higher than the unsprayed check but not significantly different among any of the rates or timings.

When comparing the 9-ounce rate at heading, a shift of as few as five days after heading lost 721 pounds per acre (Table 1). When applications were made 10 days and 15 days after heading, the plots lost 1,259 and 1,602 pounds per acre, respectively, which were not significantly different from unsprayed levels. Disease control also decreased as treatment was delayed. Head rice milling yields were significantly lower at the two late timings.

Blast

All of the Quadris rates and timings significantly reduced blast and increased grain and head rice yields over the unsprayed check. Single applications of Quadris at heading were lower yielding and had more disease than the two applications at boot and heading. These timing differences were not significant. The low and high rates were equally effective.

When the 9-ounce rate of Quadris applied at heading was compared with later timings, there was a significant decrease in disease control and a significant reduction in yield at 15 days after heading. All application timings increased head rice but not total milling compared with the control.

Timing, rate importance

The rate of fungicide necessary to control sheath blight changed as the season progressed. A higher rate was required at panicle differentiation and boot growth stages to get maximum performance because lower rates weathered off and allowed late-season disease development to occur. There was less disease control and yield increases were lower when the fungicide was delayed until heading because sheath blight was allowed to increase unchecked for several weeks. A lower rate can save farmers significant amounts of money, but only when justified by disease development at later crop growth stages, light disease levels or higher host resistance levels. Obviously, disease scouting in the field will play a key role in determining fungicide rate, timing and necessity. The Quadris label allows lower rates if disease does not develop until heading and if more resistant varieties are being sprayed. Using lower than labeled rates exposes a farmer to risk that the company will not support nonperformance complaints. It is extremely important to remember that if fungicide applications are delayed after heading, significant yield and milling losses can occur that cannot be corrected by fungicide.Blast appears to be less sensitive to fungicide rate than sheath blight. Two applications can be more effective than single applications, but it may not be economically significant to justify both a boot and heading application at low disease pressure. A heading application was much more effective than a boot application (data not shown). Although later timings may not be as detrimental as late sheath blight timings, earlier applications are more effective—especially under heavier disease pressure (2001 data).

Donald E. Groth, Professor, Rice Research Station, Crowley, La.

(This article appeared in the winter 2002 issue of Louisiana Agriculture.)