Disease prevention is key
In recent years, most disease calls have centered around sheath blight and the smuts (kernel smut and false smut). These two present very different management scenarios.
Sheath blight continues to be our No. 1 disease issue in rice. To prevent it, we want to avoid excessive seeding rates and nitrogen rates — a dense, lush canopy favors sheath blight development. Since sheath blight is also an issue in soybean as aerial blight, we know there is always a chance of issues if the environmental conditions are favorable.
Even when we handle the agronomics to the best of our ability, we can still have issues with sheath blight. Luckily, this is one disease that we have a treatment threshold for. Simply finding sheath blight is not justification for spraying. It is best to run through a treatment checklist: 1) sheath blight presence, 2) percent of positive stops, and 3) active movement / threatening upper canopy.
Once we have confirmed the presence of sheath blight, the work begins. Treatment may be warranted for moderately susceptible cultivars when greater than 50% of field stops are positive for sheath blight, or for susceptible cultivars when greater than 35% of field stops are positive.
However, finding the required percentage of positive stops is not the end. If all stops have sheath blight near the waterline (meaning no active movement up the canopy), then treatment can be delayed for additional scouting later. However, if sheath blight is actively and aggressively moving up the canopy, it may be time to treat.
The goal is to outrun sheath blight to heading. If we can reach 50% heading with the upper three canopy leaves clean, then we have outrun direct yield loss and can save a fungicide application.
For kernel smut and false smut, unfortunately, there is no threshold to follow. We must rely on our knowledge of field history and cultivar susceptibility. Fortunately, the agronomic practices to minimize issues with these diseases are similar to those for sheath blight. Use appropriate seeding and nitrogen rates to reduce problems.
If we have a field history of either of the smuts and plant a cultivar that is rated very susceptible or susceptible, then a preventative fungicide application should be considered.
Too often our triazole fungicide application goes out too late to maximize our suppression of the smuts. The optimum timing is mid-boot as flag leaves are coming out or just out. The closer we get to the start of heading, the less effective a fungicide application will be for smut prevention.
Remember, the goal is to prevent disease pressure as much as possible and rely on fungicides only when needed. Use field knowledge and thresholds to your advantage in 2023.
Deterring blast issues
Boy, what a difference a year makes. Looking back on last year’s progress, Missouri might have been 5% planted leading into the last week of April. My estimate as of April 24 is somewhere in the 70% ballpark, with many completely finished in the southern and western reaches of the Bootheel. Having this many acres planted early gives us a great chance for two things: hitting our prediction of 180,000 acres of rice and the potential for a great yielding rice crop.
While Missouri is probably last in the Mid-South in terms of blast pressure (one race we are more than happy to lose), we did see some instances of leaf blast last year. Due to more advanced breeder lines and refined production practices, we can generally keep rice blast at bay. In 2022, we were called to a field planted into a cultivar with great blast resistance, but the field was eaten up with leaf blast. This field was also a very sandy soil texture, planted into furrow-irrigated rice, and irrigated only about once per week. Putting rice into this type of situation can overcome even the greatest blast resistance we have.
Long story short, let’s not push our rice further than we know it can go in 2023. While there is good blast resistance in many of our cultivars, putting rice into some less-than-ideal situations has the potential to cultivate races of blast that can overcome the resistance genes we have.
Rice blast control begins with selecting a cultivar with some level of resistance, but that is not the only cultural control. Keeping a deeper flood (greater than four inches) or a flood, period, is one way to combat blast pressure. Furrow-irrigated rice is not a good idea in areas where we know that blast can be an issue, or with cultivars that are blast-prone. Scouting areas with an east-facing tree line or that have a longer dew period is one way to find leaf blast issues before they become widespread.
If chemical control is needed, a product containing either azoxystrobin or trifloxystrobin should be utilized prior to heading. The best neck blast control occurs from a two-application approach, with the first at late boot and the second when main tillers are about 75% out of the boot. This approach spreads out our coverage to protect the majority of our tillers. Do not wait too late and remember that if the neck is out of the boot, the fungicide application is virtually pointless.
If you have any questions, please give me a shout and we can discuss control options. As always, eat MO rice!
California disease update
After two years of severe drought, California acreage is going back up. Roughly, half the acreage will be planted in fields that were fallowed last year. Research conducted by rice specialist Bruce Linquist showed that rice planted in fields that were fallowed the previous year has a higher yield potential than rice planted in fields that were in rice the previous year.
One of the reasons for the higher yield potential following fallow may be reduced incidence of stem rot. After harvest, stem rot survives in rice straw as sclerotia, a small hardened spherical structure that resembles a grain of sand. The sclerotia become the inoculum that will infect next year’s crop. During a fallow, as straw decomposes, some of these sclerotia becomes less viable, possibly resulting in less disease the following year when rice is planted.
Another finding from Bruce’s research was that when planting rice in fields that were fallowed the previous year, nitrogen fertilizer rates can be reduced by 20 to 30 units. This means that when rice is planted in fields that were fallowed last year and the fertilizer nitrogen rate is not adjusted, there might be some extra nitrogen. This may have some implications for disease management. While the fallow may reduce stem rot, the extra nitrogen will make plants more susceptible to stem rot and blast. Blast has not been a significant problem in the past two years, but if the weather is right and blast shows up, it can significantly reduce yields in susceptible varieties.
At this point, we don’t have good guidelines to monitor for stem rot during crop development. Evaluation of the severity of stem rot is typically done at drain time; by then it is too late to apply a fungicide. Preliminary data from last year indicates that if 30% to 40% of tillers show stem rot lesions during the boot stage, the severity of the disease may reach high levels at drain time and yields may be reduced. You can use this to guide the decision to use a fungicide. High stem rot incidence may also be an indication that your soil potassium is low and you need to include it in your fertility program. Take into consideration that varieties with shorter cycles, like S-102 and M-105 are more susceptible to stem rot than varieties with longer cycles like M-209 and M-211.
For blast, scout your fields during vegetative growth for leaf lesions. If leaf blast is present, plan on using a blast fungicide at late boot or very early heading. Also, be aware if blast is present in your area. Blast spores can travel long distances, and fields without leaf blast can develop neck blast if spores from neighboring fields reach the field. Remember that the only truly blast resistant variety is M-210.
Early season disease thoughts
In Mississippi, rice planting is off to a great start as I sit down to write this at the end of April. I would easily say we have a greater percentage of acres as well as a larger amount of rice acres in the state than on average. With that being said, it is important we emphasize issues that can arise within the crop early in the year.
Planting early has always been thought to be a good idea, and I do not disagree with the thought process behind it. Current data shows that varieties tend to lose 0.25% of the yield potential after April 30, with hybrids having a slightly wider window into early May. However, it is important to understand the potential diseases in this early season environment.
My biggest thought with this early rice year is the increased potential for the environment producing conducive periods for seedling diseases such as Pythium root rot or seed decay/rot. Seedling diseases can often appear similar to glyphosate drift. Moreover, with the temperatures that have shifted toward being much cooler and with intermittent rainfall over the past few weeks, seedling disease can be compounded with a more conducive environment. Be mindful that seedling diseases are most often observed in a low or wet spot in a part of the field and do not typically occur across an entire field. In general, when seedling disease occurs, rice plants will have lesions present at the soil line. The best way to manage seedling diseases is to use a seed treatment and attempt to keep fields well-drained while rice is getting established.
As we move closer to four-leaf rice before flooding, it is important to begin scouting for leaf blast. Blast will often appear near the edge of fields, on levees, or anywhere with a less-than-four-inch flood after the flood is established. Blast can also appear similar to herbicide drift; however, lesions will appear to have more of a diamond-like appearance rather than something that smears down the leaf. Blast becomes more problematic once rice begins to head, and the disease moves from the leaf to the panicle, which can result in blank heads. The best management to prevent this is a fungicide application that includes two modes of action and generally a triazole that has a more “curative” ability and a strobilurin that is more “preventive.” Blast of the neck and panicle are the most potentially yield-limiting stages of rice blast.
This year is shaping up to be excellent for Mississippi rice production. However, if issues or questions do arise, feel free to reach out to your local Extension specialist anytime.
Rice disease management
The yield potential of any rice variety can be severely reduced under high disease levels. An integrated disease management program including the following practices should be implemented:
■ Plant resistant varieties.
■ Avoid late planting.
■ Maintain proper fertility levels.
■ Maintain adequate flood (avoid loss of flood).
■ Use fungicides at the correct growth stage if necessary.
Fungicide timing is critical for disease control. Sheath blight should be treated between early boot and heading but not beyond 50% to 70% heading. Blast must be treated at the 50% to 70% heading growth stage. Yield and grain quality increase because of disease control and quickly decrease if fungicide is applied after 70% heading. Remember, this growth stage is very difficult to detect, so it is important to scout for the rice growth stage at the same time as you scout for disease. Also, you will need to allow time to obtain a fungicide, schedule the application, and allow for poor weather conditions to apply the fungicide at the correct time. The use of foliar fungicides is justified in many cases. Some factors to consider in making this decision are as follows:
■ The history of the field.
■ Whether the variety is susceptible.
■ Yield potential.
■ Application cost and expected crop price.
■ If the rice is being grown for seed.
■ If the rice is planted late, late-planted rice is more likely to encounter foliar disease problems.
■ If a ratoon crop is planned because disease not suppressed in the first crop may cause significant damage in the second crop.
Scouting for diseases should begin early in the season. For sheath blight, cultivars that are very susceptible to susceptible will experience an economic loss of 5% to 10% if the tillers are infected during vegetative stages. For moderately susceptible cultivars, the level is 15%. At these levels, consider using a fungicide. Fields with a history of sheath blight, under rice-soybean rotation, or rice-rice rotation have a higher risk of developing severe epidemics.
For blast control, apply a foliar fungicide at early heading (50% to 70% heads emerging) when leaf blast symptoms are present. Leaf blast does not always precede rotten-neck blast, and preventive applications of a fungicide may be warranted if a blast-susceptible variety is grown. The incidence and severity of blast increases when plants are stressed (loss of flood, fertility imbalance, etc.). Draining for straighthead and/or water weevil control may increase the incidence and severity of blast. Also, blast is normally worse on later-planted rice.
Cercospora disease control and yield increases appear best when fungicides are applied between panicle differentiation and early boot growth stages. Cercospora causes narrow brown leaf spot and other symptoms. Leaf lesions are linear and reddish-brown. Resistance to narrow brown leaf spot is available. On susceptible cultivars, the lesions are wider, more numerous and lighter brown with gray necrotic centers. Spots usually appear near heading. Both young and old leaves are susceptible. This past year, rice sheaths and glumes were affected on later planted rice that was resistant to narrow brown leaf spot. It caused significant discoloration, necrosis, and yield losses.
On sheaths, the disease is referred to as “sheath net blotch” because of the brown cell walls and the tan-to-yellow intracellular areas that form a netlike pattern. Branches of the seed heads can become infected, causing premature ripening and unfilled grains. Symptoms can be confused with rotten-neck and panicle blast lesions. Narrow brown disease lesion symptoms usually are darker brown and develop in the internodal area of the neck. Grain infection appears as a diffused brown discoloration. The disease can also be very severe on the ratoon crop. The later the rice is planted, the earlier the fungicide must be applied to reduce infection. Most fungicides may not be applied to the second, or ratoon, crop.