Specialists Speaking: May 2025

Prevention is Key

For disease control in rice, prevention and avoidance go a long way in minimizing our issues. This starts with cultivar selection, relying on those cultivars that provide the best disease packages for our needs. Luckily, we continue to see improvements in the overall disease packages of cultivars we’re growing today compared to those of the past.

While a cultivar with a solid disease package is a great leg up, there’s always a disease lurking we should be mindful of. Since sheath blight is the only rice disease we have a treatment (scouting) threshold for, most of our attention is focused on avoiding and preventing the other diseases. Use of optimal seeding and fertility rates are critical in limiting disease issues.

For sheath blight, a reminder that our threshold is based on percent of positive stops while scouting and active movement threatening the upper canopy. For cultivars rated very susceptible or susceptible — 35% positive stops; for cultivars rated moderately susceptible — 50% positive stops. So, if you have both of those boxes checked, it may be time to consider a fungicide application.

For diseases such as blast, Cercospora, and the smuts, we have to depend on the disease rating of the cultivar, field history, and environmental conditions as our guide. For cultivars susceptible to blast, there isn’t typically a reason to treat for leaf blast unless severe pressure begins burning areas down. Most often, we’re worried about spraying around heading to prevent neck and panicle blast. For detail on specific fungicide timings for blast check out https://www.uaex.uada.edu/publications/pdf/mp154/Rice-Diseases.pdf and https://www.uaex.uada.edu/farm-ranch/crops-commercial-horticulture/rice/Rice-Management-Guide.pdf.

In 2025, as in 2024, there will be more acres grown of varieties that are blast susceptible. Remember that increased flood depth after mid-season can help suppress blast development. For blast-susceptible varieties in a furrow-irrigated environment, it may be wise to consider at least one fungicide application for prevention of neck and panicle blast due to the lack of a flood on the field.

Cercospora has become a more persistent issue in the past five years. Not necessarily common, just persistent, when it wasn’t even a topic of conversation prior to that in Arkansas. The best we can do is apply a fungicide between early and late boot to prevent infection from occurring on the neck and panicle, at times appearing similar to blast. Ensuring we have adequate potassium fertilization can also help.

Kernel smut and false smut also rely on cultivar susceptibility and field history to determine if preventative fungicide applications may be beneficial. The timing of these applications can be similar to those for Cercospora, which can save us an application if we feel the need to treat for both.

To wrap things up, use fungicides only when needed, avoid excessive nitrogen rates, and ensure adequate potassium rates to maximize disease management in 2025. Let us know if we can help.

What is New in California Disease Management Research

Every year, I conduct research projects to try to improve disease management in California. These projects are mostly funded by the Rice Research Board. I collaborate with the Rice Experiment Station and rice growers that let me set up trials in their fields. In this column, I will highlight some of the projects I have been working on for the past couple of years and that will continue during 2025.

One of the major needs in the area of disease management is new fungicides. California rice growers have been relying on azoxystrobin (the active ingredient in Quadris) for more than 20 years. I’ll give it to azoxystrobin — it continues to provide good control of our major rice diseases. However, we need new products so we can rotate and reduce the risk of resistance. This year, I will be testing two fungicides belonging to the group of the DMIs (Demethylation Inhibitors).

Last year I tested several fungicides that are known to induce plant defenses against pathogens. These included laminarin, cerevisane, Reynoutria extract, and others. It is too early to say for sure if any of these products will provide a benefit, but some showed promise when combined with azoxystrobin.

I have also been working on a project to improve scouting of stem rot. Stem rot is the most common disease of rice with potential to cause significant yield loss. Two years of experiments in several M-206 fields have shown that when more than 20% of tillers at mid-boot or more than 50% of tillers at drain time have stem rot lesions, the risk of stem rot yield reduction is high. I will be conducting more sampling this year to refine these thresholds.

Blast was introduced to California in the late 90s. At that time, it was determined that only one blast race was present, race IG1. It is important to know the race present in an area because the genes of resistance that are used to make a blast-resistant variety are race specific. We currently don’t know for sure if IG1 is still the only race present in California. For the past three years, I have been collecting blast samples from blast-affected fields. I am collaborating with researchers at the Dale Bumpers National Rice Research Center in Arkansas to determine the blast races in these samples. If other races are found, that information can be used by our rice breeders to develop varieties that have the adequate resistance genes.

Blast would be the most important disease of rice in California if it wasn’t for the fact that blast does not occur every year. When blast epidemics occur, they cause severe losses. It is not known for sure why some years are blast years, but weather most likely plays a large role. If we knew what specific field weather conditions allow blast development, we could warn growers so that they could intensify monitoring and use preventive fungicide applications effectively. Last year, I deployed three weather stations in areas of Glenn County known to be susceptible to blast, collecting temperature, relative humidity, and leaf wetness data. I plan on continuing gathering these data for the next few years so that we can determine what kind of field level weather patterns we observe when blast epidemics occur. This is a mid- to long-term project, but hopefully, it can help us reduce the risk of yield loss due to blast in the future.

Sheath Blight

Sheath blight has been the most economically significant disease in Louisiana since the early 1970s. The disease is caused by Rhizoctonia solani, a fungal pathogen of both rice and soybeans. On soybeans, it causes aerial blight.

Several factors have contributed to the development of sheath blight from minor to major disease status. They include the increased acreage planted to susceptible long-grain varieties and the higher rates of nitrogen fertilizers used with the modern commercial rice varieties. The disease is favored by dense stands with a heavily developed canopy, warm temperature, and high humidity. The fungus survives between crops as structures called sclerotia or as hyphae in plant debris. Sclerotia or plant debris floating on the surface of irrigation water serves as sources of inoculum that attack and infect lower sheaths of rice plants at the waterline.

Sheath blight is characterized by large oval spots on leaf sheaths and irregular spots on leaf blades. Infections usually begin during the late tillering-joint elongation stages of growth.  Lesions about 0.5 to 1 centimeter in width and 1 to 3 centimeters in length are formed a little above the waterline on infected culms. Fungus mycelium grows up the leaf sheath, forms infection structures, infects, and causes new lesions. The infection can spread to leaf blades. The lower leaf sheaths and blades are affected during the jointing stages of growth. After the panicle emerges from the boot, the disease progresses.  Infected culms are weakened, and the tillers may lodge or collapse.

The fungus can spread in the field by growing from tiller to tiller on an infected plant or across the surface of the water to adjacent plants. The fungus also grows across touching plant parts, for example from leaf to leaf, causing infections on nearby plants. Infected plants are usually found in a circular pattern in the field because the fungus does not produce spores and must grow from plant to plant. The lesions have grayish-white or light green centers with a brown or reddish-brown margin. As lesions coalesce on the sheath, the blades turn yellow-orange and eventually die. As areas in the field with dead tillers and plants increase, they may coalesce with other affected areas to cause large areas of lodged, dead, and dying plants.

Damage is usually most common where wind-blown, floating debris accumulates in the corners of cuts when seedbeds are prepared in the water. Disease severity can be reduced by integrating several management practices. Dense stands and excessive use of fertilizer both tend to increase the damage caused by this disease. Broadcast seeding tends to increase stand and canopy density. Rotation with soybeans or continuous rice increases the amount of inoculum in field soils.

Fields should be scouted for the presence of sheath blight symptoms at least once a week beginning at mid-tillering and continuing until heading. Avoid dense stands and excessive N fertilizer. Most long-grain varieties have little resistance to sheath blight. Medium-grain varieties are more resistant. Fields should be scouted by making periodic random stops throughout the field. Tillers should be examined for the presence of symptoms. When 5% to 10% of the tillers of a susceptible variety or 10% to 15% of the tillers of a moderately susceptible variety are infected, a fungicide application is justified.

Timing and rate of fungicide applications are critical for good sheath blight management. Check with your Extension agent for the latest information on fungicides.

Sheath Blight Management Options in Mississippi

As I write this article on April 16, we are right in the middle of our optimum planting window in the state of Mississippi. Growers were eager to get in the field following the rain and cold temperatures that proceeded an early planting window in late March. I wanted to highlight some of the disease work we have been doing over the past few years in furrow-irrigated rice.

Over the past five years, Mississippi has seen increases in FIR acreage, culminating in 2024 with 30% of the acreage being in FIR production. Sheath blight (Rhizoctonia solani J. G. Kuhn) and rice blast (Pyricularia grisea Cavara) are two major disease concerns for furrow-irrigated rice producers in the Mississippi Delta. In Mississippi, disease incidence and management of rice sheath blight has not been well documented in FIR. Thankfully, during my doctoral degree program, I worked to address that to an extent.

A survey was conducted over the past three years, evaluating sheath blight incidence and severity across the three separate FIR management zones. Preliminary results from that survey indicated that 64% of the fields surveyed had sheath blight in the top portion of the field, followed by 56% in the middle, and 52% in the bottom portion. These results help to back up what consultants and growers see in their fields. I attributed this increase in pathogen presence to increases in canopy temperature and humidity we observed from the lack of the flood in the top zone. The timeline of development for sheath blight seems to be very similar to a flooded rice field, which would help to answer any fungicide timing questions that may arise. When fields were surveyed prior to mid-boot, sheath blight prevalence was low to non-existent. However, following heading to soft- and hard-dough stages, sheath blight prevalence increased similar to a flooded rice system when ideal disease development conditions are met.

In FIR, growers sometimes increase their in-season nitrogen rate to account for the yield loss that can be seen in certain FIR scenarios. These higher in-season nitrogen rates can increase rice production, which can also lead to exacerbated sheath blight symptoms. We looked at several commonly used rice fungicides: Quadris, Tilt, Elegia, Amistar Top, and Avaris 2XS in combination with greater nitrogen rates. When N was applied at the recommended rate, all the fungicides provided comparable suppression of sheath blight symptoms. However, when the N rate was increased to a 2x (360 pounds of N per acre) rate, fungicide application did not provide a reduction in sheath blight prevalence.

So, a few final take home points in FIR: a moderately susceptible variety is going to be the best option when it comes to varietal selection and suppression of sheath blight, as well as using your states recommended nitrogen rate and scouting before you spray. If you have any questions, please feel free to call. God bless and Hail State!