If you had smut in 2019, plan to protect crops this season

Epidemic levels of kernel smut and false smut were prevalent in the 2019 Louisiana rice crop. While smuts have always been a common problem in northeast Louisiana and the Mississippi Delta rice-growing regions, smuts were never really a concern in southwest Louisiana. Over the past five years, sightings of smut in southwest Louisiana rice fields have increased a little bit each year. And in 2019, we had high levels of the fungi everywhere.

Levels were so high in some fields that harvested grain was rejected at some delivery points. Unfortunately, spores of kernel and false smut can overwinter in the soil and will be available to inoculate our current crop.

If you had smut last year, you need to make plans to protect your crop this year. Even if you did not have smut last year, it might be a good idea to apply a fungicide to help protect your crop from airborne spores from infected fields nearby.

Propiconazole (Tilt, PropaMax, Bumper; 6 to 10 ounces) and difenoconazole (AmistarTop; 14 ounces) fungicides are recommended to be applied at 2- to 4-inch panicle in the boot to help protect the panicle from being infected with smut.

Applications of the fungicides after boot split will not provide any protection.

Remember, these applications are preventative only; they are not curative. These applications will be going out where no signs of the disease are present. This has historically not been an application and expenditure that growers have worried about in southwest Louisiana. However, if a grower had smut last year, they should highly consider using a fungicide to prevent it this year.

Smuts have historically been low at the Rice Research Station. Because of this, Dr. Don Groth, station resident director and plant pathologist, has never had the opportunity to rate rice variety and hybrid susceptibility to the fungus.

Levels were high enough in 2019 for him to make false smut evaluations for the first time. His ratings ranged from 0 (no sign of the disease) to 9 (highest level of the disease possible). Some of the most susceptible cultivars included CL172 (4.8), Gemini 241 CL (4.3), CLXL745 (3.8), Presidio (3.2) and PLV01 (2.9).

It is important to note that this is just one dataset acquired at only one location in one year and may not be representative of a dataset replicated over multiple years at multiple locations. Nonetheless, I would be more likely to apply the 2- to 4-inch panicle in the boot application to prevent smut if I were growing these cultivars.

Of rice water weevils, kernel smut and rice planthoppers

According to the Texas Rice Crop Survey, about 80% of our crop was planted and about 70% had emerged as of April 10. The Texas rice crop is off to a good start in 2020 due in large part to a relatively dry planting season. However, only about 1% has gone to flood.

If you have not applied an insecticide to your seed to control primarily rice water weevil, you may have missed the boat. My project’s research has shown that application of pyrethroids before the flood to control rice water weevil is not as effective as in the past.

I also do not recommend draining fields to control this root-feeding pest because soil must dry to cracking. Rainfall, which is common in southeast Texas during our growing season, can prevent drying.
I sound like a broken record, but please be on the lookout for the rice planthopper this year.

In the past, this exotic pest has attacked Texas ratoon rice, although it can possibly attack earlier. There’s a chance we won’t find it in Texas this year, but we have to be proactive. If you suspect this insect in your fields, contact me at moway@aesrg.tamu.edu or 409-239-4265.

Kernel smut has become a major disease in recent years in Texas. The fungus infects and replaces the endosperm of the rice grain with a mass of black spores. Kernel smut occurs on both main and ratoon crops but usually is more severe on the main crop.

The disease is not systemic. The smut spores fall to the soil surface where they remain dormant until the following rice crop.

They can also be introduced into a field via smut-contaminated rice seed.

The spores float to the surface of the paddy water where they germinate and produce air-borne spores that infect rice florets. Disease development is favored by frequent light showers, high humidity and excessive nitrogen.

Some varieties, such as hybrids, are less susceptible. Presidio and Dixiebelle are very susceptible. A three-year crop rotation can help reduce the number of smut spores in the soil.

Kernel smut is difficult to control. Propiconazole- or difenoconazole-containing fungicides are effective, but timing is absolutely critical. Mid-boot is the best time to apply a fungicide. (Contact Dr. Shane Zhou at xzhou@aesrg.tamu.edu for more details about specific fungicides and rates). Some severely infected fields may require a second application 14 days after the first at mid-boot.

Get fungicide timing right

A common question I get regarding fungicides is what is the best timing for application. That’s going to depend on the disease you are targeting. In California, there are four diseases of importance: stem rot, aggregate sheath spot, blast and kernel smut.

Stem rot and aggregate sheath spot are similar in their cycle. The inoculum of both survive in crop residue in the form of sclerotia. After flood, the sclerotia float to the surface of the water and infect rice plants at the water level when conditions are favorable.

Stem rot penetrates the culm and causes tissue degradation, resulting in lodging and partially filled panicles.

Aggregate sheath spot affects leaf sheaths and moves up the plant, causing leaf yellowing and death. In severe cases, aggregate sheath spot can reach the panicle and cause blanking.

Both diseases can be found in most fields, but usually one is more prevalent than the other. In general, stem rot is more problematic for growers than aggregate sheath spot.

For these two diseases, fungicides should be applied at the early heading stage. Some growers like to piggyback the fungicide with their propanil application around 35-45 days after seeding to save on application cost.

Fungicide trials conducted for the past three years have shown that a fungicide treatment at the propanil timing is not as beneficial as an application at the early heading stage.

Blast can affect the foliage or panicles. I don’t recommend a fungicide application for leaf blast unless the stand is being affected. Against blast, you want to use a fungicide to protect the panicles.

The best timing for this is to treat at the early heading stage when you see the first flush of panicles emerge. As you delay treatment, the fungicide becomes less effective. If you see leaf blast, plan on treating for blast at early heading.

Kernel smut has become a concern in the past five years. In 2018, we saw several fields severely affected, especially in the northern part of the Sacramento Valley.

Kernel smut control with fungicides is challenging because signs of the disease — the black mass of spores replacing the kernels — are only visible close to harvest when it is too late to do anything.

In fields with a bad history of kernel smut, application of propiconazole should be done during the boot stage. Remember that long grains are more susceptible than medium grains and of the medium grains, M-209 seems to be the most susceptible variety.

Take a multifaceted approach to sheath blight

As I sit down to write this article April 17, less than 7% of the total Mississippi rice acreage has been planted. At this time last year, we were around 30%, while our 10 -year average for this time is greater than 50% planted.

This delay in planting makes speculating on the 2020 rice season difficult, but as we trend toward a later-planted crop, disease management may play a crucial role in the later months. Seed treatments are still vital for seedling disease prevention, even as germination periods decrease with a later-seeded rice crop.

Sheath blight caused by the fungus Rhizoctonia solani is considered the most important disease for Mississippi producers in direct-seeded, delayed flood environments. This disease requires a multifaceted approach for proper management, including choosing less-susceptible cultivars, avoiding excess N fertilization and regularly scouting fields, especially when rice is grown continuously without crop rotation.

The proper use of fungicides has aided management of this disease, but yield reductions can be significant if not controlled in a timely manner. The increasing adoption of furrow-irrigated rice throughout the Mid-South rice production region can have significant implications on rice disease management in Mississippi.

With this relatively new production technique, the lack of a permanent flood and nitrogen management strategies may greatly affect disease incidence and pathogen presence. One of the most concerning diseases in furrow-irrigated acreage is rice blast.

Rice blast is caused by the fungus Pyricularia grisea and is one of the most damaging rice diseases in Mississippi with respect to rice yield and quality. One of the most effective control strategies is to maintain a flood of at least 4 inches continually across a conventionally grown flooded field.

However, this strategy becomes unfeasible on at least portions of furrow-irrigated rice fields. With 10% to 15% of the state’s rice acreage expected to be in a furrow-irrigated production this year, increased incidence of this disease is a major concern and must be properly managed and accounted for.

Rice blast management in furrow-irrigated fields begins with planting cultivars with inbred resistance mechanisms, avoiding excessive N fertilization and planting early to avoid heavy blast pressure that can occur later in the season. Because we will not be planting early in 2020, intensive scouting so fungicide applications can be made with timeliness in blast-infested fields is paramount.