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11
Jul
2014
Fungicides for major rice diseases: Timing, rates, and contents
Author: Yeshi Wamishe, Extension Rice Plant Pathologist

Fungicides need to be applied at the right rate and time to provide the best benefit in your well managed fields. Moreover, foliar fungicides need to be mixed with adequate amount of water to provide good coverage.

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Time Fungicide Application with the Recommended Crop Growth Stage, Rate and Product

  1. Treat for sheath blight based on established thresholds

For susceptible cultivars (“S” or “VS”) treat with a fungicide when 35% of stops are positive for sheath blight. For moderately susceptible cultivars (“MS”), treat with a fungicide when 50% of stops are positive for sheath blight. In moderately resistant cultivars (“MR”) if the infection is limited to the lower half of the stem and at least the flag leaf and the upper two leaves are clean, fungicide application can be delayed. The “positive stop method” assumes scouting over the bulk of the field further from the field edges and that weather conditions favor vertical development of sheath blight. If weather conditions are not favorable, then fungicide applications may again be delayed. Consider the susceptibility of your variety, nitrogen amount applied and field history in your management decision.

Table 1. Recommended fungicides for sheath blight disease of rice with minimum and maximum rates and active ingredients.

Fungicide Rate oz/acre Strobin Triazole* Other
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
Sercadis** 4.5 – 6.8 Xemium

*Propiconazole is not effective against sheath blight at these rates but is included for suppression of smut diseases if applied at the proper timing.

**Sercadis is a new fungicide with different mode of action that helps prevent the spread of resistant sheath blight. Although resistant sheath blight has not been reported in Arkansas, rotating fungicides with different mode of action are known to delay resistance.

  1. Timing is crucial to effectively control neck blast using preventative fungicides

In most cases leaf blast can be managed with a flood depth of at least 4 inches. However, blast-Prone-fields with history, planted with susceptible varieties provided with excessive nitrogen require preventative fungicide treatment. Fields in low lying areas, with heavy tree lines especially on the east side, that do not hold water or are difficult to water, fields that get foggy due to close by rivers or waterways and that are planted late can be categorized as “blast prone fields”. While planning preventative fungicides for neck blast,” timing in relation to the crop stage” is crucial. Fungicides applied after the head is completely out of the boot will not be effective enough to control neck blast disease. The first application need to be made from late boot stage to 10% heading and the second from 50-75% heading. If the crop is “already headed”, that is too late. The main idea is the neck should still be in the boot to protect the crop from neck blast using the correct chemical products.

Table 2. Recommended fungicides for blast disease of rice with minimum and maximum rates and active ingredients.

Fungicide Rate oz/acre Strobin Triazole
Quadris 8.5 – 12.5 azoxystrobin
Stratego 16 – 19 trifloxystrobin propiconazole
GEM RC 3.8 – 4.7 trifloxystrobin

Research has shown GEM as slightly more effective than Stratego to suppress neck blast. To protect the crop from neck blast, the higher rates are preferred. Waiting long to tank mix the fungicides with insecticides against stink bugs is a bad idea.

  1. Timing, rate and coverage are important to suppress kernel smut and false smut

Kernel smut and false smut affect quality and yield loss in fields with history. The smuts are unpredictable diseases. They may occur in hot-dry years or warm and wet years. They are severe in late-planted rice than early-planted rice. Under favorable conditions most of our cultivars are susceptible to smuts. Fungicides containing Propiconazole (Tilt equivalent) have been used in previous years and still they are the preventative options. False smut appeared to be less sensitive to propiconazole fungicides than kernel smut. For this reason the fungicide rate has been increased from 4 fl oz of propiconazole to 6 fl oz rate. When everything is done right with timing, rate and coverage, kernel smut was suppressed 90% to 95% and false smut 50% to 70%. Boot to mid boot is the right timing. Waiting until boot split is too late (Picture 1).

Table 3. Recommended fungicides for kernel smut and false smut disease or rice showing triazole and strobin rates

Fungicide Rate oz/acre Strobin Triazole Strobin rate oz/acre Triazole rate oz/acre
Quilt 21 – 34.5 azoxystrobin propiconazole 6.3 -10.3 6.1- 10
Quilt Xcel 21 – 27 azoxystrobin propiconazole 11.9 -15.3 5.9 -7.6
Stratego 16 – 19 trifloxystrobin propiconazole 4.0 – 4.7 4.6 -5.5

Tilt (Bumper or Propimax) at 6 oz/acre could be included in the list for suppression of the smuts.

Check the minimum strobin rates in the mix if treatment is intended for sheath blight and blast. All rates are approximate. To read more on rice fungicides and rates go to http://www.arkansas-crops.com/2014/07/11/fungicides-diseases-contents/. See labels for restrictions and more.

Picture 1. Fungicide application to suppress kernel smut and false smut after late boot is too late. Early or late boot split is also too late (Photo by Dr. Rick Cartwright).

Picture 1. Fungicide application to suppress kernel smut and false smut after late boot is too late. Early or late boot split is also too late (Photo by Dr. Rick Cartwright).

 


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