Missouri Rice Disease Control
By Sam Atwell
University of Missouri Extension
Rice blast is one of the earliest known foliar diseases. The blast fungus survives in various ways but often is seedborne. To reduce seedborne blast, research suggests seed treated with Dynasty fungicide (azoxystrobin) at a rate above 0.75 fluid ounces per hundredweight.
However, note that this seed treatment will not guarantee protection later in the season. Therefore, we highly encourage field scouting, deep flood management and foliar fungicides as needed.
Treating For Blast
In blast-prone fields (lighter soils, tree-lined, low-lying, etc.), plant a hybrid or resistant variety. This takes care of the disease for the most part. Where susceptible varieties are planted in the wrong field, keep a deep flood of four inches on them at all times after initial flood.
Fungicides work best if applied twice for blast. The first application should be made at late boot to beginning panicle tip emergence and the second when panicles are 50 to 75 percent out of the boot on most of the main tillers. Higher rates are best. If the field is very uniform and disease potential is low to moderate, the best timing would be when panicles are emerging with about 35 percent of the length out of the boot on most of the main tillers.
In fields that are maturing unevenly, spray on the earlier maturing parts of the field if disease pressure is substantial. Also, two fungicide applications would be automatic on these types of fields. Again, proper flood management will really help with blast management and improve performance of the fungicides.
Sheath Blight Fungus
For many years now, strobilurin fungicides have been used to manage sheath blight disease of rice and have been the backbone for managing fungal diseases of rice in Southern rice-producing states. Current fungicides are most effective under low or moderate disease pressure.
The challenge comes when varieties are highly susceptible and environmental conditions are very favorable for disease development. When we have sheath blight we recommend our producers use strobilurin + propiconazole fungicide mixtures to combat sheath blight and the smuts. It is important to note that the sheath blight fungus can be moved from field to field in soil and water and by equipment.
Smuts And Pre-Flood Nitrogen
Smuts were bad in some fields in 2011. Fields sprayed properly with propiconazole-containing fungicides minimized these diseases. In some cases, too much nitrogen was applied to affected fields and, in other cases, the fungicide was applied too late in the booting stage for maximum effect.
Rice smuts cannot be scouted for, so preventive treatment with propiconazole-containing fungicides is the only chemical control option. Fields with a strong history of smuts, or those that have been knowingly over-fertilized with nitrogen are most at risk. Some growers have learned to “back off” their pre-flood nitrogen over time so they minimize smut without sacrificing yield.
Timing and rate of the fungicides to prevent the smuts are critical. Fungicides need to be applied at early to late boot but before heading begins on any plants in the field. Earlier is usually better in the booting stage, especially for false smut. The minimum rate of six fluid ounces of Tilt or Tilt equivalent is now required for most effective results under current conditions, but no application will provide 100 percent control.
Hybrid fields and medium grains are unlikely to benefit from fungicide applications due to their tolerance, although some fields do when pushed with a lot of fertilizer. Fungicides should be applied if your effective scouting indicates more than 35 percent positive stops in susceptible varieties and more than 50 percent positive stops in moderately susceptible varieties.