Sheath blight and blast cause significant rice yield and quality reductions every year that cost Mid-South farmers millions of dollars. Disease resistance is the best control, but often it is not available or breaks down after varietal release. Many of our long-grain varieties are susceptible to sheath blight, and several major varieties are susceptible to blast.
Cultural control can reduce disease development, but is not all ways completely effective. As a result, rice farmers often rely on fungicides to control diseases. Several new fungicides are available, and rate and timing are 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 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 from 1997 to 2002. It was found that 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 than heading applications. Lower rates weathered off sooner and allowed late-season disease development to occur.
There was less disease control and a lower yield increase 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 plays a key role in determining fungicide rate, timing, and need.
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 several days after heading, significant yield and milling losses can occur that cannot be corrected by the fungicide.
Blast appears to be as sensitive to fungicide rate as sheath blight. Two applications can be more effective than single applications, but one may be all that is necessary at low to moderate disease pressure. Heading applications are much more effective than boot applications.
Later timings (10 to 15 days) are more detrimental than late sheath blight timings.
Donald E. Groth is the rice pathologist at LSU AgCenter's Rice Research Station. e-mail: firstname.lastname@example.org.