By Yeshi Wamishe, Extension Rice Pathologist
Cultural disease management strategies are beneficial to reduce kernel and false smut disease pressure. Historically, both kernel and false smut are severe in late planted susceptible rice cultivars raised in a field with history and applied with excessive nitrogen fertilizers. Under favorable conditions most rice cultivars including hybrid rice are susceptible.
Scouting for kernel or false smut is not recommended to plan for fungicide application. Once either or both of these smuts show up, it is already too late for fungicide applications. The fungicides should be protective and need to be applied at the correct timing for optimum suppression. Triazole fungicides containing Propiconazole (e.g. Tilt, Bumper, Propimax) are still the only available and partially effective fungicide options.
Applied at the correct timing, rate and canopy coverage, the kernel smut pathogen is more sensitive to these fungicides with up to 90% to 95% disease suppression in past studies. While false smut is a newer rice disease these fungicides have been shown to suppress the disease only 50% to 70%, when applied properly.
As a result of field observations, the minimum recommended rate for Tilt has been increased to 6 fl oz per acre for both kernel and false smut. Field investigations have indicated that fungicide applications mostly are made too late – the rice is too far along in development for optimum disease suppression. So, it is recommended to apply a minimum of 6 fl oz/acre of Tilt or Tilt equivalent from early boot (1/2” IE) to mid-boot (swollen) but prior to the late boot stage (flag leaf collar emergence) (Figure 1). Fungicide applications before early boot are too early since the fungicide has a limited lifespan in and on plants, and will “give out” before flowering.
Fungicide application timing once plants have reached the late boot stage (flag leaf fully emerged and leaf collar obvious) is too late for best suppression since the fungicide needs time to do its job. If the field is in active boot split (Figure 2), then it is too late for best results.
When attempting to use premix fungicides for control of multiple diseases, in general the recommendations for timing of best smut suppression align most often with those for sheath blight management timing. Sheath blight can at times be more aggressive and need to be treated earlier, and separately, from the smuts.
Leaf blast is often managed by increasing flood depth. However, if fungicide application is needed, it can possibly be aligned with the need for smut management. However, neck blast should generally need to be managed separately from the smuts. The optimum first application timing for managing neck blast is late boot to 10% heading. In fields where smut concerns are minimal and the blast treatment is made at late boot, some smut suppression can be achieved but efficacy may be greatly reduced compared to earlier application timings.
The most common mistakes are 1) waiting too late to make a fungicide application, and 2) using inadequate water volumes. For adequate canopy coverage, use a minimum of 10 gallons of water per acre. Stand-alone fungicides such as Tilt, Bumper, or Propimax; or combination (premix) fungicides such as Quilt Xcel or Stratego can be used.
Tips for Maximizing Benefit from Fungicide Application
- Well-managed fields benefit more from fungicide application.
- Fungicides applied at recommended timing and rate maximize their benefit.
- Fungicides mixed and applied using adequate volume of water provide better coverage.
- If tank mixing, fungicides needs to be checked for compatibility.
- Rotating chemistries with different modes of action help delay the development of fungicide resistance (there are no options for smuts’ however)
- Avoiding automatic application with no disease potential cuts expense.
- Tank mixing compatible products by adjusting rates as required may cut cost over pre-mix fungicides.
- Using recommended rates and scouting for sheath blight help to make the most effective and economic decision.
- Check the active ingredients of fungicides to make sure of appropriate rate (Table 1). For example the highest rate for Stratego contains 5.5 fl oz/acre rate of Tilt equivalent. Lower field rates of Stratego have less Tilt content (Table 1).
- Refer to fungicide labels. Labels are the rule.
- To read more on fungicide rates and timing to major rice disease read Rates and Contents of Fungicides; Fungicide Timings for Selected Rice Diseases.
Summary: For most consistent results in suppressing kernel and false smuts, we recommend fungicides that contain propiconazole with a minimum Tilt equivalent rate of 6 fl oz per acre, applied during booting. Applications made during late boot and beyond will be less likely to provide suppression of either disease. Applications can be timed or combined to also control sheath blight in many cases. Fungicide applications to control neck blast are generally too late for best smut suppression. Recommended fungicides and rates for best results include: Tilt 6 fl oz per acre; Bumper 6 fl oz per acre; Propimax 6 fl oz per acre (alone or in combination with Quadris or GEM); Stratego 19 fl oz (smuts and sheath blight); Quilt Xcel 21 fl oz per acre (smuts and sheath blight).